1911 Encyclopædia Britannica/Neuropathology

by Frederick Walker Mott

NEUROPATHOLOGY, the general name for the science concerned with diseases of the nervous system. As regards the anatomy and physiology, see the articles Nerve, Nervous System, Brain, Spinal Cord, and Sympathetic System. The morbid processes affecting the nervous system are numerous and varied, but usually they are clinically divided into two great groups of (1) organic disease, (2) functional disturbance. Such a classification depends upon whether or not symptoms observed during life can be associated with recognizable changes of the nervous system, gross or microscopical, after death. Sometimes this is the morbid process itself, sometimes only the ultimate result of the process. It must be remarked, however, that many diseases which we now look upon as functional may be found due to recognizable changes when suitable methods of investigation shall have been discovered. The paroxysmal neuroses and psychoses may be considered a priori to be due to temporary morbid functional conditions. Our knowledge of the first group is naturally much more advanced than of the latter, for, given certain symptoms during life, we are able, as a rule, to predict not only the nature of the morbid process, but its particular locality.

The histological elements which make up the nervous system may also be divided into two groups: (1) the nervous units or neurones, (2) the supporting, protecting and nutrient tissues. Organic diseases may start primarily in the nervous units or neurones and cause their degeneration; such are true diseases of the nervous system. But the nervous units may be affected secondarily by diseases starting in the supporting, protecting and nutrient tissues of the nervous system; such are essentially diseases within the nervous system, and include diseases of the blood-vessels, lymphatics, membranes and the special nervous connective tissue, neuroglia (a residue of the embryonal structure from which the nervous system was developed). Tumours and new growths must also be included.

The modern conception of the “neurone” as an independent complex cell with branching processes, in physiological rather than anatomical association with other neurones, has modified our ideas of the morbid processes affecting the nervous system, especially as regards degenerations of systems, communities or collections of neurones subserving special functions. It was formerly believed, and generally taught, that the primary systemic degenerations were due to a sclerosis; thus locomotor ataxy was believed to be caused by an overgrowth of the supporting glia tissue of the posterior columns of the spinal cord, which caused a secondary atrophy of the nervous tissue. We now know that this overgrowth of glia tissue is secondary to the atrophy of the nervous elements, and the only true primary overgrowth of glia tissue is really of the nature of the new growth (gliosis). But even in this case it is doubtful if the mere proliferation of the glia tissue elements could destroy the nervous elements, if it were not for the fact that it leads to changes in the vessel walls and to haemorrhages.

The symptoms manifested during life depend upon the nature of the morbid process and the portion of the nervous system affected. A correct understanding of neuropathology involves the study of (1) the causes which give rise to morbid conditions, which are often complex and due to various combinations of factors arising from without and within the body, and (2) the changes in the structure and functions of the nervous system brought about by intrinsic and extrinsic causes.

The causes of pathological processes occurring in the nervous units (neurones) may be divided into internal and external, and it may be remarked that in all cases except direct injury the two groups are generally more or less combined.

A. Internal Causes.—Of all the causes of nervous disease hereditary predisposition stands pre-eminently first; it may be convergent, paternal, maternal; from grandparents or even more remote ancestors. Moreover, no study of heredity is complete that does not take into consideration collaterals. Especially does this statement apply to functional neuroses, e.g. epilepsy, migraine, hysteria and neurasthenia; and to psychoses, e.g. delusional insanity, mania and melancholia, manic-depressive, recurrent or periodic insanity and dementia-praecox or adolescent insanity.

In 70% of 150 cases of idiocy or imbecility in the London county asylums, Dr Tredgold found a family history of insanity in some form or another. Strictly speaking, it is the tendency to nervous disease rather than the disease itself that is inherited, and this is frequently spoken of as a neuropathic or psychopathic taint. There are, besides, a number of inherited diseases, which, although somewhat rare, are of interest inasmuch as they affect members of a family, the same disease frequently commencing in each individual at about the same age. These are termed family diseases, and include hereditary ataxia (Friedreich’s disease), myotonia (Thomsen’s disease), hereditary (Huntingdon’s) chorea, amaurotic idiocy and various forms of idiopathic muscular atrophy. Alcoholism, tuberculosis and syphilis in the parents, especially if one or both come from a neuropathic or psychopathic stock, frequently engender idiocy, imbecility, epilepsy and general paralysis in the offspring, by the production of defects in the vitality of the germinal plasm, causing arrest, imperfect development or premature decay of groups, communities or systems of neurones, especially those which are latest developed—the symptoms manifested depending upon the portions of the nervous system affected. To explain the hereditary neuropathic tendency morphologically, we may suppose that there is an inherited defect in the germinal plasm which is concerned in the formation of the neurones. We may regard the neurone as a complex cell, and the nervous system as a community of neurones arranged in systems and groups having special functions. Like all cells, the neurone nourishes itself and is not nourished; certainly it depends for its development, life and functional activity upon a suitable environment, but it must also possess an inherent vital energy by which it can assimilate and store up nutrient material which may be regarded as potential (latent nerve energy), to be converted into nerve force as required. A constant constructive and destructive bio-chemical process occurs in the neurones of a healthy nervous system, latent nervous energy is high and the sense of fatigue is the natural indication for sleep and repose, whereby it is constantly recuperated. In the neuropathic or psychopathic individual it may be conceived that in some portion of the nervous system, especially the brain, there may exist communities, systems or groups of neurones with inherited low potential, readily becoming exhausted, and, under the influence of altered blood states or stress, especially liable to functional depression, from which arise function-paralysis and melancholia. Again, the bio-chemical substance which represents potential in the nervous system may be in a chemically unstable condition, so as readily to fulminate when excited by abnormal conditions (e.g. toxic conditions of the blood), thus acting as a centre of discharge of nervous energy, which may be manifested by mental or bodily symptoms. We know that in strychnia and tetanus poisoning the most localized peripheral excitation will cause general muscular spasm; in both toxic conditions the spread is probably due to a bio-chemical change in the protoplasm of the spinal neurones, whereby the excitability is greatly increased and a slight stimulus is sufficient to fulminate the whole system of motor neurones. In epilepsy and other paroxysmal neuroses and psychoses it is possible that some altered condition of the blood, when associated with an inherited bio-chemical instability of certain groups, systems or communities of neurones, may act as a fulminating agent. In neuralgia and local hyperaesthesia the slightest general or distant local irritation suffices to produce pain; thus coughing, the vibration of a passing train or the slamming of a door may produce pain by the stimulation of the hyper-excitable neurones. Moreover, it must be borne in mind that the symptoms of nervous disease are due as much to normal physiological functional activity improperly applied, as to actual loss of function occasioned by disease. Thus squint, caused by paralysis of one of the muscles of the eyeball, causes less trouble to the patient than the double vision occasioned by the physiological activity of the two retinae, upon the corresponding points of which the images are prevented by the paralysis from falling.

B. The external causes producing morbid changes in the nervous elements are: I. Abnormal conditions of the blood and lymph, by which the neurones are poisoned and their metabolism morbidly affected. II. Excess or deficiency of normal stimulation, or existence of abnormal stimulation. III. Injury or diseases of supporting, enclosing or vascular tissues.

I. Abnormal Conditions of the Blood and Lymph.—The immediate environment of all the cellular elements of the body is lymph, and in the central nervous system there is a special form of lymph, the cerebro-spinal fluid, which is secreted by the choroid plexus in the venticles of the brain. The neurones, like other cellular elements, are bathed in the lymph, and extract from it the materials necessary for their growth and vital activities, casting out the waste products incidental to the bio-chemical changes which are continually taking place. The lymph, therefore, serves as a medium of exchange between the blood and the tissues, consequently the essential causes of change in environment of the nervous elements (neurones) are: (1) Deficiency or absence of blood-supply to the nervous system in general (as after severe haemorrhage), or to some particular portion, owing to local vascular disturbance or occlusion. (2) Alterations in the normal condition of the blood, due to (a) deficiency or absence of certain essential constituents, (b) excess of certain normal constituents, (c) the presence of certain abnormal constituents produced within the body, or entering it from without.

(1) Quantity of Blood Supply.—Syncope or fainting occurs when the blood supply suddenly fails to reach the higher centres of the brain; this usually arises from sudden reflex arrest of the heart’s action. If a portion of the central nervous system is cut off from its arterial blood supply by embolic plugging or by clotting of the blood in a vessel with diseased walls, the portion of the brain substance thus deprived of blood undergoes softening, the nervous elements are destroyed, and the systems of nerve fibres, which have had their trophic and genetic centres in the area destroyed, undergo secondary degeneration. Clotting of the blood in the veins may also give rise to destructive softening of the brain, and similar secondary degeneration.

(2) Quality of Blood Supply.—(a) Insufficiency of oxygen, due to poverty of the colouring matter or of the number of the red corpuscles, which constitutes the various forms of anaemia, leads to functional depression, lassitude and mental fatigue. Impoverishment of the blood in women by frequent pregnancies and excessive lactation causes neuralgia, nervous exhaustion and, in the neuropath, hysteria, neurasthenia, melancholia and mania. The mental depression, and the tendency that the various neuroses and psychoses have to occur and recur at the time of the menstrual and climacteric periods in women, suggests the possibility of an alteration in the composition of the blood, either in the nature of an auto-intoxication or “sub-minimal deficiency,” as the probable contributory factor of the mental disturbance. It may be remarked that eclampsia, puerperal and lactational mania are relatively common forms of insanity in women; although sometimes of septic origin, they more frequently are occasioned by some morbid metabolism as yet little understood. The most striking examples we have, however, of the effect of absence or “sub-minimal” deficiency of a normal constituent of the blood upon the development and functions of the nervous system are afforded by cretinous idiots, who are born without thyroid glands, and whose brains never develop in consequence; and by those people who suffer from the disease known as myxoedema, occasioned by the absence of iodothyrin, a product of the internal secretion of the thyroid gland. The proof of this is shown by the disappearance of the nervous phenomena, slowness of thought, slowness of speech, &c., after a preparation of the gland has been continuously administered by the mouth. Even cretinous idiots when subjected in early life to thyroid treatment improve considerably. The removal of the testicles in the male may produce a profound effect upon the nervous temperament; for probably there is an internal secretion of this gland in the male, as of the ovary in the female, which has some subtle influence upon the functional activity of the nervous system. The seminal fluid contains a large amount of complex phosphorus-containing substances, which, lost to the body by sexual excess or onanism, have to be replaced by the blood; the nervous system, which also needs these complex organic phosphorus compounds, is thereby robbed, and neurasthenia ensues. Brown-Séquard’s testicular injection treatment for many nervous complaints, based upon this idea, has not, however, met with much success.

(b) Excess of certain Normal Constituents in the Blood.—Excess of carbonic acid causes drowsiness, and probably in asphyxia is one of the causes of the convulsions. All the series of the nitrogenous waste products—the most highly oxidized, most soluble and least harmful of which is urea—are normal constituents of the blood; but should the oxidation process be incomplete, owing to functional or organic disease of the liver, or should these substances accumulate in the blood, owing to inadequate function of the kidneys, a toxic condition, called uraemia, may supervene, the nervous manifestations of which are headache, drowsiness, unconsciousness or coma, epileptiform convulsions and sometimes symptoms of polyneuritis. Again, in Graves’s disease, nervous phenomena, in the form of exophthalmos, fine tremors, palpitation and mental excitement, have by some authorities been explained by the excess of thyroid internal secretion, due to the enlargement and increased functional activity of the gland. The successful treatment of Graves’s disease by the administration of the blood serum and milk of animals (goats), which had the thyroid glands removed, supports this theory.

(c) The presence of abnormal constituents in the blood is a most important cause of disease of the nervous elements. We may consider the subject under the following headings: Poisons produced within the body (α) by perverted function of organs or tissues, auto-intoxication; (β) by the action of micro-organisms, protozoa and bacteria, upon the living fluids and tissues of the body; (γ) poisons introduced into the body from without, in the food and drink, or by inhalation.

(α) Poisons resulting from perverted Function of the Organs.—In the process of digestion a number of poisonous substances, e.g. albumoses, &c., are produced, which, although absorbed in the alimentary canal, are prevented by the living epithelium, and possibly by the liver, from entering the systemic circulation. Fatigue products, e.g. sarcolactic acid in prolonged muscular spasms, may lead to auto-intoxication. Excess of uric acid in the blood is associated with high arterial pressure, deposits of lithates in the urine, headache and nervous irritability; it is an indication of imperfect metabolism and auto-intoxication, as shown by the fact that marked improvement occurs by suitable diet and treatment. Phosphoruria, oxaluria and glycosuria, tokens of deranged metabolism, may be associated with various nervous phenomena. Bile in the blood, cholaemia, resulting from obstructive jaundice, may be attended by stupor and psychical depression; and the term melancholia, signifying “black bile,” indicates the importance which has long been attached to the liver as an organ the derangement of which causes nervous depression. The rapidly fatal results attending acute yellow atrophy of the liver, namely, the profound changes in the urine, the jaundice and the nervous phenomena of delirium, motor irritation, delusions, stupor and coma, demonstrate the important part this organ plays in preserving the normal quality of the blood. The delirium and coma which sometimes supervene in diabetes, heralded by acetonaemia, is another instance of auto-intoxication. The coma is very possibly due to the saturation of the sodium salts of the blood by aceto-acetic and oxybutyric acids, products of imperfect proteid metabolism. The effect of this would be an interference with the elimination of carbonic acid in the processes of tissue and pulmonary respiration. Again, in pernicious and certain grave anaemias, the degenerative changes in the spinal cord found in some cases is due, not so much to the defect in the red corpuscles, as to some neuro-toxin, which probably arises from imperfect metabolism or absorption from the alimentary canal. In this question of auto-intoxication, it must be remarked that all the tissues of the body are mutually interdependent. If one suffers, all suffer, and a disease of one organ or tissue is thereby apt to establish a vicious circle which is constantly enlarging; therefore nervous symptoms manifesting themselves in the course of a disease add much to the gravity of the complaint.

(β) Poisons produced by Infective Micro-organisms.—Some of these poisons have a general devitalizing influence, by an alteration of the blood and the production of fever. In the course of the acute infectious diseases, typhoid, typhus, smallpox, scarlet fever, measles, influenza, also tuberculosis and septicaemia, delirium is a frequent complication; it may be the result of high fever or prolonged fever, or directly due to the poison, or the two combined. In severe cases stupor and coma may occur, and it has been shown that in this extreme stage the nerve cells undergo an acute morbid bio-chemical change. These particular poisons have no selective toxic action upon a particular part of the nervous system, and symptoms not only during, but after, the acute illness are liable to supervene, especially in a neuropathic individual. Thus many cases of neurasthenia, insanity, neurosis, also neuritis, date their origin from an acute specific fever. In cerebro-spinal meningitis, tubercular meningitis, acute delirious mania and leprous neuritis, the inflammation of the membranes of the brain and spinal cord is due to the growth of the specific organism in the lymph and interstitial tissue elements.

Poisons may have a selective influence upon some part of the nervous system. The syphilitic poison is the most important factor in the production of two progressive degenerations of the nervous system—one affecting especially the afferent conducting tracts of the spinal cord, namely, locomotor ataxy, and the other affecting especially the frontal and central convolutions of the cerebral hemispheres, namely, general paralysis of the insane. A striking instance of the selective action of the syphilitic poison is shown in the fact that only in persons affected with acquired or inherited syphilis is a symptom known as Argyll-Robertson pupil found; this is the absence of the pupil reflex contraction to light, while that for accommodation persists. Seeing that this is the most common objective phenomenon

in the two diseases mentioned, it strengthens the presumption,

based on experience, that the syphilitic poison is the cause of these diseases in the majority of instances. Again, syphilis, when it attacks the supporting, enclosing and nutrient vascular tissues, shows a predilection to affect structures about the base of the brain, and paralyses of the third nerve are almost pathognomonic of this disease. In rabies, although the whole nervous system is charged with the poison, the medulla oblongata (as shown by the symptoms) is especially affected. Again, in tetanus the bacilli are only found in the wound; they must therefore be comparatively few in number, but they elaborate a virulent poison, which affects particular groups of neurones. The fact that lockjaw nearly always occurs first, shows that the poison selects the motor nucleus of the fifth nerve; but it is remarkable that experiment has shown that the tetanus toxin, if mixed with an emulsion of nervous matter before injection into an animal, loses its toxicity. This fact indicates its affinity for nervous matter, and also a power of absorption of the poison by some chemical substance in the nervous matter. Another example is offered by diphtheria. A neuro-toxin is produced by the local action of the bacilli, for they do not become freely generalized in the blood and tissues. Whether the poison is a direct production of the bacilli themselves, or is an auto-toxin created in the body itself, by an influence exerted on the living fluids and tissues by a ferment-like product of the bacilli, is not determined. But whatever may be the source of the toxin, its effects upon the neurones are constant, as shown by the sufferings of the patients—paralysis of the soft palate, with nasal speech and regurgitation of fluids through the nose when swallowing is attempted; inability to read, owing to the paralysis of the muscle of accommodation; weakness and inco-ordination of the limbs, which may amount to paralysis; absence of the knee-jerks; and often skin anaesthesia.

The relation of protozoa to the existence of widespread diseases affecting men and animals is becoming yearly of greater importance and interest. Certain hitherto obscure diseases in which the nervous system is profoundly affected are now explained by the invasion of the tissues of the body by these lowly organisms, for example, Protozoa and diseases of the nervous system. Sleeping Sickness, the cause of which has been definitely proved to be the Trypanosoma gambiense (see Plate II. fig. 1.)

The discovery by Schaudinn of the presence of the Spirochaete Pallida (see Plate II. fig. 2) in the primary and secondary lesions of seventy successive cases of syphilis, and the general acceptance of this organism as the cause of the disease, taken together with the fact that in many respects it simulates the trypanosome in its mode of division and other characters, tend to prove that syphilis is also a protozoal disease.

The bacterial invasion of tissues is generally characterized by a migration of polymorpho-nuclear leucocytes, but protozoal invasion is characterized by a formative hyperplasia of the fixed cell tissues, endothelial, epithelial and conjunctival, and there is a close similarity in the defensive reaction of the tissues to all forms of protozoal invasion (see Plate II. with explanatory text).

If the cause of rabies be regarded as proved since the discovery of Negri bodies, we may assume that just as in malaria the Haematozoon malariae undergoes its endocellular development in the red blood corpuscle, the protozoon of rabies undergoes its endocellular development in the nerve cell.

Only a short time has elapsed since Negri showed that in cases of rabies, whether experimental or otherwise, curious bodies measuring from 1 μ to 20 μ. could be constantly found in the nerve cells, and that these bodies are not found in the nerve cells in any other disease; so that even if the theory advanced that they are endocellular forms of protozoa prove not true, yet the discovery affords a valuable and expeditious means of determining whether a suspected animal suffered with rabies or not. It is known that the salivary glands and saliva contain the virus, even before the animal shows symptoms. It is known too that the central nervous system contains the virus and that it multiplies there. Experimental inoculation can be made either from the saliva or an emulsion of the central nervous system of an animal suffering with rabies. Moreover, the virus can pass through a Berkfeldt filter; and if the filtrable product be injected into an animal, the animal thus inoculated will die of rabies and exhibit the Negri bodies. There are only two conclusions to be drawn from these observations: (1) If it be a protozoal disease, the organism at one period of its developmental cycle must be so small as to be able to pass through the pores of the Berkfeldt filter. (2) Negri bodies are the result of intra-cellular degenerative changes caused by an elective affinity of the virus for the protoplasm of the nerve cell. The virus, whatever it may be, does not exist in the blood and other organs and tissues. Seeing that the Negri bodies cannot be found in the saliva, although the saliva contains the virus, nor can they be found in the peripheral nerves, although the virus passes by the lymphatics of the nerves to the nerve cells of the spinal ganglia and central nervous system, it must be concluded that the filtrable virus travels to the central nervous system and there increases.

It is a remarkable fact that before the discovery of the Negri bodies, the diagnosis of rabies was made by microscopic examination of the spinal and sympathetic ganglia, particularly the ganglia of the vagus and fifth nerves. Changes were found similar to those met with in other protozoal diseases, namely, sleeping sickness, dourine and syphilis. These changes were proliferation of the interstitial connective tissue cells forming the supporting structure of the ganglion and hyperplasia of the lymphatic endothelial cells forming the capsule containing the nerve cells.

The diagram here given (fig. 1) after Volpino explains the supposed developmental cycle of the protozoon which is presumed to be the cause of rabies. The weak link in the chain is the assumed sporozoit which is so small as to be capable of passing through a Berkfeldt filter. It has taken twenty years to lead to the complete knowledge of the life history of the malarial parasite and its relation to the disease, and all we can say is that there is now a certain amount of evidence forthcoming which tends to show that rabies is due to a protozoon, which Calkins, who discovered a similar body in the epithelial cells of variola, places among the rhizopods.

There are certain chronic trypanosome infections in which the nervous symptoms form a special feature of the disease, notably sleeping sickness (see Plate II. fig. 1) and a disease affecting horses, termed mal de coit or dourine.Trypanosome diseases and affections of the nervous system.

The chronic trypanosome affections resemble in many respects syphilis; they are characterized by local infection, enlargement of the nearest lymphatic glands, a general polyadenitis and successive eruptions, accompanied by fever. The tissue changes are the same whether we examine the primary seat of infection, papular eruptions on the mucous membrane or the skin, or the lymphatic glands.

From a coloured plate in Centralblatt für Bakteriologie, by permission of Gustav Fischer.

Fig. 1.

When the nervous system is affected a local or general chronic meningo-encephalitis is set up, characterized by a meningeal and perivascular infiltration with lymphocytes and plasma cells, occasioned by a chronic irritative process, presumably caused in the case of sleeping sickness by the presence of trypanosomes in the cerebro-spinal fluid (see fig. 8, Plate II.). The same perivascular and meningeal infiltration with plasma cells and lymphocytes is found in syphilitic and parasyphilitic diseases of the nervous system (see Plate II., figs. 7 and 9).

The significance of pathological changes in the cerebro-spinal fluid has recently become of great importance in the diagnosis of nervous diseases, and a short account of the subject in this article will therefore not be out of place. The cerebro-spinal fluid is clear like water; it has a specific gravity of 1006 and resembles in its composition the blood minus its Pathology of the cerebro-spinal fluid. corpuscular and albuminous constituents. It is secreted by the choroid plexus, and if any cause, such as tumour or meningitis, should interfere with its escape from the ventricles it gives rise by pressure to internal hydrocephalus and cerebral anaemia which may occasion epileptic convulsions and various degrees of drowsy stupor, lethargy, unconsciousness and even coma. Withdrawal of the fluid by lumbar puncture and by tapping the ventricles of the brain has been employed in treatment, but without very satisfactory results. If, however, lumbar puncture has proved of but little use in treatment, it has proved of inestimable service in the diagnosis of various diseases of the central nervous system. The fluid withdrawn may be examined in various ways which are complementary to one another.

It should be centrifuged and the deposit examined microscopically if necessary after staining by suitable methods; the existence of cells in a fluid which normally contains no cellular elements indicates disease of the central nervous system. In general paralysis, syphilis of the nervous system and tabes dorsalis even in early stages of these diseases, the deposit is seen to consist almost entirely of lymphocytes. Some evidence of the progress of the disease and the effect of treatment may be obtained by counting the number of cells at different periods. In tubercular meningitis there are also lymphocytes in abundance although usually tubercle bacilli cannot readily be found, yet bacilli are present, for injection of the fluid into a guinea pig is a certain means of determining whether it is tubercular meningitis or not; for if it is, the animal is sure to develop tubercle. In epidemic cerebro-spinal meningitis the cells in the deposit are polymorphonuclear leucocytes, and in the leucocytes can be seen the specific organism Diplococcus intracellularis with its characteristic staining and cultural characters. Septic, pneumonic and pyogenic organisms may also invade the central nervous system giving rise to meningitis, and in these cases the deposit will be polymorpho-nuclear leucocytes, and perhaps the specific organisms may be seen in stained preparations; but if not, they can be obtained by cultural methods. In all operations of this kind antiseptic precautions must be adopted both for the safety of the patient and the reliability of the findings, otherwise organisms in the skin may contaminate the fluid withdrawn.

Other formed elements which may be found are large cells, macrophages containing blood pigment; these cells indicate that some haemorrhage has occurred. One of the most important uses of lumbar puncture has been the discovery of the cause of sleeping sickness. The fluid withdrawn and centrifuged contains, as one would expect from the lesions in the brain and spinal cord, large numbers of lymphocytes and plasma cells (see Plate II. fig. 10), but besides, the actively moving organisms (Trypanosoma gambiense) (see Plate II. fig. 1) which are the essential cause of the disease. It has been remarked that the normal cerebro-spinal fluid is devoid of proteins, but in the various forms of disease above described as containing cells in the centrifuged deposit, there is also in the fluid an appreciable amount of proteins. If pathological cerebro-spinal fluid be added to an equal quantity of saturated solution of sodium sulphate there will be a distinct turbidity indicating the presence of proteins in appreciable quantity. This appreciable quantity. of proteins is especially significant in the case of fluid withdrawn from cases of general paralysis or tabes, for it goes pari passu in amount with a reaction which is known as the Wassermann sero-diagnostic reaction for syphilis; a reaction, however, which is too complicated to explain here, but which is of the greatest importance for the diagnosis of general paralysis and tabes dorsalis. The finding of the Trypanosoma gambiense in the cerebro-spinal fluid in sleeping sickness led to the belief that the specific organism of syphilis, Spirochoete pallidum might be found in the cerebro-spinal fluid in syphilitic diseases of the nervous system, but although in a few instances successful inoculation of animals with syphilis by injection of the cerebro-spinal fluid has been effected, yet the organism has only once been found in the fluid withdrawn by lumbar puncture. It has long been a puzzle why only certain individuals, about 5%-8% of those infected with syphilis, should subsequently suffer with diseases of the nervous system. The skin and mucous orifices are the most common sites of secondary and tertiary lesions and after this the nervous system, but no tissue or structure in the body is exempt. It is probable that the virus attacks tissues when in a low state of resistance in a random metastatic manner. It is necessary to distinguish between these true syphilitic lesions which are the result of the reaction of the tissues to the living virus and the parasyphilitic affections, which own a different cause. The former may be most successfully treated with mercury, which has the power of devitalizing the specific virus and preventing its multiplication, the same as atoxyl prevents the multiplication of the trypanosomes. Iodide of potassium favours the absorption of the degenerative products of the cells, and syphilitic tumours may rapidly resolve and disappear under the influence of these drugs. Nervous symptoms even so severe as to threaten a rapidly fatal termination may disappear with energetic treatment when they are due to the syphilitic virus producing an inflammatory reaction of the tissues; not so, however, when the symptoms are slow, insidious and progressive, due to a primary decay of the neurones, e.g. the parasyphilitic affections tabes dorsalis and general paralysis of the insane, which are really one and the same disease owning the same cause. We can understand that it may be a chance whether a man suffers with true brain or spinal cord syphilis, because it may be a chance whether the virus is carried there by the blood-vessels and lymphatics, and if carried there finds a suitable nidus to develop. But the parasyphilitic affections appear to be due to a premature primary decay of the neural elements owing to bio-chemical changes in the body induced by reaction to the syphilitic virus. There are a good many facts now forthcoming which show that the subjects of parasyphilis present mild symptoms of syphilis, and upon an average it is not until ten years later that they develop nervous symptoms, which are aggravated rather than benefited by mercury. Such subjects are immune to a second attack of syphilis, and the examination of the blood and cerebro-spinal fluid by the Wassermann reaction of the deviation of the Complement reveals the fact that there is a bio-chemical change; the presence of this reaction may be correlated with the fact that these fluids contain lipoid substances and a globulin in excess. The cerebro-spinal fluid contains these lipoid substances and globulin in proportion to the degree of decay of the neural structure; they arise from the destructive metabolism of the neural elements. But the same lipoid substances and globulin are found only in the blood of syphilitic individuals, consequently it must be supposed that in general paralysis and tabes certain groups and systems of neurones undergo decay from excessive metabolic activity which is brought about by two factors (1) a bio-chemical stimulus, the syphilitic poison, (2) excessive physiological stress, which in non-syphilitic individuals would only lead to cerebral or spinal neurasthenia.

Sleeping Sickness is characterized by a progressive lethargy, paresis, tremors and the signs and symptoms of neural exhaustion without neural destruction; it comes on slowly and insidiously often years after infection and eventually terminates fatally by intercurrent disease or paralysis of the bulbar centres. Examination of the central nervous system explains the fatal lethargy; the perivascular and meningeal, lymphatics are filled with lymphocytes and plasma cells (Plate II. fig. 6.); moreover, the neuroglia supporting cells have undergone a rapid formative proliferation (Plate II. figs. 3 and 5). The effect of this morbid process is to deprive the neural elements of oxygen and nutrition; the neurones in consequence, although not destroyed, are nevertheless unable to function for more than a brief period.

(γ) Poisons introduced into the Body.—The most widespread and potent cause of nervous and mental disease is the abuse of alcoholic stimulants. At least 20% of the inmates of the asylums of London are admitted with a history of alcoholism. In not more than 10% is alcohol the efficient cause of the mental disease; in many it is only a contributory factor, and in not a few the lapse from moderation to intemperance is the first sign of the mental breakdown. Most of the patients admitted inherit the neuropathic tendency, and it is a rare thing, among such, to find cirrhosis of the liver with ascites, condition which indicates long persistent spirit-drinking. The writer, from a very large experience as pathologist to the asylums of London, only remembers one such case, and that was in a notorious woman who was convicted nearly four hundred times for drunkenness before she could be certified as of unsound mind, a fact which indicates that she inherited a very stable nervous constitution. To people with unstable nervous systems a relatively small quantity of alcohol may act as a poison. Thus epileptics, imbeciles, criminals, potential lunatics, hysterics, neurasthenics and the subjects of head injury are liable to become anti-social and dangerous to themselves and others by indulgence in quantities of alcohol which would have no harmful effect upon the mentally stable and sound individual. Alcohol may produce acute delirium, with fine tremors, and, generally, visual hallucinations of a horrible nature, indicating acute toxic influence upon the brain. This apparently acute form of alcohol poisoning is met with in chronic inebriates especially; it is much commoner in men than in women, and it is remarkable how a severe injury or illness, such as pneumonia, will bring out delirium tremens in a drunkard. Chronic alcoholism manifests itself in a variety of ways according to the inborn temperament of the individual. The well-fed man with an inborn stable well-balanced mental organization is able to consume daily large quantities of alcohol with no other obvious effect than the lowered moral sense of indulgence in a vicious habit. However, chronic alcoholics form a large proportion of those convicted for crimes of violence, homicide, suicide and sexual offences. Alcohol acts especially upon the higher centres of the brain, and a drunken man may exhibit “the abstract and brief chronicle of insanity, going through its successive phases in a short period of time” (Maudsley). The effect on the nervous system of chronic tippling may be dementia, a very characteristic manifestation of the mental degradation being absence of knowledge of time and place, personal illusions and loss of memory of recent events, indicating a failure of receptivity and of the formation of memory-pictures in the higher centres, mental confusion, delusions of persecution, and especially a morbid jealousy with suspicions of fidelity of the husband by the wife or of the wife by the husband. A certain amount of improvement may occur when total abstinence is enforced, which shows the poison has damaged but not destroyed the nervous elements. There is also a form of mental disease characterized especially by hallucinations of hearing and vision, associated with delusions usually of a persecuting nature, unaccompanied by other marked mental disorder. Abstinence and proper control generally ends in recovery, but such cases so frequently relapse that it is fairly certain that alcohol is an exciting factor to a morbid or insane temperament. Besides mental symptoms of chronic alcoholic poisoning, there is frequently paralysis, affecting especially the lower limbs (structures suffer most where vitality is least), although the upper limbs, and even the respiratory muscles, may be affected in severe cases. The patient, usually of the female sex, becomes helpless and bedridden, and death frequently occurs from heart failure. Characteristic features of this affection are great tenderness on pressure of the muscles, especially of the calves, absence of reflexes, a variable degree of skin anaesthesia, wasting of muscles and alteration of the normal electrical reactions, and frequently pyrexia. There is no loss of control over the bladder and bowels, unless there is very marked dementia. This “complex of symptoms” points to a peripheral polyneuritis, although frequently changes occur also in the ganglion cells, from which the axis cylinders of the nerves have their origin (vide figs. 2, 3, 4, and 5). Alcoholic polyneuritic psychosis affecting women in many ways resembles delirium tremens; the fact that neuritis occurs much more frequently in women is probably associated with a greater liability to the influence of microbial toxins by absorption from the organs of reproduction. Many other poisons, notably lead and arsenic the specific fevers before mentioned, syphilis and alterations of the blood due to imperfect metabolism, such as occur in diabetes and gout, may produce, or become important factors in producing, peripheral neuritis. The outbreak of arsenical neuritis from beer containing this poison in Manchester in 1900 is of interest, from the fact that the symptoms closely resembled acute alcoholic neuritis. A distinctive feature, however, was the pigmentation of the skin and the severity of the nervous symptoms. A disease which is common in the East, termed Beriberi, is a form of neuritis, the cause of which is not exactly known (see Beri-Beri). Anaesthetic leprosy is an interstitial inflammation of the nerves due to the Lepra bacillus. Among the nervous diseases due to occupation may be cited lead-poisoning. This is peculiar in selecting the nerve which supplies the extensor muscles of the wrist and fingers, so that dropped wrist is almost characteristic of this form of toxic neuritis. Lead also produces a chronic inflammation of the cerebral cortex, Encephalitis saturnina, causing a complex of symptoms, namely, dementia, loss of memory, weakened intellect, paresis and epileptiform seizures, hallucinations of sight and hearing, and mental exaltation or depression. Mirror-makers suffer with characteristic fine tremors, from the slow absorption of mercury into the system. Workmen at indiarubber factories may suffer from severe mental symptoms, owing to the inhalation of the fumes of bisulphide of carbon. Serious nervous symptoms have followed carbon monoxide poisoning. Cases which have recovered from the immediate effects have suffered with dementia and symptoms of disseminated sclerosis, the result of multiple haemorrhagic softenings.

Figs. 2, 3, 4 and 5.—Spinal motor cells in various stages of destruction, from a case of acute alcoholic poly-vacuolation. Compare with the appearances of a normal cell, fig. 12.

There are a certain number of poisons, besides alcohol, which act upon the nervous system when continually entering the body as the result of a habit, namely, absinthe, ether, cocaine, opium, morphia, hashish and tobacco. Each of these poisons produces a train of symptoms denoting a selective influence upon certain parts of the nervous system. In illustration thereof may be mentioned impairment of central vision in tobacco amblyopia.

The disease pellagra, an affection of the skin associated with degenerative changes in the brain and spinal cord and characterized by melancholy with suicidal impulses, sometimes mania associated with paresis, was long considered to be due to the eating of bad maize. But in 1910 the recent research on this disease, still in progress, seemed to negative this theory (see Pellagra. Another disease, ergotism, in an epidemic form, has affected poor people in Russia and North Germany when obliged to subsist upon bread made of rye which has been attacked by the ergot fungus. The poison thus introduced into the system produces progressive degenerative changes in the brain and spinal cord, which are manifested by psychical disturbances, such as slowness of thought, weakness of memory, dulness of perception, sometimes delirium and incoherence; other symptoms are blunted sensibility, dilated pupils, muscular spasms, perhaps even epileptiform seizures and ataxy, and, lastly, stupor deepening into coma. Sausage disease, due to eating decayed meat and fish infected with Bacillus botulinus, is associated with symptoms which frequently terminate fatally, and it has been shown that the symptoms are due to a poison which has a very destructive effect upon the nerve cells (fig. 6).

Fig. 6.—Cell illustrating swelling of nucleus and chromatolysis in acute toxaemia produced by poison of bacillus botulinus. Compare with the appear­ances presented by a normal cell, fig. 12.

II. Normal and Abnormal Stimulation.—The nervous system, in order to develop and manifest functional activity, requires suitable stimulation from without. Structure and function are mutually reciprocal and interdependent; for a structure which is not used will gradually lose its function, while its nutrition will also suffer, and in time atrophy may occur. Consciously and unconsciously, a continuous stream of impulses is pouring into the nervous system from without by the sensory channels, which are the avenues of experience and intelligence, and our somatic and psychical life depends upon the existence of such stimuli. The nervous system in the form of systems, groups and communities of neurones, each with special functions, yet all woven together in one harmonious whole, develops in a particular way in consequence of the awakening influence of these stimuli from without. Consequently nervous structures which are not used are liable to undergo regressive metamorphosis and atrophy; thus amputation of a limb in early life causes atrophy of the nervous structures which presided over the sensation and movement of the part. This is seen both in the grey and white matter of the spinal cord; there is also an atrophy of the psychomotor neurones of the brain presiding over the movements of the limb.

A healthy physical, intellectual and moral environment of the individual is an essential factor in the prevention and cure of psychoses and neuroses, because it tends to develop and strengthen body and mind, deliberation, judgment and the higher controlling functions of the brain. A function not used will gradually disappear, and become more and more difficult to evoke. This fact is of importance in functional neuroses and psychoses, e.g. hysterical paralysis, melancholia and delusional insanity, because the longer mental or bodily function is left in abeyance, the more likely is the defect to become permanently installed. The converse is also true; the longer a perverted function exists, the more unlikely it is to disappear. Thus auditory hallucinations, a very important and frequent symptom in the insane, commence with indistinct noises: these are followed by “voices,” which eventually become so distinct and real that the greater part of the patient’s psychical existence is concentrated upon, and determined by, this abnormal stimulus from within, indicating progressive strengthening and fixation of the perverted functions of the mind, and progressive weakening and dissolution of the normal functions.

Mental pain in the form of grief, worry, anxiety, fright, shock, violent emotions (pleasurable or painful), disappointed love, sexual excesses or perversions, and excessive brain work, frequently precede and determine, in persons with the insane or neuropathic taint, various forms (a) of psychoses, e.g. mania, melancholia, delusional insanity; (b) of neuroses, e.g. chorea, hysteria, epilepsy, hysteroepilepsy; (c) or organic brain disease, e.g. apoplexy, thrombosis, general paralysis.

Visceral reflex irritation affords many examples of neuroses and psychoses, the symptoms of which are set up by irritation of the viscera, e.g. intestinal worms. Teething and indigestible food are often the exciting cause in infants and young children of convulsions, spasms of the glottis and tetany. Various functional and organic diseases of the female reproductive organs act as exciting causes in the production of hysteria, hystero-epilepsy, melancholia and mania; moreover, paroxysmal attacks in these diseases are more liable to occur at the menstrual period or menopause. The irritation of a carious tooth may produce spasmodic tic and trigeminal neuralgia. Wax in the ear may occasion vertigo and tinnitus; and errors of refraction in the eyes may be the cause of attacks of migraine, and even tend to excite epileptic fits in a person suffering from epilepsy. Numerous other examples of peripheral disturbance could be mentioned as exciting causes of nervous affection in neurotic individuals. Irritation of the terminals of the vagus in almost any part of its widespread visceral distribution may lead to vomiting. The characteristic pain of angina pectoris, which radiates down the inner side of the left arm, is explained by the fact that the cardiac branches of the sympathetic arise from the same segments of the spinal cord as the sensory branches of the ulnar nerve; consequently the pain is referred to the corresponding skin area supplied by this nerve. This is one example of a great number of referred pains.

III. Injury or disease of enclosing or supporting structures may lead to paralytic or irritative lesions of the nervous system, or the two may be combined. Blows or wounds of the head and spine may damage or destroy the nervous structures by shock or direct injury. Concussion of the brain or spinal cord may occur, as a result of injury, without any recognizable serious damage of the enclosing structures or even the central nervous system. Shock, due to concussion, can only be explained by a molecular or bio-chemical change in the nervous structures.

Direct injury or a fall fracturing the skull, driving the fragments into the brain, will cause direct destruction of the nervous tissue; but wounds and diseases of the enclosing and supporting structures, if producing simple non-infective inflammation, give rise only to such symptoms as accord with the nerve structure irritated or destroyed. Should, however, the wound or diseased structure become infected with micro-organisms, the disease spreads and becomes generalized likewise the symptoms. Of all the causes of infective inflammation, middle-ear disease, on account of its frequency and insidious onset, is the most important. It is very liable, when neglected, to be followed by a septic meningitis, encephalitis and brain abscess, the most frequent seat of which is in the adjacent temporal lobe, but it may be in other parts of the brain, e.g. the cerebellum and frontal lobe (Plate I. fig. 3). The peripheral nerves may be destroyed or irritated by direct injury, disease or new growth in adjacent tissues, or they may be involved in the callus thrown out round the seat of a fracture.

Diseases of the blood-vessels are among the most frequent causes of organic brain disease. Arteries or veins—more frequently the former—may become blocked or ruptured from various causes. The immediate effect is a disturbance or loss of consciousness, and the individual may be “struck down” (see Apoplexy) and never regain consciousness (see Coma). Should the individual recover consciousness more or less permanent loss or disturbance of function will be the result. Paralysis of some form, especially hemiplegia, is the commonest result, but the loss or disturbance of function will depend upon the seat of the injury.

The cerebral arteries may be occluded by embolism; a portion of a clot or vegetation from a diseased valve of the left side of the heart may be detached, and escape into the circulation; and this is carried into one of the arteries of the brain, usually the middle cerebral, more often of the left side than the right. The area of brain tissue supplied by that artery is deprived of blood, and undergoes softening in consequence, resulting in paralysis of the opposite half of the body (hemiplegia) associated with aphasia when the paralysis affects the right side in a right-handed person (Plate I. figs. 5 and 9). When the embolus is infective, as it frequently is in ulcerative endocarditis, its lodgment in an artery of the brain, not only blocks the vessel but leads to an infective inflammation and softening of its coats, with the formation of an aneurism. The aneurism may suddenly rupture into the substance of the brain and produce apoplexy. In fact the majority of cases of apoplexy from cerebral haemorrhage recurring in young people are due to this cause. Softening may also arise from coagulation of the blood (thrombosis) in the arteries or veins. There are many causes which generally combine or conspire together to produce thrombosis, viz. a weak acting heart and altered conditions of the blood, and sometimes independently of vascular disease spontaneous coagulation in a vessel of the brain may occur. It is sometimes met with in the cachexia of certain grave diseases, viz. in phthisis and cancer, in typhus and pneumonia, after parturition and in marasmus at all periods of life, but especially in the very young and very old. But thickening, roughening and a degenerated condition of the cerebral arteries known as atheroma when associated with a weak acting heart is especially liable to give rise to thrombosis and softening, and this is a very common cause of apoplexy, paralysis and dementia in people who have passed middle life. General disease of the arteries of the body, associated especially with chronic Bright’s disease and high arterial pressure, is frequently attended with the formation of minute miliary aneurisms upon the cerebral arteries, which may rupture and cause apoplexy. Haemorrhage into the brain from this cause is especially liable to occur in certain situations; one vessel in particular, supplying the basal ganglia, most frequently gives way, the effused blood tearing through the motor efferent fibres, which, proceeding from the cerebral cortex in the shape of a funnel, become aggregated together to form the neck between the two masses of grey matter—the optic thalamus and the corpus striatum (Plate II. fig. 6). The result is hemiplegia of the opposite side of the body. Disease of the arteries of the central nervous system, occurring in a person under forty, is generally due to syphilis, the virus of which produces an inflammation of the coats of the vessel, especially the inner (see Plate II. figs. 4, 9, 10). The thickening and narrowing of the lumen with loss of elasticity of the arteries of the brain generally, may suddenly or gradually set up conditions of cerebral anaemia and give rise to semi-comatose and comatose or even apoplectic states. Occlusion by the inflammatory proliferation or by the sudden clotting of blood in the diseased vessel may occur, the immediate effect of which may be an epileptic or apoplectic fit; the result is softening; and seeing that any or all the arteries of the brain may be affected successively, simultaneously, or at random, the symptoms may be manifold. They may be general or local, and not uncommonly are associated with inflammation of the membranes. The disease, under treatment, may abate, and the paralytic or mental phenomena partially or completely disappear, indicating the restoration, or partial restoration, of the circulation in the diseased arteries; sometimes with the lapse of treatment and sometimes without, new symptoms, such as paralysis of a fresh group of muscles or of the opposite side of the body, may manifest themselves, showing that the disease has attacked a fresh set of arteries. Disseminated sclerosis (insular) is another random morbid process, affecting especially the white matter, with certain characteristic symptoms of a progressive character, the pathology of which is not understood fully, but is probably due to some toxic cause.

Fig. 7.—Diagram of left cerebral hemisphere, showing localization of function. The motor region is situated in front of the central sulcus, and is arranged in a series from “toe to larynx” downwards, corresponding in an inverse manner to the spinal series. Irritation of any part of this area will cause localized convulsive spasms, which may spread in a definite march to the whole motor area, as in Jacksonian epilepsy. Destructive lesions will cause paralysis. The centre for “taste and smell” is represented at the tip of the uncinate convolution. The centre for “half-vision” is only in small part represented, for the larger part is on the mesial surface. “Hearing” is represented occupying the posterior half of the first temporal convolution, but only a small part of the centre is seen, for the greater part lies above within the fissure of Sylvius. Included in this area, but in the left hemisphere only, is the centre for “auditory word memory”; destruction of this causes inability to understand the meaning of words uttered, although the patient is able to read aloud. Behind this, in the angular gyrus, is the centre for “visual word memory”; destruction of this causes loss of power of understanding of written or printed words—therefore inability to read. In front of the motor area is Broca’s convolution, the centre of “motor speech”; destruction of this produces motor aphasia, or inability to articulate words. Above this is a centre which is connected with written speech. These four centres concerned with verbal and written language are connected by commissural fibres, and destruction of these connexions leads to various defects in verbal and written language. It will be understood from this diagram that diseases of the left hemisphere in right-handed persons are associated with results of more significance than similar affections of the right hemisphere.

Islands of nervous tissue undergo a morbid change, commencing in the myelin sheath and ending in an increase of the supporting neuroglia tissue at the expense of the true nervous tissue.

Fig. 8.—Diagram of section of the spinal cord in the upper cervical region, showing recent degeneration of the crossed pyramidal tract of the right side and direct pyramidal tract of the left side. The black dots indicate the degenerated fibres stained by the Marchi method. This degeneration is secondary to haemorrhage into the internal capsule of the left hemisphere, and it will be observed by the number of degenerated fibres that the greater bulk have crossed over to the right side of the spinal cord, thus agreeing with the fact that the paralysis is of the right half of the body.

Tumours and new growths in the central and peripheral nervous systems may be primary or secondary: the former arise in the supporting, enclosing or nutrient tissue elements; the latter are metastatic deposits from tumours originating elsewhere. Tumours may be single or multiple, the special symptoms occasioned depending upon the seat of the tumour and whether it destroys or only irritates the adjacent nervous tissue. Tumours situated within the cranial cavity cause general symptoms, namely, optic neuritis, severe headache and vomiting; these symptoms, which are caused by increased intracranial pressure, are more severe in rapidly-growing vascular tumours, even though small, than in large slow-growing tumours.

Fig. 9.—A diagram to indicate afferent, efferent and association systems of neurones. It will be observed that there are three nervous circles indicated by the arrows—spinal, cerebellar and cerebral. In every perfect co-ordinate movement impulses properly adjusted are flowing along these three systems of neurones. In systemic degenerations one or more of these systems may be affected, and the symptoms will depend partly upon the function which is lost or disturbed, and partly upon the disturbance of equilibrium of the three co-ordinated systems.

Fig. 10.—Diagram of spinal cord, fifth lumbar segment, from a case of advanced tabes dorsalis. The posterior column is shrunken, and but faintly stained, except in the anterior part; the shrinkage and the loss of stainability are due to the absence of fibres of the posterior roots, which normally form the greater part of this region of the cord. The fibres which are seen in the anterior part of the posterior column are derived from cells within the spinal cord, and belong to spinal association neurones.

Fig. 11.—Diagram illustrating the relative number and wealth of cells and fibres in the cerebral cortex in the normal brain, in amentia and dementia. The horizontal systems of fibres are association systems, and it will be observed that these are especially diminished in amentia, and still more in dementia, whereas the radial fibres are less affected. In the normal, there are five layers of cells arranged in columns (Meynert’s); in the pathological conditions it will be observed that the pyramidal-shaped cells no longer have their apical processes pointing vertically upwards. The processes are broken off, the cells are distorted in shape and diminished in numbers, and the degree of dementia in a wasted brain is proportional to the atrophy and destruction of the small and medium-sized pyramids of the whole cerebral cortex, and the disappearance of the superficial layers of fibres. This is specially manifested in paralytic dementia and the dementia of chronic insanity.

Some tumours are highly vascular and a large thin-walled vessel may suddenly rupture and cause an apoplectic fit. If the growth is situated in a portion of the cortex having some special localizing function, e.g. the motor area (vide fig. 7), it may give rise to epileptiform convulsions, starting in a limb or definite group of muscles; but the irritation usually spreads to the whole motor area of the same side, and even extends to the opposite hemisphere, by an overflow of the discharge through the corpus callosum. In such case there is loss of consciousness. If, however, the tumour destroys the cerebral cortex of a particular region, it may give rise to a paralytic lesion, e.g. paralysis of the arm (vide Plate I., fig. 4).

Organic diseases of the blood-vessels, or of supporting and enclosing tissues, produce secondary degenerations of the nervous system. The symptoms, like the lesion, are obvious, coarse and obtrusive; frequently arising suddenly, they may in a short time terminate fatally, or tend towards partial or complete recovery. Various forms of motor and sensory loss and disturbance of function may arise, indicating destruction or disturbance of particular regions of the central nervous system; and degenerations in certain tracts and systems of fibres arise, corresponding in histological character with those observed when a nerve fibre is separated from its cell of origin by section (secondary degeneration of Waller and Türck) (vide fig. 8, with explanation). This form of degeneration must be distinguished from primary degeneration, which is due to an inherent nutritional defect of the nerve cell and all its processes (the neurone), in which a regressive metamorphosis occurs; it starts in the structures of the neurones latest developed (namely, the myelin sheath and the fine terminal twigs of the axis cylinder and dendrons), and proceeds back to the main branches and trunk, eventually destroying the trophic and genetic centre itself, the nerve cell. These primary degeneration processes are insidious in origin, progressive in character, and nearly always fatal in termination; they affect definite systems, groups and communities of neurones in a progressive manner, and, therefore, are associated with a progressive evolution of symptoms, related to the structures affected (vide figs. 9 and 10).

To cite some examples: (1) Locomotor ataxy, on the one hand, is a primary degeneration affecting the afferent system of neurones; it is characterized by muscular incoordination without wasting, inability to stand with the eyes shut, lightning pains in the limbs, absent knee-jerks, Argyll-Robertson pupils, and other symptoms pointing to a morbid process affecting especially the afferent sensory system-of neurones. (2) Progressive muscular atrophy, on the other hand, is a disease of the efferent motor system of neurones of the brain and spinal cord, characterized by progressive wasting of groups of muscles innervated by groups of neurones which are undergoing degeneration. A fatal termination to this disease frequently arises from affection of the medulla oblongata, causing what is known as bulbar paralysis. Infantile paralysis is an acute inflammation of the anterior horns of the spinal cord, causing destruction of the spinal motor neurones of the anterior horn. It differs from the above chronic disease in its sudden onset and non-progressive character; it resembles it in producing paralysis of muscles without sensory disturbance. (3) General paralysis of the insane is a degeneration which begins in the association system of neurones of the cerebral cortex, but which may be, and frequently is, associated with degeneration of the afferent or efferent systems (fig. 9).

Neuroses and psychoses have not hitherto been satisfactorily explained by definite morphological changes in the brain (Plate I., fig. 1). We know little or nothing accurately about the morbid histology of insanity, except as regards the morphological changes met with in cases of amentia and dementia. The conditions of amentia, namely, idiocy and imbecility, are associated with arrest of development of the brain, as a whole or in part, the naked-eye evidence of which may be afforded by small size and simplicity of convolutions of the brain as a whole or in part (Plate I., figs. 2, 8 and 10); and the microscopical evidence by arrest of development, or imperfect development, of structures connected with the higher functions of the mind. namely, the association neurones in the superficial layers of the cerebral cortex (fig. 11). Conditions of dementia, primary or secondary, are associated with progressive decay and atrophy of the superficial layers of the grey matter of the cortex, and nakedeye evidence thereof is afforded by partial or general wasting of the cerebral hemispheres, accompanied with thickening of the pia-arachnoid membrane, atrophy of the convolutions, and with deepening and widening of the intervening sulci (Plate I., fig. 7).

Motor Cells, drawn from Microphotographs of Preparations stained by Nissl method to show Microchemical Changes produced by various diseases.

 Fig. 12.—Normal motor cell from cerebral cortex, showing a mosaic pattern of the cytoplasm due to a substance stainable by basic aniline dyes; this stainable substance exists also on the dendrons. By comparing the appearances of this cell with the other figures a just idea can be obtained of the morbid changes which result in various pathological conditions.

 Fig. 13.—Cell from a case of hyper-pyrexia—disappearance of the mosaic pattern, substance uniformly stained; absence of the chromatic elements on the dendrons, due to a precipitation of cell-globulin by the heat.

 Fig. 14.—Cell in an advanced stage of coagulation necrosis, complete absence of mosaic pattern; diffuse fine dust-like stain; breaking off of the processes; all caused by softening of the brain from vascular obstruction.

 Fig. 15.—Another specimen from the same brain in a still more advanced stage of destruction, and showing a phagocyte attached to the cell and devouring the decayed structure.

 Fig. 16.—A cell with enormously swollen nucleus, the result of hydration clue to absorption of fluid after ligature of cerebral vessels. Such a cell will probably recover.

The cerebro-spinal fluid fills up the space in the cranial cavity caused by the atrophy of the brain; consequently there is a great excess of this fluid. Before general paralysis was recognized as a disease some of the cases which died suddenly in a fit were doubtless termed serous apoplexy. This wasting so characteristic of general paralysis is especially due to atrophy of the cells and fibres of the superficial grey matter of the cortex, sections of which, examined microscopically, after suitable methods of staining have been employed, show great poverty, or complete loss, of three sets of delicate myelinated fibres, namely, tangential, super-radial and the inter-radial corresponding to the line of Baillarger. This degeneration of the superficial association fibres of the cerebral cortex affects especially the frontal and central convolutions, and is the earliest and most constant microscopical change in progressive paralytic dementia; it is accompanied usually by meningeal and vascular changes, atrophy of the nerve cells, and proliferation of the neuroglia (fig. 11); especially characteristic is the perivascular infiltration with lymphocytes and plasma cells (see Plate II., fig. 7). It was indeed thought that this condition of the vessels was pathognomonic of general paralysis; it certainly is not, for it is found throughout the central nervous system in sleeping sickness and cerebro-spinal syphilis (Plate II., figs. 8 and 9). It sometimes occurs in the neighbourhood of cerebral tumours but it is not found in uraemia or lead encephalitis. Possibly new methods may enable us to show changes of structure in diseases such as epilepsy and delusional insanity, in which hitherto no naked eye or microscopical structural defects accounting for the symptoms have been certainly demonstrated.

Fig. 17.—Diagram to illustrate various stages in degeneration and regeneration of medullated nerve fibres.
1,	Normal medullated nerve with node of Ranvier.
2,	Degenerated nerve, ten days after section, showing degenerated myelin stained black; disappearance of axis-cylinder.
3,	Central end of cut nerve, showing at the top an axis-cylinder budding out, proliferated neurilemmal cells, and still some degenerated myelin in sheath.
4,	Peripheral cut end of same, showing proliferated neurilemmal cells, still some degenerated myelin.
5,	Complete absorption of degenerated myelin, proto-plasmic basis of new fibre formed out of neurilemmal cells.
6,	A new fibre, with axis-cylinder.
7,	Central end of cut nerve at junction, showing an axis-cylinder sprouting and forming a number of axis-cylinder processes, which grow into the peripheral end to form new channels of conduction.
8,	Is a new regenerated fibre resembling a sympathetic fibre in having as yet no myelin sheath; as the nerve becomes excitable and stimulus passes, a myelin sheath is formed.

In conditions of acute mania there is usually considerable vascular engorgement. We should, however, probably be more correct in assuming that insanity (especially those forms in which there is neither amentia or dementia) is due to alterations in the quality rather than the quantity of blood in the brain. The primary dementia of adolescence, which in 80% of the cases occurs before the age of 25, in which hereditary taint is most common, and which frequently is accompanied by, or terminates in, tuberculosis, can be explained by the effect of toxaemic conditions of the blood on cerebral neurones with an inborn low specific energy and metabolic activity. The histological changes found in the brain do not serve to explain the symptoms and we must look to bio-chemical changes in the body acting upon an innately unstable brain to explain the problems of the disordered mind in this disease.

Microscopical Changes in Degeneration of the Neurone.—About 1850, Waller demonstrated that a nerve fibre underwent degeneration to its termination when separated from its cell of origin; hence the term “Wallerian degeneration.” Embryological researches by Professor His showed that the axis-cylinder process (the essential conducting portion of the nerve fibre) is an outgrowth of the nerve cell. The cell, therefore, is the trophic and genetic centre of the nerve fibre. Acute alterations and death of the nerve cells may occur from toxic conditions of the blood; from high fever (107°-110° F.); arrest of the blood supply, as in thrombosis and embolism; or actual destruction by injury, haemorrhage or inflammation. These morbid processes produce, as a rule, bio-chemical as well as morphological changes in the nerve cell and its processes. Space will not allow of a full description, but some of these changes are indicated in figs. 18-22, with explanatory text. When a nerve cell dies, the nerve fibre undergoes secondary degeneration and death; that is to say, the whole neurone dies, and regeneration, at any rate in the higher vertebrates, does not take place. Restoration,
 Fig. 18.—Diagram drawn from photo­micrograph to show different forms of neuroglia cells in a patch of sclerosis secondary to degeneration and disappearance of the neurones. Ob­serve the large branched cells of Deiters. or partial restoration, of function is due to other structures taking on the function, and the more specialized that function is, the less likely is restoration to take place. If, however, a peripheral nerve is divided, its component fibres are merely severed from their cells of origin. All that portion of the nerve which is in connexion with the nerve cells of origin practically undergoes no change. The peripheral portion undergoes degeneration, but from the central end of the nerve new axis cylinders again grow out and a new nerve is formed. With this regeneration comes restoration of function, which may be hastened by suturing the ends of the cut nerve. A similar regeneration, however, does not occur after section of fibres of the white matter of the central nervous system, and this may be due to the fact that the nerve fibres of the white matter of the cerebro-spinal axis possess no nucleated sheath of Schwann, which, by the light of recent investigations, is shown to play an important part in regeneration; in the writer’s opinion, the neurilemmal sheath of the old fibre forms a new protoplasmic basis, into which the axis-cylinder from above grows, the passage of stimulus determining its function. Fig. 17, Nos. 1-8, with explanatory text, shows the changes which occur in degeneration and regeneration of a peripheral nerve after section, with loss of function; and subsequent union, with restoration of function. The writer, in conjunction with Professor Halliburton, has shown that the characteristic microscopical changes in the myelin sheath which occur in the process of degeneration are due to a splitting up of the complex phosphoretted substance “protagon” into glycero-phosphoric acid, choline and-oleic acid by a process of hydration. The Marchi reaction, which has been found so useful for demonstrating degeneration of the central and peripheral nervous systems, is dependent upon the fact that the myelin sheath, after hardening in a solution of bichromate of potash, does not turn black when acted upon by osmic acid, whereas the simpler non-phosphoretted fatty product of degeneration is stained black. When the Marchi reaction of degeneration is fully developed, it has been ascertained that the nerve yields no phosphorus. The degeneration resulting from section of a nerve is termed secondary, to distinguish it from another, primary, due to slow and progressive decay of the whole neurone, beginning usually at the terminal twigs and proceeding back towards the cell body with its contained nucleus. These primary degenerations involve systems of neurones, correlated by function rather than by anatomical situation. Examples are afforded by locomotor ataxy and progressive muscular atrophy, the former being a degeneration of the afferent sensory system of neurones, the latter of the motor efferent system. The cause of primary degenerations is probably a defect inherited or acquired in the “vita propria” of the neurones affected. They slowly atrophy and disappear, and their place is filled up by an overgrowth of the supporting neuroglia tissue (figs. 10 and 18). This overgrowth of dense tissue is termed sclerosis, and was erroneously considered to be the cause, instead of the effect, of the atrophy of the nervous tissue.

For further information the reader may consult the Croonian Lectures on the Degeneration of the Neurone, by F. W. Mott, published in the Lancet (1900); and the same writer’s “Introduction to Neuropathology,” in Albutt’s System of Medicine. Also Gower’s Handbook of the Nervous System, von Monakow’s Gehirn Pathologie, Ford-Robertson’s Pathology of Mental Diseases and Mott’s Archives of Neurology, vols. 1, 2, 3 and 4.  (F. W. Mo.)