SYSTEM.] PHYSIOLOGY the cord outwards. Again, section of a number of posterior roots is followed by loss of sensation of a part of the body on the same side, and, if the proximal ends of the divided roots those next the cord be irritated, painful sensations are excited. The posterior roots, therefore, contain sensory fibres, carrying impressions into the cord from the periphery. As we have seen, these roots are connected with the grey and white matter of the cord, and it is practically impossible to trace all their ramifications. Recourse must there fore be had to the evidence supplied by experiment (cutting, or by the Wallerian method, p. 26) and by pathological observation. In tracing the path of fibres, what may be called the " developmental method " has been pursued. It has been shown by Flechsig that, " if the development of the cord be carefully observed, the medul lary substance of the nerve -fibres is formed later along certain tracts of the white columns than in the rest of the white matter, so that in transverse sections of the cord these tracts are easily dis tinguishable by their more transparent grey appearance " (Quain, vol. ii. ]). 277). If the anterior columns be cut by an incision ex tending into the grey matter, leaving the posterior columns intact, voluntary movements disappear in the parts below the section. Again, section of the posterior columns and grey matter, leaving the anterior uninjured, enfeebles but does not destroy the power of voluntary movement below the section. Finally, section of an antero-lateral column on one side paralyses voluntary motion on the same side. From these facts it is inferred (a) that the motor tracts passing from the brain to the periphery are in the antero- lateral columns, and (b) that the fibres forming these tracts are chiefly distributed to the same side of the body. These inferences are supported by pathological observation. In diseases where the anterior horns of grey matter are affected paralysis ensues, with complete flaccidity of the limbs ; and if, from haemorrhage, soften ing, or the pressure of tumours, the anterior portion of the cord be irritated there are spasmodic twitchings of muscles. Complete transverse section of the posterior columns does not abolish sensi bility in the parts below ; but there is a loss of the power of making co-ordinated movements. Section of the posterior columns and of the antero-lateral columns, leaving only the grey matter in the centre of the cord intact, does not abolish sensibility. Again, section of the antero-lateral columns and of the whole of the grey matter, leaving only the posterior columns uninjured, is followed by complete loss of sensibility in the parts beneath. The inference therefore is that sensory impressions pass through the grey matter. As already seen, many of the sensory fibres connected with the posterior roots decussate in the grey matter. This explains some of the results obtained by Brown-Sequard, that hemi-section of the cord, involving the grey matter, enfeebled sensibility on the opposite side more and more as the section cut deeply into the grey matter ; that a vertical section in the bottom of the posterior median fissure caused loss of sensibility on both sides ; and that a lateral section, whilst it caused loss of sensibility (anaesthesia) on the opposite side, was followed by increase of sensibility (hyper- resthesia) on the same side, a curious fact, explained by Brown- Sequard as being due to irritation caused by paralysis of the vessels of the cord on the side of the section. It would appear also that tactile impressions travel, for a certain distance at all events, in the posterior columns. This has been inferred chiefly from the fact that in certain cases of paralysis involving the posterior columns, where the sensation of touch was absent, the patient could still feel a painful sensation, as when a needle was thrust into the skin ; whilst in other cases, in which these columns were not affected, the converse held good. In the disease known as locomotor ataxia (see ATAXY and PATHOLOGY, vol. xviii. p. 392) the patient first passes through a period in which there are disorders of general sensibility, especially lancinating pains in the limbs and back. By and by there is unsteadiness of gait when the eyes are closed or in the dark, and to a large extent the patient loses the power of co-ordinating movement. Especially he is unable to judge of the position of the limbs without seeing them ; in other words, the so-called muscular sense is enfeebled. At last there is a stage before death in which there is almost complete paralysis. A study of this disease has thrown much light on the physiology of the cord. It is known to be caused by a slow disorganization or sclerosis of the posterior root-zones, the posterior columns, slowly passing on to affect the columns of Goll, the lateral columns, and the anterior grey horns, and ultimately involving the cord. The disordered sensations at an early stage, the staggering gait at a later, show that the posterior part of the cord has to do with the transmission of sensory impres sions. The man staggers, not because he is paralysed as regards the power of movement, but because, in consequence of the sensory tracts being involved, he does not receive those peripheral impres sions which excite or indirectly regulate all well-ordered movements of locomotion. 2. As a Reflex Centre. The grey matter of the lower cervical, dorsal, and lumbar regions of the cord may be regarded as composed of reflex centres associated with the general movements of the body, whilst in the upper cervical region there are more differentiated centres corresponding to special actions. The initial excitation may commence in any sensory nerve ; the effect passes to the cord, and sets up changes in the nerve-cells of the grey matter, involving time, and resulting in the transmission outwards along motor fibres of impulses which excite particular groups of muscles. There is an exact co-ordination, with a given strength of stimulus, between certain areas of skin and certain groups of muscles, and thus movements may be so purpose-like as to simulate those of a con scious or voluntary character. Thus irritation near the anus of a decapitated frog will invariably cause movements of the limbs towards the irritated point. The activity of reflex centres may be inhibited, as already shown, by higher centres, or possibly by certain kinds of sensory impressions reaching them directly from the periphery. Hence removal of these higher centres is followed by apparently increased reflex excitability. Strychnia and the alkaloids of opium increase it, whilst aconite, hydrocyanic acid, ether, chloral, and chloroform have an opposite effect. In certain pathological conditions also, as in tetanus, or in some slow progressive diseases of the cord, reflex excitability may be much increased. In tetanus the slightest touch, a movement of the bedclothes, the closing of a door, the vibration caused by a footstep, may throw the patient into severe and prolonged con vulsions. The earlier formed ganglionic cells are those specially concerned in reflex acts. Special reflex centres have been clearly made out in the cord. (1) A cilio- Special spinal centre, between the sixth cervical and third dorsal nerves, associated re fl ex with the movements of the iris. The fibres controlling the radiating fibres of j - .... . . centres. ing fibres to the sympathetic which ultimately reach the heart, and irritation of these centres quickens the movements of that organ. (3) Respiratory centres. The movements of respiration, of a reflex character, involve the action of many thoracic and abdominal muscles. Section of the cord above the eighth dorsal paralyses the abdominal muscles ; above the first dorsal, the intercostals ; above the fifth cervical, the serratus magnus and the pectorals ; and above the fourth cervical, by paralysing the phrenics, it arrests the action of the diaphragm. (4) Genito-spinal centre. This is in the lumbar region. Irritation causes erection, &c. ; destruction or disease is followed by loss of virile power. (5) Ano-spinal and vesiculo-spinal centres. These, connected with the movements of the sphincter ani and of the bladder, exist in the lower portion of the dorsal and upper portion of the lumbar regions. Disease or injury involving these centres causes involuntary evacuation of the bowel and com plete paralysis of the bladder, with non-retention of urine. The bladder may be full whilst the urine constantly escapes in small quantity. 3. As a Trophic Centre. The ganglion-cells in the anterior Cord as cornua undoubtedly have a trophic or nutritive influence upon trophic muscles. This has been determined chiefly on pathological evidence, centre. If these cells undergo atrophy or degenerative changes, the muscles, even though they may be kept periodically in a state of activity by galvanism, become soft and fatty changes take place. There is thus a correlation between the nutritive condition of muscle and nerve-centre, and influences affecting the one affect the other also. It has been supposed that the cells in Clarke s vesicular column may form the centres in visceral innervation. They are bipolar, like those in the sympathetic, and not multipolar as in the rest of the cord, and the columns are absent in the lumbar and cervical enlargements. The cells are found where nerves come off that influence the viscera, and similar cells are found at the roots of the vagus in the medulla, a nerve also much concerned in the iunerva- tion of viscera. Inhibition of Reflex Actions. The reflex actions of the spinal Inhibi- cord may be inhibited or restrained to a greater or less extent by tion of the action of centres in the encephalon, so that pure reflex actions reflex only occur after removal of the cerebrum, or during profound sleep, actions, when the cerebrum is inactive. Thus a strong effort of the will may restrain from scratching an irritated part of the skin, whilst the same amount of irritation would certainly cause reflex movements if the will were in abeyance. Such power of voluntary control, however, is limited with respect to most reflex actions, whilst some reflex acts cannot be so influenced. Any movement that may be orginated by the will may be inhibited or restrained to a certain extent when the movement is of a reflex character ; but, if the movement be invariably involuntary, it can never be inhibited. Thus the ejaculation of semen cannot be voluntarily induced, whilst the reflex act once provoked cannot be arrested (Hermann). That these inhibitions of reflex actions of the cord depend on mechanisms in the brain is proved by the fact that removal of the brain is fol lowed by an increase in the reflex excitability of the cord, and that even section of the cord permits of increased reflex excitability below the plane of section (Setschenoff). Further, after section of the spinal cord in the cervical region, irritation of the lower end arrests reflex movements dependent on reflex centres in the lower cervical, dorsal, and lumbar regions (M Kendrick). Medulla OUongata. This is the prolongation into the cranium Medulla of the spinal cord so as to unite it with the brain. Strictly speak- oblon- ing, the medulla spinalis and the medulla oblongata form one organ, gata. The columns of white matter of the cord undergo changes in form, structure, and relative position when they pass into the medulla (see vol. i. p. 870). Without again detailing the minute anatomy, it is necessary to show, as in the following table, the connexions
of the cord and of the medulla with the rest of the brain.