Popular Science Monthly/Volume 11/July 1877/On Heredity in Nervous Diseases
|ON HEREDITY IN NERVOUS DISEASES.|
THE facts which I propose to consider in this paper have been brought to light by means of the experimental method. They are very interesting, both physiologically and psychologically viewed. I shall occupy myself with the physiological aspects only, and their bearing on human pathology. Psychology is beyond my province. Moreover, I conform myself to the saying of Montaigne, that deductions are very difficult to draw in psychological science, for "comment cognoit on la semblance de ce de quoi on ne cognoit point l'essence?"
Most of the facts observed, of hereditary transmission of nervous disorders, were put on record many years ago by my eminent friend and teacher Dr. Brown-Séquard. Some I have observed, together with him, during the years that I was his assistant, and others I have discovered quite lately.
The disorders which were inherited had all been artificially induced in animals for the sake of experimentation. Very great care was taken in all cases to avoid causes of error, and I am positive that they were got rid of completely.
It will be seen that lesions which affect not only nutrition of parts, but also the higher functions of the nervous system, can be developed by hereditary tendencies, when artificially provoked, through those nerves which minister to organic functions—belonging to the so-called sympathetic system.
It is well known that a system of nerves exists in all animals which have a circulation, and which apparently has no other functions but to control the blood-vessels, to regulate the flow of the blood through them. This nervous system is called, therefore, the vaso-motor system; it is also termed the sympathetic.
It has been discovered that when a branch of that nervous system is sectioned, is separated from its centre, the blood-vessel with which it is connected almost at once enlarges: its calibre increases, more blood passes through it during a given time, etc. But if, now, this separated end of the nerve is irritated, the reverse phenomena are seen: the blood-vessel contracts, its calibre diminishes. When small arteries are experimented upon, the calibre becomes so small that blood-corpuscles no longer succeed in passing through it. So that it becomes evident that that nervous system has a great function to perform with regard to the nutrition of all the parts, or rather of every organ in the body, including, of course, the central, i. e., the spinocerebral nervous system, for the "blood is the life of the anatomical elements." The materials which compose animal bodies are endowed with properties which differ in every different tissue: for instance, we say that the muscular tissue has the property of contracting. These properties of tissues develop functions: for example, the contraction of the ciliary muscle permits correct vision; but it is evident that if the tissue does not keep up the process of nutrition, i. e., assimilation and disassimilation, which it can only do by the agency of the blood which carries to it new materials, and removes effete elements, its properties are impaired and its functions are consequently perverted. This point being understood, I proceed to relate the experiments:
If in a Guinea-pig, for instance, that portion of the vaso-motor branch which is in connection with the carotid artery in the neck,—which, therefore, regulates the blood-supply of some part of the brain, of the ear, of the face, and of the eye—be divided, or, better still, if the ganglion from which that branch springs be removed, we see that the entire half of the head of the animal, on the side on which the operation has been performed, becomes hotter, and on examining more closely we discover that the increase of heat is due to the fact that the blood-vessels allow more blood to pass through them, that the nutrition of the parts is increased, and therefore the heat also increases; and we see that the upper eyelid of the animal drops a little, being in a state of hyperæmia—that is, its capillaries are distended—that the secretion of tears is increased so that the eye is wet, that the pupil of the eye is contracted because of more blood in the ciliary system, etc. The ear also becomes hotter, and, if the animal is white, we can see that the ear which before was white, with some blood-vessels stretching across, is now become red, and presents a very rich network of capillaries, which have become apparent, being of enlarged calibre.
Now, all these phenomena may disappear after a while, except a few. The eye always remains smaller, although the blood-supply of the eyelid is more regulated; the pupil remains a little contracted, and the secretion of tears continues, and also the that state of the eye persists, and when the animal dies, or is sacrificed, it is seen that this eyeball is smaller than its fellow.membrane remains in a congested state. No matter how long the animal lives,
If, now, such an animal were allowed to breed with another—whether operated upon in the same manner or not—it would be seen that young which are born, apparently perfectly healthy, present a few days after birth all the phenomena observed in their changed parent or parents. They have the same smaller eyes, but on both sides; the same ear thickened and enlarged, etc. The only phenomena which they do not show are those which have been transient, the increased heat and the increased sensation which depended upon the increased amount of blood present, etc.
Those young can be made to breed in-and-in for several generations—I have watched them for five generations—and always the same characteristics will be discovered in the young.
If, now, an examination is made of the parent, the first one, it will be seen that the nerve which had been sectioned, or its ganglion which had been extirpated, is not regenerated; while, if an autopsy is made of one of the offspring of any of the subsequent generations, it is seen that they all possess the nerve and the ganglion intact. The acutest or most minute microscopic examinations do not discover any difference between their structure and those of other animals of the same family and species.
If a puncture be made into that portion of the upper part of the spinal cord which anatomists call the restiform body, in Guinea-pigs, it will be seen that the animal presents at once an increased vascularity of the ear on the corresponding side; the ear becomes gorged with blood, chiefly toward the periphery: sometimes in a very short time, indeed, that portion of the ear falls off, destroyed by dry gangrene. I have the record of a case in which the ear was thus partially destroyed in less than nine hours. The eye on the same side becomes larger and protrudes; it protrudes first, and becomes larger in the course of time.
If a pair of Guinea-pigs thus operated upon be allowed to breed, and even if only one parent is thus diseased, the other being healthy, when young are born, these young always present the phenomena observed in the parents; but the phenomena just described only come shortly after their birth. It is seen that their eyeballs increase in size and protrude from their sockets, their ears after a few days become diseased, just like those of the parents, the subjects of experimentation, and drop off, eaten by dry gangrene.
When the parent or parents are sacrificed, and their restiform bodies are examined microscopically, nothing is detected but a cicatrix in the envelopes of the spinal cord, which appear a little thickened at that point, but the nervous tissue itself does not differ apparently from surrounding elements of the same nature and structure. If an examination is also made of one of the young, nothing at all is discovered. Those young can be allowed to breed in-and-in, and always the same phenomena will be observed in each subsequent generation. I have sometimes noticed that if a male or a female belonging to any one of the successive generations is allowed to breed with another healthy animal, very generally some of the young present the same hereditary peculiarities. I have followed animals thus operated upon through seven generations.
But what is still more remarkable is the transmission of an epileptic malady artificially induced. Dr. Brown-Séquard, as is well known, has for nearly thirty years made experiments on this subject of epilepsy, and his researches have discovered an array of facts of the highest value, to lighten the obscurity which has at all times rendered the true causes of that disease unattainable. He has made the like of what renders illustrious the names of the foster-fathers of experimental physiology—a synthesis: he has produced an epileptic malady in the Guinea-pig, which presents all the characteristics observed in that disease in the human species.
Dr. Brown-Séquard found that when the spinal cord of a Guinea-pig is pricked, or a portion of it is destroyed, or one of the sciatic nerves—that is, one of the largest nerves of the hind-leg—is either sectioned or torn off from the spinal cord, the animal in the course of a few weeks develops the epileptic malady. First, after a week or a little more, all traces of the operation have disappeared, as far as the wound is concerned. When the spinal cord has been operated upon, sometimes the feeling of pain is found lacking to a slight degree on the opposite side, and exaggerated below the point of the body at which the wound has been made; but most generally no such symptoms at all exist.
When the sciatic nerve has been cut or torn away, the greater portion of the leg is paralyzed as to motion and sensation, which is natural enough, because the muscles can no longer obey the mandates of the will, being deprived of the nerves which carry them, and also the centres no longer receive impressions which formerly came from those muscles and the skin covering them, because the nerves which carry the impressions have been destroyed.
It happens, also, in this case, that those parts which are thus deprived of motion and of sensation are dragged on the ground, are easily hurt, and become inflamed and enlarged. As soon as the skin has been broken, the animal begins to eat away all the parts of its leg which it does not feel; but it cautiously stops at the very limit where sensation still persists, so that, as, out of its three toes which terminate its posterior limb, the inner one is animated by nerves which do not come from the sciatic, that one toe has preserved sensation and motion. Therefore, when all the insensible tissues have disappeared, the wound heals very rapidly, and the animal has a limb which terminates by one toe only, the inner. But another specially interesting fact with regard to epilepsy is this, that, a few days after the operation, whether made on the spinal cord or the sciatic nerve, it is found that an area of skin on the same side with the nerve destroyed or the spinal cord injured, which is limited by the line extending from the anterior extremity of the eye to the end of the nose, thence, comprising the upper lip, running along the neck to the shoulder, and then in a straight line to the posterior extremity of the ear, and, lastly, to the posterior angle of the eye, by degrees loses certain faculties and acquires new ones. The sensation of pressure, of squeezing, of heat, cold, or electricity, of pain in a word, all disappear; only the faculty of feeling tickling persists, and that appears exaggerated. In an experiment which I made some years ago, this effect followed within two hours of the operation on the upper spinal cord.
It is seen at the same time that tickling this zone gives rise at first to involuntary twitchings in the muscles of the jaw, of the eyes, and of the nose, on the same side; by degrees those twitchings become more strong and more general, then they manifest themselves on the other side also, and after a few weeks the animal has regular convulsions after each tickling of the zone, which lastly culminate in a regular attack of epilepsy, of which the features are the following: When the attack begins, the head is drawn first, and with great violence, at times toward one shoulder, at times toward the other. This has been explained by the contraction of the muscles of the neck. The mouth is drawn open by the same cause, and the muscles of the face and of the eyes, which had twitchings, now contract violently also. At this period it would appear that the muscles of the larynx are contracted to some extent. At all events, it appears that the vocal cords are contracted, for not unfrequently the animal utters an inarticulated, unnatural, sharp cry, which may be taken for the passage of air through the obstructed larynx. It then falls. The muscles of the legs are contracted stiff, those of the chest are thoroughly so; very soon all the muscles are the seat of convulsions. Respiration, which was in no little degree impeded during the time that the muscles of the chest were rigid, now becomes more frequent but very irregular. After a while the animal recovers, and remains in a state of stupidity for some time. It is not unfrequent to observe in these epileptic seizures fits of insanity—if I may use such a term when speaking of Guinea-pigs, but that word only will make my meaning understood. When these animals have been suffering for some months, it is seen that they have fits without apparent provocation; that is to say, spontaneously.
When they recover from the epileptic taint, all the phenomena observed about the zone of skin in the neck and face recur in the reverse order; that is to say, all the different sensations return by degrees, at the same time that the hair of that region falls, and new hair grows gradually. The fits become simple convulsions, then mere twitchings, and lastly the animal can no longer be distinguished from another healthy one but by the fact that it has only one toe at one of its hind-legs, when the operation has been performed on the sciatic nerve; and nothing whatever remains when the origin of the disease was a prick in the spinal cord.
All these different facts, of which I have just spoken, are the characteristics of an epileptic seizure in our own species. That zone of skin about the neck and face is the analogue of those areas of the body of the epileptic from which sensations of all possible natures and kinds arise a short time before the attacks, and which are called by the general name of "aura."
To make the analogy greater, let me say that, just as in man, when such an "aura" is discovered to start from such a part of the body that it can be acted upon directly, so as to be removed, the disease is cured, so also, if that zone, of which I have spoken, in the Guinea-pig be cauterized, the fits do not occur—the animal is cured.
I have given these details with some minuteness, because they show still more strongly the effects of hereditary transmission of the disease. All the young of Guinea-pigs thus made the subject of experiment do not become epileptic; Dr. Brown-Séquard has observed several. I have had occasion to observe only a very few, and I have been able to learn that the young are born healthy, apparently. Sometimes—almost always, in fact—they are born with only one toe in the hind-leg; that is the case when the parent had lost its toes in the manner that I have already stated. Perhaps two months or more after their birth—they become adult very rapidly—the same phenomena develop in those young as in their parents; to use the words of Dr. Brown-Séquard himself, "we see the gradual increase of the affection, the diminution of the sensibility in the zone, just as with the parent, the coming of a period of complete attacks of epilepsy, and then the loss of hair, and the gradual diminution of the nervous complaint." Now, it is to be observed—and this is the important feature in this transmission of disease—that in the parent operated upon the cure, when it is spontaneous (that is, when it does occur without any treatment, which is the case when the sciatic nerve and not the spinal cord has been divided), only supervenes because the alteration in the nerve is cured: if it has been sectioned, the two ends meet again and are reunited after a few weeks; if it has been torn away, the parts still remaining attached to the outer heal together; we can therefore understand how, the cause of the disease, i. e., the alteration of a nerve, being removed, the disease, i. e., epilepsy, also disappears. But in the young which have inherited the taint no such explanation obtains. Their nerves have not been operated upon, not torn nor cut. How are we to explain their cure, and this fact, which is, as I have said, the peculiar feature of this heredity, that the cure in the parent which has so characteristic stages should have exactly the same stages in the young as to circumstances and time?
I have said that I would only deal with this subject of heredity in its physiological aspects, but I cannot refrain from recalling to the attention of the reader that such facts in the human species need to be studied by psychological physicians, because they occur in insane patients, as is well known. Those alterations of the ears of which I have spoken at first are frequently met with in the ears of demented patients; physicians call them bæmatoma. I have satisfied myself that it is even more frequent than suspected in the inmates of asylums. That state of stupor, or stupidity, and of insanity, in the Guinea-pig, is of very frequent occurrence in the human epileptic. I could tell a long story of similar phenomena observed in our own species.
I believe that, if any conclusion can at present be drawn from those facts, a physiologist or a physician will state that it is not a disease which is inherited, which is transmitted, but the power to develop such a disease. On the one hand, a physiologist is bound to accept that the disease was originally developed as a consequence of an anatomical alteration of a certain organ or nerve-cell or fibre; and on the other he is bound to consider that the same disease or consequence develops itself in a young one which has no such circumstances of anatomical alteration of a certain organ or nerve-cell or fibre—at least detectable by any means of investigation at present employed.
This question of heredity is one of the most vexed, and I shall not say much more about it at present; but as I have stated that I am sure that there were no causes of error in the facts themselves or the deductions drawn from them, I take this opportunity to say that Mr. Galton has committed a grave error, in his very remarkable paper on "Heredity" published not very long ago, in which he stated that the Guinea-pigs which had epilepsy without alteration induced in their nervous system may have acquired the disease by imitation, just as, it is well known, is too frequently the case in the human species.
First, all the children or adults who happen to live with epileptic patients and witness their paroxysms do not develop epilepsy; only a very few do so. This fact would show, therefore, that those who do develop epilepsy have some tendency; but the argument of Mr. Galton does not hold good with regard to the first two series of facts which I have reported, and specially in the case of the Guinea-pigs which inherit epilepsy. How will he account, on the strength of his hypothesis, for the fact that, out of a couple of hundreds of young which were born from epileptic parents during the lapse of several winters in Dr. Brown-Séquard's laboratory in Paris, and which I have very studiously watched, only three became epileptic, although all lived together and all witnessed the fits of their parents, and only those three developed the malady which were born toeless? I regret, on closing this paper, not to be able to take the psychological aspect of the question, because it is very interesting in a forensic point of view, and as bearing on the question of responsibility.
- How can one know the like of that of which one knows not the essence?