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.