Are the nerves reproduced when they have been cut? The experiments of many distinguished anatomists evidently prove that they are. What is the manner of this reproduction? If we examine the results of these experiments it is easy to see that there is nothing peculiar in the nervous system, that it is a simple cicatrization analogous to the callus of bones, to the cicatrix of the skin, &c. When a nerve has been cut, its two ends inflame, the cellular texture that it contains sends forth granulations by the property of reproduction that it possesses. These granulations meeting, form adhesions that unite the two divided ends of the nerve. As the cellular texture, the means of union, grows from the cut extremity of the nervous coat, as well as from that which is between the cords, it partakes of the nature of the nervous coat, and becomes a parenchyma of nutrition, whose mode of organic sensibility is analogous to that of the nerves, and whose vessels deposit there medullary substance, which gives a new appearance to the nervous cicatrix, and makes it resemble very nearly the texture of the nerves themselves. However, as the granulations arising from the divided ends are not made in a regular manner, there is never at the place of union a thread-like arrangement as there is in the nerve itself. Thus the callus of a long bone, though analogous to this bone, is never regularly arranged like it in longitudinal fibres; thus a cutaneous cicatrix has always an irregularity in its organization, which arises from the irregular manner in which the parenchyma of cicatrization has been developed.
The cicatrization of nerves is then analogous to that of bones. In the first period there is inflammation; in the second, growth of the cellular texture which is to serve for the nutritive parenchyma; in the third, adhesion of those parts that have grown; in the fourth, exhalation of the medullary substance into the parenchyma. It is this medullary substance that makes this cicatrix differ from the osseous, in which phosphate of lime and gelatine are deposited, from the muscular, in which there is fibrin, &c. Sometimes there is an enlargement in the form of a ganglion, at the place of the reunion of the nerves; this depends upon the greater granulation of the cellular texture. Thus sometimes the callus is enlarged; at others, if the contact has been exact, we perceive but a slight difference; these are varieties that do not affect the nature of cicatrization.
It follows from this, that the regeneration of the nerves, which has lately been the object of much research, and which Cruikshank, Monro, &c. have particularly demonstrated, has nothing peculiar in it; that it is only a consequence of the general laws of cicatrization, and a proof of the constant uniformity of the operations of nature, though these operations present at first sight different results. A nerve, that is cut out in its whole course, is never reproduced like a nail, or the hair, which take a length, form, and appearance exactly the same as they had before they were removed. It is under the point of view that we have presented them, and not under this last, that the nervous reproductions should be described.
ARTICLE FOURTH.
DEVELOPMENT OF THE NERVOUS SYSTEM OF ANIMAL LIFE.
I. State of this system in the Fœtus.
The nervous system of animal life is one of the first that is developed. If the heart is the first that has motion, the brain is the first that has any considerable size. The disproportion of the head to the other parts is remarkable in the first periods after conception; its size is monstrous when compared with that of the subsequent ages. Now it is evident, that it is the brain that produces this, and that the increase of the size of the bones and the membranes that surround it, is owing to it.
We may say that by creating first the heart and the brain, and developing them much sooner than the other organs, nature wished first to establish the foundations of the organization of the two lives. For on the one hand, it is the brain which is the centre of animal life; it is to this that all the sensations are referred; it is from it that all the voluntary motions proceed. On the other hand, by sending the blood towards all the organs, the heart evidently presides over the circulation, the secretions, exhalations, nutrition, &c. which compose by their union organic life. When these two essential bases exist, nature begins to build, or rather develop around them the double organized edifice, which produces on the one part a communication between the animal and external bodies, and on the other nourishes it.
Notwithstanding these early developments, the brain is not like the heart constantly active; its two great functions, relative to sensation and motion, are almost nothing. The intellectual functions also have but a very obscure action, if they have really commenced at all. The brain is then, if we may so say, in the expectation of action; it has not acted; it requires the excitement of external bodies. I do not say, however, that its inactivity is necessarily entire. It can undoubtedly perceive certain internal motions that take place in the body, and especially the pains that arise there; for if the organic diseases can produce the death of the fœtus, why does it not suffer pain in these diseases? Perhaps the brain is so much the more sensible to it, as it is not diverted by the external senses. The difference of the external and internal sensations, is a question that deserves to be attentively considered. We have seen that the first are uniformly transmitted by the nerves and that the mode of transmission of the second is uncertain. On the other hand the phenomena, the sensation, the impression, &c. are not the same in each; so that an examination of their relations and their differences is essential. This examination would have much influence upon the knowledge of the kind of animal life that the fœtus can enjoy. Whatever it may be, there can be no doubt but that it is infinitely more contracted than after birth.
The softness of the brain is very great in the fœtus; it is truly a kind of fluid, that the arteries, or rather the exhalants that arise from them deposit in their interstices. These arteries are then extremely numerous; as the brain has a very evident reddish tinge. When it is cut in slices, numerous streaks of this colour are observed in its substance. The two portions, the cortical and medullary, are infinitely less distinct than afterwards, because the second is much less white. The caustic alkali dissolves them at this period of life with great ease. The first effect before a complete solution, is to change the cerebral substance into a glutinous, transparent and viscous matter, a little reddish however, and ropy, almost like the white of an egg. I discovered nothing similar to this in my experiments with the brain of an adult when treated with caustic alkali. The acids coagulate the cerebral substance of the fœtus, it does not however attain by them a degree of hardness equal to what they produce in the subsequent periods of life.