The nerves of animal life have a development proportional to that of the brain. All of them are very large compared to the other parts; thus the fœtus and the young infant are the most proper for the study of the nervous system, as the less development of the other systems renders this more apparent. Their medullary substance is, like the cerebral and that of the spinal marrow, very soft and even almost liquid under the finger; in this state we can see it in the anterior part of the optic, in which it is very evident though contained in the canals of the nervous coat, in the posterior part of the same nerve, and in the olfactory where it is found by itself, in the auditory in which it predominates, and finally at the origin of each pair, where its proportion to the nervous coat is very evident. In all the other nerves it is much more difficult to examine well this medullary substance, because the nervous coat that contains it, is as much or even more developed in proportion to what it will be afterwards. Hence it is that the nerves are very hard and resisting in the fœtus; and that they can support weights proportionably very great. Maceration in water, at a moderate temperature, increases this resistance as in the adult, and renders the nerve harder without increasing its size. We should say that this fluid acts at first upon the nervous coat, in an opposite manner to what it does upon the other animal substances; finally it softens it also, and renders it almost liquid. The blood vessels are proportionably much larger in the nerves of the fœtus than in those of the adult. These nerves have in their whitish colour, a livid tinge that arises from the kind of blood that enters them; it is the same phenomenon as that of the brain.