from each of which the pairs of nerves go off; that it is from the size of these ganglia, and from their separation, as well as from the smallness of the brain, that we endeavour to explain the divisibility of the individual, which is remarked in all these species, at least for some minutes, and which goes so far in some, such as the earth-worms and the naides, as to make two lasting individuals from one, by means of division. Nothing resembling this had been perceived in man, whose spinal marrow has no sensible contraction, and only swells out at those places from which it gives off nerves to the arms and thighs; but Drs. Gall and Spurzheim have showed us a preparation of the spinal marrow of a calf, in which is observed a kind of slight swelling between each pair of nerves. It would be curious to ascertain exactly in what animals this structure is found, and if it has any relation with the power of executing certain voluntary actions, without the brain; if the turtles, for example, which live and walk several months successively without this viscus, have the spinal marrow more knotty than the other animals with red blood, &c. One of our number has begun inquiries with this intention, which have not afforded him sufficient results to be laid before this class; but he has already ascertained that there are no sensible knots in quadrupeds even very nearly allied to the calf. The third article is subdivided, for examination, into as many heads as there are pairs of nerves. The general result which the authors propose to prove is, that all the nerves arise from the medulla oblongata or spinal marrow, and not from the brain. There is no difficulty as to the spinal nerves, which are only conjectured to come from the brain, but the roots of which undoubtedly no human eye can trace into it, nor even perceive a tendency to run into it. There can be no more with regard to the last pairs of the encephalon, counting from the par vagum, and under it, for they arise by transverse filaments, as the spinal nerves, although they have not two bundles; and no anatomist has observed these filaments reflected towards the brain after having entered the spinal marrow. There is still less doubt with regard to the nervus accessorius of Willis, which evidently re-ascends. We have only, therefore, to attend to the first eight pairs, reckoning the facial nerve as a separate pair. The seventh pair of Willis is, in fact, now generally acknowledged as forming two nerves, distinct at their origin as well as in their course. The soft portion, or the auditory nerve, arises transversely from the corpus restiforme, called also processus cerebri ad medullam. This nerve has been for a long time considered to be formed by the small white filaments traced over the ceiling of the fourth ventricle, and this is still the opinion of Haller,* of Vicq-d'Azyr,† and of Soemerring. However, as these filaments vary in number, and even in directionas a part of them are sometimes seen re-ascending towards the corpus restiforme, or passing through it towards the pons varolii, and as it is not very rare to find them wanting, their continuation in the auditory nerve has begun to be doubted. Prochaska, the two Wenzels,¶ &c. have formally denied it. The latter** and Dr. Gall have farther remarked, that these striæ are generally wanting in animals. The two Wenzelst† observed, in 1791, for the first time, a small grey band slightly prominent, placed also transversely upon the corpus restiforme, and which constantly covers a part of the base of the auditory nerve, which it unites with the fourth ventricle. Prochaska is hitherto the only one who has given a representation of it. It is likewise observed in ani * Phys. t. iv. p. 225. Explication des Planches, p. 95. De Fabrica Corp. Hum. t. iv. p. 256. § We have had a very marked example of the latter structure in the course of the researches occasioned by this memoir. Oper. Min. t. i. p. 388. ** Prod. ¶ Prod. p. 22. #Oper. Min. t. i. tab. iii. fig. 1. mals; and Dr. Gall, who adopts, in this respect, the opinion of the Wenzels, remarks that it is the more swelled in every species the larger the ears are, and the more acute the hearing is. In the horse, the stag, the sheep, it is a tubercle almost as large as the eminence called testis. We have found this circumstance to be true. It is besides evident, that if the origin formerly admitted were the true one, the auditory nerve would not, therefore, arise less transversely from the medulla oblongata, and that its visible roots would always come rather from below upwards than from above downwards. The facial nerve, or portio dura of the seventh pair, and the abductor or the sixth pair, are then the first which can leave any doubt, whether they arise from the medulla oblongata, or from the brain, from behind or from before. In man they both arise from the body of the medulla oblongata, immediately behind the posterior edge of the pons varolii, and so near to it, that several anatomists make them derive a part of their filaments from it. The facial nerve, in particular, goes off some lines farther outwards than the other, at the angle formed by the pons varolii and corpus restiforme, at about a line from the point where the auditory nerve is detached from the corpus restiforme, which it had, as it were, surrounded. The abductor seems to arise from the fissure which separates the pons varolii from the pyramidal eminences; and there are some anatomists who derive all its roots from the pons, others from the pyramids, others from both parts. There are others again who are not explicit with regard to this point. According to the idea generally adopted, that the nerves descend from the brain, M. Soemmerring supposes that the abductor nerve has its roots in the pedunculi, and that they separate from them, turning backwards, after the pedunculi have crossed the pons varolii to form the pyramidal eminences. This is nearly the opinion of Vieussens, but it is evidently the result of hypothetical reasonings, and not of actual observation. To ascertain the true direction of the roots of these nerves, it is necessary to have recourse to the herbiferous animals, in which the pons varolii does not cover them, since it is not so large as in man. This is what Drs. Gall and Spurzheim have done, and they found at once that the abductor, in them, arises some way behind the pons varolii, and appears to be the continuation of a small bundle which ascends between the corpus pyramidale and olivare. The filaments which produce it are longer at the back part and shorter before, so that they have, in miniature, the same arrangement as those of the accessorius of Willis. There is no reason, then, to suppose that it descends from the brain. This observation concludes the discussion whether or not the nerve receives some filaments from the pons varolii; since it is only on account of the breadth of the pons in man that it approaches to its posterior edge. in We have not found any positive traces of this remark in the authors whom we have consulted, but we have convinced ourselves that it is true with respect to the herbiferous animals; and one of our number had even observed it some time ago the horse. In the carnivorous animals and the ape, the pons varolii and the sixth pair have a greater resemblance to what is observed in man. With regard to the facial nerve, a transverse medullary band is seen in the herbiferous animals, behind the pons varolii, which begins exactly at the outer edge of the abductor, and passes over the root of the par trigeminum, or where it is continuous with the auditory nerve. The facial nerve appears to pierce the band obliquely from behind forwards. Thus it would seem to arise from the under part of the medulla oblongata, al * Neurogr. Univ. p. 176. most in the same manner as the auditory nerve arises from the upper part, and two pairs of nerves would be formed, the origin of which is, in fact, separated from each other by the whole thickness of the medulla oblongata, although they afterwards approach so as to touch each other. Nor have we remarked, that any author has pointed out this fact before Dr. Gall; but we are certain of its accuracy, and one of us had seen it, and delineated it some time ago, in the stag, horse, sheep, and rabbit. The brute animals also show more clearly than in man the origin of the par trigeminum, or fifth pair. It is generally said simply to arise from the lateral parts of the pons varolii, or from the extremity of the pedunculi cerebelli. This is still adhered to by Vicq-d'Azyr* and Meckel.† Haller reckons this pair in the number of nerves, which may come both from the cerebrum and cerebellum. It is, however, ascertained, that it neither comes from the one or the other, and that it may be traced deep into the medulla oblongata, to nearly an inch farther back than where it comes out. Santorini asserts that he has traced the roots of it to above the corpora olivaria, and says, that it is not more astonishing to see this nerve ascend from below, than the accessorius of Willis; but he then supposes, that a part of the fibres of the pedunculi which do not enter the corpora pyramidalia, they being, in fact, too small to contain them all, go on further, and then those, among others, are reflected which form this nerve; a very gratuitous supposition, and which nothing sensible to the eye can justify. M. Soemmerring seems not to have understood Santorini at the time he wrote his treatise De basi encephali;|| but he relates, that¶ accident had made him afterwards trace the origin of this nerve into the substance of the medulla oblongata, even as far as the bottom of the fourth ventricle; and, according to his fa * Explication des planches, p. 52. † No. 46 et 47. Phys. t. iv. p. 387. § Observat. Anatom. p. 64, 65. P. 135. De fabric. corp. hum. p. 212. No. 7. |