The annexed diagram, (fig. 405.) with the letters attached to it, will assist the reader in following this otherwise intricate description. It is taken from a drawing by Morganti in the essay referred to ; but it has been reduced in size and simplified, so as better to allow of its introduction here.
The same author has examined into the comparative anatomy of the ganglion and the nerves connected with it in many of the other marrimalia, as the dog, calf, lamb, mule, and dormouse.
The general results of these examinations abundantly verify his description of the ar rangement in the human subject. Indeed, these animals offer by far the most favourable subjects for exemplifying the truth of the preceding description, being, as Morganti remarks, natural preparations of these parts. Not only is the dense and intimately adherent sheath of fibrous tissue, which is present in man, much looser in the ganglion and nerves of these animals, but the position of this body with respect to the nerve is considerably altered. The much less marked anterior bend of their portio dura occurs at some little distance from the hiatus Fallopii ; and the ganglion, which is in immediate proximity to this aperture, is thus no longer geniculate in its position, being removed from the knee of the facial. Hence it is, as it were, out of the way of the facial branches, and ceases to be entangled amongst them, as in the human s ubj ect.
The author of this article can bear testi mony to the accuracy of these statements; indeed, any one may easily verify them for himself, in most of these animals with scarcely more trouble than renioving the brain and osseous roof of the Fallopian canal, and then stripping off the comparatively lax neuri lemma from the subjacent ganglion and nerves. The accompan)im, sketch (fig. 406.) was taken from the left side of a sheep's head. With as little artificial separation as possible, it represents the arrangement of the ganglion and nerves in sira, especially the manner in which the trunk of the portio intermedia crosses the facial nerve without joining it, and the apposition or proximity, without mingling, of the ganglion and the latter nerve.
The varieties of arrangement which obtain in the different animals whose nerves Mor ganti examined, are chiefly, as might be expected, differences in the degree of inter lacement of the adjacent nerves. In cular, that of the portio intermedia with the Auditory, Facial, and Intermediate Nerves of a Sheep as seen in sit& Magnified about 2i diameters.
a, portio dura; b, portio intermedia; c, portio monis; e, origin of the superficial petrosal nerves; f, chorda tympani; g, geniculate ganglion.
vestibular nerve is sometimes so complete and intricate, as to render it in such instances difficult to ascertain from their examination only, whether the former of these nerves gives branches to the latter, or, vice versci, this to that. In the mule, he exhibits a fila
ment from the facial to the ganglion ; but thinks this a possible restitution of one or both of the two previously given to it by the portio intermedia.
The general anatomical conclusion to be drawn from these details is, that the facial nerve—as implying in this term both the portio dura and the portio intermedia— arises by two roots. Upon the smaller of these a ganglion is formed, while the latter is entirely devoid of such a structure. The branches of the facial nerve in the Fallopian canal are mixed nerves, being formed partly by filaments from the ganglion ; partly also by filaments from the aganglionic root ; the latter being in considerably lesser numbers. And the trunk of the facial itself, beyond the ganglion, is also a mixed nerve, since, although by far the greater part of its bulk consists of fibres from the greater root, yet it also con tains one or two filaments which come from the ganglion. The analogy of this arrange ment to that of the spinal nerves is sufficiently obvious, and will be hereafter again re ferred to.
It deserves to be mentioned in this place, that many other accounts of the arrangement of these nerves might easily have been added from various authors, but that all of them are more or less at variance, both with the above description by Morganti, and with each other. It has seemed fit, however, to assign these a very subordinate position in the present short article, since the verification of' a ganglion belonging exclusively to the portio intermedia includes not only the denial, but I think we may add the disproof, of many of these descriptions. So far as our knowledge of the structure of ganglia at present extends, and whether the late brilliant researches of' Ru dolph Wagner* apply universally or not, we are at least justified in viewing with great incredulity any account of the unaltered pas sage of a nerve through a ganglion, as viewed by the unassisted eye ; and, in the particular instance before us, the microscope disproves this supposition. So, also, concerning the various theories of the derivation of the super ficial petrosal nerves which have been set forth as based on dissections. Let it be granted that there are two ganglia,—the sphenopalatine and the geniculate,— which are united by an intervening nervous cord : in such a case, I cannot see how any merely anatomical skill would enable one to predicate a definite di rection as taken by the connecting nerve. Indeed, any statement of this kind really amounts to asserting a special direction or quality of the nervous force, and to the affir mation or denial of such a view, the scalpel affbrds no assistance.