The cranial nerves[171]. The earliest stages in the development of the cranial nerves have been most satisfactorily studied, especially by Marshall (No. [354]), in the Chick, while the later stages have been more fully worked out in Elasmobranchii, where, moreover, they present a very primitive arrangement. In the Chick certain of the cranial nerves arise before the complete closure of the neural groove. These nerves are formed as paired outgrowths of a continuous band composed of two laminæ, connecting the dorsal end of the incompletely closed medullary canal with the external epiblast. This mode of development will best be understood by an examination of [fig. 270], where the two roots of the vagus nerve (vg) are shewn growing out from the neural band. Shortly after this stage the neural band, becoming separated from the epiblast, constitutes a crest attached to the roof of the brain, while its two laminæ become fused. The relation of the cranial nerves to the brain then becomes exactly the same as that of the posterior roots of the spinal nerves to the spinal cord.
It does not appear possible to decide whether the mode of development of the cranial nerves in the Chick, or that of the posterior roots of the spinal nerves, is the more primitive. The difference in development between the two sets of nerves probably depends upon the relative time of the closure of the neural canal. The neural crest clearly belongs to the brain, from the fact of its remaining connected with the latter when the medullary tube separates from the external epiblast.
It is not known whether the cranial nerves originate before the closure of the neural canal in other forms besides the Chick.
The neural crest of the brain is continuous with that of the spinal cord, and on its separation from the central nervous axis forms on each side a commissure, uniting the posterior cranial nerves with the spinal nerves, and continuous with the commissure connecting together the latter nerves.
Anteriorly, the neural crest extends as far as the roof of the mid-brain[172]. The pairs of nerves which undoubtedly grow out from it are the third pair (Marshall), the fifth, the seventh and auditory (as a single root), the glossopharyngeal, and the various elements of the vagus (as separate roots in Elasmobranchii, but as a single root in Aves). Marshall holds that the olfactory nerve probably also originates from this crest. It will however be convenient to deal separately with this nerve, after treating of the other nerves which undoubtedly arise from the neural crest.
The cranial nerves just enumerated present in their further development many points of similarity; and the glossopharyngeal nerve, as it develops in Elasmobranchii, may perhaps be taken as typical. This nerve is connected by a commissure with those behind, but this fact may for the moment be left out of consideration. Springing at first from the dorsal line of the hind-brain immediately behind the level of the auditory capsule, it apparently loses this primitive attachment and acquires a secondary attachment about halfway down the side of the hind-brain. The primitive undifferentiated rudiment soon becomes divided, exactly like a true posterior root of a spinal nerve, into a root, a ganglion and a nerve. The main branch of the nerve passes ventralwards, and supplies the first branchial arch ([fig. 271] gl). Shortly afterwards it sends forwards a smaller branch, which passes to the hyoid arch in front; so that the nerve forks over the hyobranchial cleft. A typical cranial nerve appears therefore, except as concerns its relations to the clefts, to develop precisely like the posterior root of the spinal nerve.
Most of the cranial nerves of the above group, in correlation with the highly differentiated character of the head, acquire secondary differentiations, and render necessary a brief description of what is known with reference to their individual development.
The Glossopharyngeal and Vagus Nerves. Behind the ear there are formed, in Scyllium, a series of five nerves which pass down to respectively the first, second, third, fourth and fifth branchial arches.
For each arch there is thus one nerve, whose course lies close to the posterior margin of the preceding cleft; a second anterior branch, forking over the cleft and passing to the arch in front, being developed later. These nerves are connected with the brain by roots at first attached to the dorsal summit, but eventually situated about halfway down the sides. The foremost of them is the glossopharyngeal. The next four are, as has been shewn by Gegenbaur[173], equivalent to four independent nerves, but form together a compound nerve, which we may briefly call the vagus.