Relations of the nerves to the head-cavities. The cranial nerves, whose development has just been given, bear certain very definite relations to the mesoblastic structures in the head, of the nature of somites, which are known as the head-cavities. Each cranial nerve is typically placed immediately behind the head-cavity of its somite. Thus the main branch of the fifth nerve lies in contact with the posterior wall of the mandibular cavity, as shewn in section in [fig. 272] V. 2pp and in surface view in [fig. 271]; the main branch of the seventh nerve occupies a similar position in relation to the hyoid cavity; and, as Marshall has recently shewn, the main branch of the third nerve adjoins the posterior border of the front cavity, described by me as the premandibular cavity. Owing to the early conversion of the walls of the posterior head-cavities into muscles, their relations to the nerves are not quite so clear as in the case of the anterior cavities, though, as far as is known, they are precisely the same.
Fig. 272. Transverse section through the front part of the head of a young Pristiurus embryo.
The section, owing to the cranial flexure, cuts both the fore- and the hind-brain. It shews the præmandibular and mandibular head-cavities 1pp and 2pp, etc.
fb. fore-brain; l. lens of eye; m. mouth; pt. upper end of mouth, forming pituitary involution; 1ao. mandibular aortic arch; 1pp. and 2pp. first and second head-cavities; 1vc. first visceral cleft; V. fifth nerve; aun. ganglion of auditory nerve; VII. seventh nerve; aa. dorsal aorta; acv. anterior cardinal vein; ch. notochord.
Anterior nerve-roots in the brain.
During my investigations on the development of the cranial nerves I was unable to find any roots comparable with the anterior roots of the spinal nerves, and propounded an hypothesis (suggested by the absence of anterior spinal roots in Amphioxus[178]) that the head and trunk had become differentiated from each other at a stage when mixed motor and sensory posterior roots were the only roots present, and I supposed the cranial and spinal nerves to have been independently evolved from a common ground form, the resulting types of nerves being so different that no roots strictly comparable with the anterior roots of spinal nerves were to be found in the cranial nerves.
The views put forward by me on this subject, though accepted by Schwalbe (No. [357]), have in other quarters not met with much favour. Wiedersheim holds that it is impossible to believe that the cranial nerves are simpler than the spinal nerves. Such simplicity, which is clearly not found, I have never asserted to exist; I have only stated that the cranial nerves, in acquiring the complicated character they have in the adult, do not develop anterior roots comparable with those of the spinal nerves. Marshall also strongly objects to my views, and has made some observations for the purpose of testing them, leading to some very interesting results, which I proceed to state, and I will then explain my opinion concerning them.
The most important observation of Marshall on this subject concerns the sixth nerve. In both the Chick and Scyllium he has detected a nerve (the first development of which has unfortunately not been made out) arising by a series of roots from the base of the hind-brain. By tracing this nerve to the external rectus muscle of the eye he has satisfactorily identified it as the sixth nerve. “Neither in the nerve nor in its roots are there any ganglion cells." This nerve he finds to be placed vertically below the roots of the seventh nerve; and it is not visible till much later than the cranial nerves above described.
In addition to this nerve Marshall has found, both in the third nerve and in the fifth nerve, a series of non-gangliated roots, which arise in a manner not yet satisfactorily elucidated, considerably later than, and in front of, the main roots. These roots join the gangliated roots on the proximal side of the ganglion or in the ganglion[179]; and Marshall believes them to be homologous with the anterior roots of spinal nerves, while he holds the sixth nerve to be an anterior root of the seventh nerve.
In addition to these nerves Marshall holds certain ventral roots, which occur in Elasmobranchs close to the boundary of the spinal cord and medulla, and which probably form the hypoglossal nerve of higher types, to be anterior roots of the vagus. It is very difficult to prove anything definitely about these nerves, but, for reasons stated in my work on Elasmobranch Fishes, I am inclined to regard them as anterior roots of one or more spinal nerves.