THE SPINAL AND CEANIAL NEHVES. 129 



nerve is perhaps to be added to these. T he nptin nerve dA^rolripa 

 i n a very special mana er ; and the mode of development of the 

 t hird, fourth, and sixth nerves in the fmg has not y*-. Viggp 

 determined with gp.oiu- anv. 



The trigeminal, facial, glosso-pharyngeal, and pneumogastric 

 nerves, although arising from the neural ridges in the same way 

 as the dorsal roots of the spinal nerves, yet differ from these, and 

 agree amongst themselves, in certain important features, of 

 which the following are the principal : — 



i. T he nerves in question, in place of growing downwards, 

 like the spinal nerves, alongside the central nervous system, 

 g row outwards, clo se to the surface of the embryo, between the 

 epiblast and the mesoblast. 



ii. E ach of these four nerves acquires a new connection w ith 

 the surface epiblast s ome considerable distance beyond the roo t 

 of origin fr om the brain, and at about the horizontal level of the 

 notochord ; at this place, and at anv rate in part from the surface 

 ep iblast itself , the ganglion of the nerve is formed . 



iii. The nerves have special re la tions to the gill-slits, e acli 

 nerve dividing int o two main branches, whi ch embrace between 

 th em one of the gill-slits. 



iv. A special system of cutaneous nerves is developed fro m 

 the sur face epiblas t in conne ction with these four nerves, form- 

 ing t he lateral line system of nerves . 



In dealing with the several cranial nerves individually it will 

 be convenient to consider them in order from behind forwards. 



X. The pneumogastric, vagus, or tenth cranial nerve. This 

 grows rapidly m the early stages, and soon attains an enormous 

 size. In embryos of about 3 mm. length (cf. Figs. 58, C, and 

 59), when the neural folds have not quite met in the hinder part 

 of the head, and the neural groove is, therefore, still open, the 

 pneu mogast ric nerves are already present as a pair of win g- 

 like expansi ons of the neural ridges . The root of attachme nt 

 of the nerve, in the re-entering angle at t he top o f the brain, 

 between th e epiblast and the brain wall, is slende r ; b ut the rest o f 

 the' nerve is of great thickne ss. It extends more than half way 

 down the side of the pharynx, lying between the mesoblast and 

 the surface epiblast, very close to the latter but distinct from 

 it along its entire length (cf. Fig. 79, x). The nerves of the 

 two sides are in some cases unequally developed at this stage. 



K 



