XERVOCti XYSTEM OF THE VERTKERATA. 379 



obviously comparable to the main branch of the posterior nerves. The 

 superficial ophthalmic branch is clearly equivalent to the ophthalmic branch 

 of the seventh. The superior maxillary is usually held to be equivalent to 

 that branch of the posterior nerves which forms the anterior limb of the fork 

 over a cleft. The similarity between the course of this nerve and that of 

 the palatine branch of the seventh, resembling as it does the similar course 

 of the ophthalmic branches of the two nerves, suggests that it may perhaps 

 really be the homologue of the palatine branch of the seventh, there being 

 no homologue of the typical anterior branch of the other cranial nerves. 



The third nerve. Our knowledge of the development of the third 

 nerve is entirely due to Marshall. He has shewn that in the Chick 

 there is developed from the neural crest, on the roof of the mid-brain, an 

 outgrowth on each side, very similar to the rudiment of the posterior nerves. 

 This outgrowth, the presence of which I can confirm, he believes to be the 

 third nerve, but although he is probably right in this view, it must be 

 borne in mind that there is no direct evidence on the point, the fate of 

 the outgrowth in question not having been satisfactorily followed. 



At a very considerably later period a nerve may be found springing 

 from the floor of the mid-brain, which is undoubtedly the third nerve, and 

 which Marshall supposes to be the above rudiment, which has shifted its 

 position. It is shewn in Scyllium in tig. 271 B, ///. A few intermediate 

 stages between this and the earliest condition of the nerve have been im- 

 perfectly traced by Marshall. 



The nerve at the stage represented in fig. 271 B arises from a ganglionic 

 root, and "runs as a long slender stem almost horizontally backwards, then 

 turns slightly outwards to reach the interval between the dorsal ends of the 

 first and second head cavities, where it expands into a small ganglion." 

 This ganglion, as first suggested by Schwalbe (No. 359), and subsequently 

 proved embryologically by Marshall, is the ciliary ganglion. From the 

 ciliary ganglion two branches arise ; one branch continuing the main stem 

 of the nerve, and obviously homologous with the main branch of the other 

 nerves, and the other passing directly forwards ''along the top of the first 

 head cavity, then along the inner side of the eye, and finally terminating 

 at the anterior extremity of the head, just dorsal of the olfactory pit." 



The partial separation, in many forms, of the. ciliary ganglion from the 

 stem of the third nerve has led to the erroneous view (disproved by the 

 researches of Marshall and Schwalbe) that the ciliary ganglion belongs to 

 the fifth nerve. The connecting branch of the fifth nerve often becomes 

 directly continuous with the anterior branch of the third nerve, and the 

 two together probably constitute the nerve known as the ramus oph- 

 thalmicus profundus (Marshall). Further embryological investigations 

 will be required to shew whether this nerve is homologous with the nasal 

 branch of the fifth nerve in Mammalia. 



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 



