NERVOUS SYSTEM OF THE VERTEBRATA. 461 



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 fig. 271 B, ///. A few intermediate 

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

 imperfectly 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 ophthalmicus 

 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 to the hyoid cavity ; and, as Marshall has recently shewn, the 

 main branch of the third nerve adjoins the posterior border of the front 



