51 G COMPAEATIVE ANATOMY. 



In the Selachii and Gano'idei, some only cross over, and this is the 

 arrangement which is seen in all the higher Vertebrata. 



Neither of these sensory nerves have any character in which 

 they resemble spinal nerves, nor can they be referred to metameres. 

 They belong to that portion of the cranium which is not formed by 

 the concrescence of vertebral segments (cf . § 340) ; and they may 

 correspond to those nerves which we found passing to the same 

 organs in the Invertebrata. 



§ 388. 



The second division consists of the nerves which are arranged 

 on the type of the spinal nerves. It is sometimes possible to dis- 

 tinguish two roots : their dorsal branch is often very feebly developed, 

 in correlation with the small size of the area to which it is dis- 

 tributed. The ventral branch is consequently the important one ; it 

 sends nerves to the branchial arches, and the parts derived from them. 

 The visceral branch goes to the wall of the pharynx. The nerves 

 of this division arise at the base of the fourth ventricle, and partly, 

 also, from its continuation in the aqueductus Sylvii. They are 

 given off from the myelencephalon, and leave the cranial cavity by 

 perforating the vertebral portion of the cranium (§ 340). While these 

 relations are best seen in the cephalic nerves of the Selachii, which 

 most resemble the primitive condition, they undergo greater changes 

 the farther the organism has risen above this low stage, or has been 

 differentiated in another direction. 



We may note a number of special characters in the various 

 nerves, proceeding on the supposition that the series are homody- 

 namouswith spinal nerves. Some branches of a nerve become much 

 larger than others, which undergo degeneration ; or the roots of a 

 nerve take an independent course, and give the appearance of 

 independent nerves. In this way a nerve is broken up ; but other 

 nerves may undergo concrescence, so that what were primitively 

 complexes of nerves acquire the form of a single nerve. 



This latter arrangement is seen in two groups of the cerebral 

 nerves, which I have distinguished as the trigeminal and as 

 the vagus group ; the names of which are taken from that of the 

 most important nerve in each group. 



§ 389. 



The trigeminal group supplies the anterior, and larger 

 portion, of the head. The following nerves belong to it : 



First, the trigeminus, the largest nerve of the group, which, in 

 correspondence with the great differentiation of the regions to wliich 

 it is sent, has the characters of a greatly developed spinal nerve 

 (Fig. 290, Tr). Its dorsal branch is formed by the ophthalmic, which 

 supplies the orbits and the ethmoidal region. A branch for the cranial 

 cavity, which is found in the Teleostei, also belongs to the dorsal 



