GENERAL MORPHOLOGY OF THE NERVOUS SYSTEM. 



cephalon. In this region the dorsal and ventral nerves remain 

 separate from one another throughout life. Of the dorsal nerves 

 the most caudal which can be recognized in all vertebrates corre- 

 sponds to the tenth cranial or vagus nerve of human anatomy. 

 This arises by numerous small roots some of which are sensory, 

 others motor. The motor are situated slightly ventral to the 

 sensory. The roots all unite into one large trunk which descends 



R. palat.VII 



R. hyom. 

 R. praetr.Vl 



R. pharyng. 



R. palat.IX R- posttr. 

 R. prastr. 



B 



FIG. 5. Simple diagrams of the branchial nerves of lower vertebrates as seen 

 from the left side. A, in a cyclostome; B, in a true fish. In B the trigeminus 

 nerve is not shown. 



to the dorsal border of the second gill slit in fishes and there bears 

 a ganglion (Fig. 5). From this ganglion a main trunk continues 

 caudally and bears a ganglion over each gill slit. From each 

 ganglion two rami arise. The first runs ventrally in front of the 

 slit and is called the ramus praetrematicus. It gives the large 



