THE NERVOUS SYSTEM 453 



superficial, and at the same time, the most aberrant, is the ra- 

 mus lateralis, which belongs to the system of lateral line nerves 

 and supplies the lateral line itself, which extends typically 

 to the end of the tail. It is thus the longest nerve in the body, 

 co-extensive with the spinal cord itself. The lateral nerves of 

 the two sides are connected with one another by the supra-tem- 

 poral branch, which forms a connecting loop over the top of the 

 head ; in the Dipnoi, though not in other fish, lateral communi- 

 cating branches connect it with the superficial ophthalmic nerve 

 of the Facialis, thus uniting the two parts of the system. The 

 ramus lateralis has at its proximal end a ganglion of its own 

 (ganglion laterale), although it arises in connection with the 

 combined ganglionic mass of the Vagus. In spite of this asso- 

 ciation, however, it is probable that the ramus lateralis did not 

 originally belong to the Vagus alone, but was built up as a 

 collecting trunk from branches supplied by each metameric 

 nerve of the body, beginning with the Vagus. The gradual 

 loss of these metameric connections, beginning posteriorly, 

 would, in time, leave the most anterior one alone, the condition 

 found at present. 



Ventral to the ramus lateralis appear five elements, similar 

 to one another, each associated with a gill-slit, and possessed of 

 its own ganglion. This extremely primitive condition is seen 

 in a few forms only (e.g., the rays and skates}, but these ani- 

 mals are in other respects so primitive, and the condition is 

 so exactly what one would expect as an early one, that it may 

 be taken as undoubtedly the starting point. The first of these 

 elements is more distinct than the others, and forms the Glosso- 

 pharyngeus, treated as a separate nerve; the remaining four 

 are Vagus elements and in all but very primitive forms arise 

 from a single ganglionic mass, the ganglion jugulare, formed 

 of a fusion of the four primary ones. It is with this that the 

 ganglion laterale of the ramus lateralis is associated. Each 

 of these five elements (Glossus-pharyngeus, and the four Vagi) 

 possesses an identical distribution. From the ganglion the 

 .main stem passes downwards, and forks into two branches, 

 including a gill-slit in the fork. The two branches, one in 



