Integrative Systems 169 



the ear is a highly specialized part of the lateral-line system. Nerve 

 VIII may be regarded as part of the lateral-line division of VII. 



IX and X are closely allied in origin and distribution. 



XI and XII occupy a confused "no-man's land" in the region of 

 transition between brain and cord. 



While there is considerable overlapping in the territorial distribu- 

 tion of V and VII and of IX and X, it is possible to recognize a terri- 

 torial center for each nerve. V is concerned mainly with the region of 

 the mandibular arch; VII, with the spiracular and hyoidean region; 

 IX, with the first branchial chamber; and X serves the entire branchial 

 territory behind the first gill-chamber, however many the gill-chambers 

 and arches may be. In terrestrial vertebrates, these nerves retain their 

 primitive territorial relations, serving such structures as may have 

 persisted from gill-breathers or new structures which may have arisen 

 in the respective old territories of the several nerves. The extraordinary 

 feature of X is the extreme posterior extent of its somatic lateral-line 

 trunk and its important intestinal division. 



One striking feature common to V, VII, IX, and X is that each has 

 its pretrematic and post-trematic divisions (trema meaning a 

 "hole"). V sends a trunk to the upper jaw and one to the lower jaw. 

 (In sharks the upper jaw is anterior to the lower jaw.) Similarly the 

 spiracle is straddled by VII, the first gill-chamber by IX, and each of 

 the remaining gill-chambers by a division of X. 



SPINAL NERVES 



The spinal nerves are segmentally arranged — a pair to each seg- 

 ment of the body-muscle. Two trunks or "roots" intervene between 

 the cord and the main nerve, a dorsal and a ventral root (Figs. 144, 

 151). The exit of the nerves from the vertebral column is usually inter- 

 vertebral, but in sharklike fishes the roots pierce cartilaginous elements 

 of the column and in some other vertebrates the nerves may pierce 

 bony vertebrae. 



The dorsal root is composed mainly or entirely of sensory fibers 

 which arise from cells of a spinal ganglion situated on the root near 

 its junction with the ventral root (Fig. 151). Each cell of the ganglion 

 produces a bifurcating process (Fig. 152), one of whose branches passes 

 along the root into the cord, where it divides usually into a long as- 

 cending and a short descending fiber. From each of these pass very 

 fine branches into the dorsal column of the "gray matter," where 

 synaptic connection with other neurons is made. The other of the two 

 primary branches of the ganglion-cell passes outward into the main 

 trunk of the spinal nerve and proceeds directly to some receptor. The 



