GENERAL CUTANEOUS SUBDIVISION. 



107 



tively simple. The arrangement is shown in a generalized scheme 

 in Figure 50. In cyclostomes (Fig. 51) the dorsal spinal nerves 

 in the branchial region send dorsal rami to the skin of the back 

 and their ventral rami join the epibranchial trunk. From this 

 the cutaneous fibers are distributed by the posttrematic rami to 

 the skin of the lateral and ventral surface of the gill region. The 

 cutaneous component in the vagus root goes to the first post- 

 trematic ramus of the vagus. In true fishes and all higher forms 

 the cutaneous component of the X root is distributed by dorsal 

 rami to the dorsal surface of the head. In fishes provided with 

 an operculum general cutaneous fibers are not found in the bran- 

 chial rami, but a ramus from X helps to supply the skin on the 

 operculum. The cutaneous components in the branchial nerves 



N. ophth. pro 

 N. terminal! 



Olf.sac 

 R. max. 

 Eye 



(. opercularis 

 mand. 



N. spin, dors- 



" FIG. 50. A simple diagram of the general cutaneous components in the cranial 

 and spinal nerves of a fish. The dotted lines (except R. opercularis) represent 

 components present in cyclostomes but not yet found in other fishes. 



of selachians have not been described. The cutaneous component 

 in the IX nerve in cyclostomes goes by way of the ramus post- 

 trematicus to the skin of the region of the first branchial arch. 

 In other vertebrates only a dorsal ramus is present. 



The absence of a general cutaneous component from the VII 

 nerve in most vertebrates requires to be explained. In forms 

 provided with an operculum, the operculum is innervated by rami 

 from the V and X nerves and rami from the V nerve supply the ven- 

 tral surface in the gill region. In cyclostomes, where there is no 

 operculum, a simpler and more primitive arrangement is found. 



