SOMATIC AFFERENT DIVISION. 



CHAPTER VI. 



SOMATIC AFFERENT DIVISION. GENERAL 

 CUTANEOUS SUBDIVISION. 



In the skin of all vertebrates certain fibers belonging to the 

 dorsal nerves end by free branches between the cells of the epider- 

 mis (Fig. 48). These are the fibers of the sense of touch. In 

 the trunk these fibers form the largest component of the dorsal 

 spinal nerves and are distributed by way of both dorsal and 

 ventral rami. The rami reach the skin in the myosepta and may 

 be distributed forward or backward from the myoseptum in which 



FIG. 48. General cutaneous endings in the cyclostome Lampetra Wilderi. B is 

 taken from the border of a neuromast pit. 



they run, or both forward and backward, i.e. to the two adjacent 

 segments. A diagram to show the central relations of the general 

 cutaneous components is given in Fig. 49. 



Passing forward into the head we find a region between the 

 permanent first spinal nerve and the vagus in which general 

 cutaneous nerves are apparently absent. In cyclostomes, however, 

 the full number of cutaneous nerves are present, and the same is 

 apparently true in the primitive selachian Heptanchus, as shown 

 in Figure 2. In the embryo of other selachians there is one 

 vestigeal ganglion. for each segment, which probably represents 

 a cutaneous nerve. Even in mammalian embryos, as in the pig 

 (Fig. 20) and in man (Fig. 32), rudimentary ganglia which may 



