SPINAL NERVES IN VERTEBRATES 155 



tinue dorsally to supply the skin. Ventrally, a mixed ramus 

 ventralis divides at the lower border of the myotome, sending 

 the R. cutaneus medius laterad and dorsad along the inner 

 surface of the skin and a mixed ramus, the R. cutaneus inferior 

 of Cole, to the constrictor mus(3les of the mucous sac and to the 

 ventral integument, (d) A few spinal nerves situated in the 

 region of the cephalic end of the M. cordis caudalis present the 

 following peculiarities. The long motor roots of each nerve 

 leave the neural arch behind their corresponding ganglion. 

 Each dorsal motor ramus follows the course of its succeeding 

 sensory ramus rather than its own, and every ventral motor ramus 

 joins its corresponding sensory ramus from the rear instead of 

 in front. This union also occurs much further peripherally, 

 taking place below the level of the caudal vein, (e) A dissec- 

 tion of the spinal nerves of one specimen (fig. 13) discloses that 

 the change relating to the ventral motor rami joining the ventral 

 sensory rami from behind, is brought about abruptly, through 

 the third ventral sensory ramus in front of the M. cordis caudalis 

 dividing on a level with the lower surface of the notochord into 

 cephalic and caudal branches, which furnish sensory fibers for 

 two rami ventrales. In the first the motor ramus joins from in 

 front and in the second it joins from behind; thereby introducing 

 a different arrangement for the following caudal nerves, (f) At 

 least seven of the spinal nerves in the region of the M. cordis 

 caudalis differ from the more cephalic nerves in that their 

 sensory and motor rami remain separate throughout. The 

 motor rami frequently cross the sensory rami, but always with- 

 out exchange of fibers. Tn most of the spinal nerves of the tail 

 region the dorsal and ventral sensory rami extend to the dorsal 

 and anal fins, and when outside the liixiits of the myotomes they 

 give off lateral cutaneous branches, designated as the R. cutan- 

 eus lateralis superior and the R. cutaneus lateralis inferior. 

 (g) The nerve supply for the M. cordis caudalis, the pulsating 

 muscle for the caudal heart, is from certain of the rami ventrales. 

 (h) Several variations were found in the last three spinal nerves. 

 These nerves were without dorsal motor rami and had but one 

 motor root, while their spinal ganglia received more than one 



