SPINAL NERVES IN VERTEBRATES 153 



periphery of some of the sensory rami. A description and dis- 

 cussion of these cells and similar cells of the glossopharyngeal 

 and vagus nerves is reserved for a separate subhead. 



It is clear from the previous description that there is a striking 

 difference in the mode of union of the ventral motor and sensory 

 rami of the cephalic region and those of the caudal region, where 

 such union takes place. In the cephalic nerves (figs. 5 and 6) 

 the motor root foramina and the ventral motor rami are all 

 cephalad of their corresponding sensory rami and ganglia, and 

 the ventral motor rami join their respective sensory rami from 

 in front, while in the caudal region (figs. 1 and 3) the foramina 

 and the ventral motor rami are caudad of their corresponding 

 sensory rami and ganglia, and the motor rami join the sensory 

 from the rear. The point of union is much further peripherad 

 in the caudal nerves. In figures 5 and 6 it is opposite the noto- 

 chord, while in the tail region (fig. 1) it took place considerably 

 below the notochord. It occurred to the writer that a dissec- 

 tion of the spinal nerves of an adult might explain how the 

 change from one type to the other happened; whether it was 

 gradual or abrupt, and if the latter, where it took place. Fig- 

 ure 13 is a drawing of a part of this dissection. It includes the 

 distribution of the sensory and motor components of six of the 

 spinal nerves situated immediately in front of the M. cordis 

 caudalis. It will be seen that the third ventral sensory ramus 

 divides at the lower surface of the notochord into two branches, 

 which supply sensory fibers for two rami ventrales. The cephalic 

 branch receives the ventral motor ramus from in front and the 

 caudal one, from behind. All of the ventral motor rami cephalad 

 of this branched sensory ramus join their corresponding sensory 

 rami from in front and all caudad, that unite with sensory rami, 

 join them from the rear. It is clear in these six spinal nerves 

 that the point of union between the ventral motor and the ventral 

 sensory rami is rapidly becoming more proximal in passing 

 cephalad. So that it is only in a few of the caudal nerves that 

 the union of the ventral sensory and motor rami occurs below 

 the level of the notochord. 



