SPINAL NEEVES IN VERTEBRATES 149 



four segments. Apparently in this specimen it was a very im- 

 portant effective or motor nerve for the M. cordis caudahs. 

 The first four ventral motor rami in figure 1 cross their respective 

 sensory rami as the three more caudal rami did, but immediately 

 after crossing they bend ventrad and probably join the sensory 

 rami or branch of the same. It is impossible to state whether 

 the fibers mix or not, although there is nothing to prevent them 

 from so doing. A little below the ventral border of the myo- 

 tomes the motor components of these ventral rami form a sort 

 of plexus from which branches are given off to the M. cordis 

 caudalis (R.M.C.C), to certain longitudinal muscle bundles 

 (L.M.B.) and to the M. transversus caudalis. 



Regarding the distribution of the dorsal motor rami (R.D.M.) 

 of the tail region, at least one important difference from the 

 cephalic region should be noted. It is that the motor rami do 

 not follow the general course of their respective sensory rami so 

 closelj^ as they do their succeeding sensory rami, a characteristic 

 which was also recorded for the last three ventral motor rami. 

 As in the cephalic region there is considerable variation concern- 

 ing the mode of origin of these nerves. They usually branch off 

 from or with the ventral motor rami at the point of union of 

 the cephalic and caudal motor roots or they may arise directly 

 from the motor roots upon emerging from the neural arch. 

 Near the point of origin a lateral branch of considerable size is 

 given off from each of the dorsal motor rami to the myotomes. 

 It is much larger than any of the other lateral branches of the 

 nerve and might be designated as the motor ramus lateralis. 

 No dorsal motor rami were found corresponding to the last 

 three ventral motor rami, resembling in this respect the occipito- 

 spinal nerves. This may have been brought about by the scar- 

 city of dorsal musculature in the tail region. As was noted in a 

 previous paper, that portion of the spinal cord which is situated 

 behind the exit and entrance of the last motor and sensory roots 

 is non-nervous. It consists of supporting and undifferentiated 

 cells. From sections, the spinal cord was found to end in a 

 number of processes, which intermingle and become lost in a mass 

 of connective tissue at the end of the membranous neural canal. 



