SPINAL NERVES IN VERTEBRATES 165 



sory rami dorsales fibers traverse the inner surface of the myo- 

 tomes along the intermuscular septum joining the first and the 

 second following myotomes. 



In a reconstruction of two of the lower abdominal spinal 

 nerves of a 32 mm. Squalus (fig. 22) it will be seen that the ar- 

 rangement of the nerve components have reached practically 

 adult conditions. The most noticeable advance over the pre- 

 vious stage is an increase in length of the rami dorsales and a 

 pronounced caudal shifting of both the dorsal and the ventral 

 rami. In the dorsal rami this takes place much more abruptly. 

 It involves a movement of both the nerves and the myotomes 

 for a distance of a segment, and is probably caused by a more 

 rapid growth of the myotomes than of the skeletal and neural 

 axes. Even at this late stage the motor and sensory fibers in 

 both the dorsal and ventral rami remain separated for some dis- 

 tance. Each motor ramus dorsalis assumes at first a general 

 caudal course upon branching from the motpr ramus ventralis, 

 and after crossing the outer surface of its ganglion, it is accom- 

 panied by sensory fibers {R.D.S.), arising from the caudal sur- 

 face of the ganglion at the point where the motor nerve crosses. 

 The two dorsal rami continue caudad, side by side, to a point 

 in front of the foramen for the following motor root, then bend- 

 ing dorsally, they cross somewhat dorsally the outer surface 

 of the succeeding spinal ganglion. Soon after lea\dng this 

 ganglion the two rami join in forming a mixed ramus dorsalis 

 (R.D.), which continues in a general caudo-dorsal direction 

 along the inner surface of the corresponding intermuscular 

 septum, which has apparently shifted caudad for a distance of a 

 segment from its original embryonic position. Throughout the 

 proximal course of a ramus dorsalis, where sensory and motor 

 nerves were described as running side by side, they will be 

 found to be sharply separated by connective tissue or neuri- 

 lemma. Figure 26, which passes through the exit of the sen- 

 sory ramus dorsalis from the ganglion, shows very clearly its 

 relationship with the corresponding motor component, while a 

 more distal section (fig. 25) through the two rami shows them 

 to be well-separated and to lie in nearly the same horizontal 



