SPINAL NERVES IN VERTEBRATES 159 



It is evident from figure 23 that the distribution of the motor 

 and sensory components of the spinal nerves of an adult Squalus 

 differs in a number of details from other vertebrates. As far 

 as could be determined each spinal nerve has but one ventral 

 ar motor root {V.R.), which may take origin from several root- 

 lets, arising from the ventro-lateral surface of the spinal cord, a 

 little in front of the spinal ganglion of this nerve. The dorsal 

 or sensory root fibers (D.R.) upon leaving the dorsal poles of 

 their respective ganglia, immediately enter the neural arch 

 about on a level with the dorsal surface of the spinal cord, and 

 without pursuing a cephalic course within the neural canal 

 terminate in the dorso-lateral surface of the spinal cord. Im- 

 mediately after leaving the neural arch a motor root separates 

 into a dorsal and a ventral ramus. The motor ramus dorsalis 

 {R.D.M.) pursues at first, a general caudal course and after 

 crossing the lower caudal surface of its ganglion, is joined by 

 the sensory ramus dorsahs fibers {R.D.S.), which take exit from 

 the lower caudal surface of the ganglion (instead of from the 

 dorsal pole as in cyclostomes). The mixed ramus dorsalis, 

 thus formed, in place of following its corresponding intermus- 

 cular septum, continues caudad opposite the neural arch for a 

 distance of a segment, to the foramen for the motor root of the 

 following nerve, where it bends obHquely dorsad and crosses 

 the upper outer surface of the succeeding spinal ganglion, to 

 assume a general dorsal caudal course along the inner surface 

 of the second following intermuscular septum, which is one 

 segment behind the one it would follow in most vertebrates. 

 This pecuhar disposition of the ramus dorsalis in Squalus recalls 

 a somewhat different arrangement found in some bony fishes, 

 where the ramus dorsalis is formed from a sensory filament of 

 its own nerve and a motor branch from the preceding nerve. 

 The motor ramus ventralis {R.V.M.) or ventral continuation 

 of many of the motor root fibers takes a general ventro-caudal 

 course across the skeletal axis and, when part way across, is 

 joined by the sensory ramus ventralis {R.V.S.), which fibers 

 arise from the ventral pole of its ganglion. The course of the 

 mixed ramus ventralis (R.V.) is then along the inner surface of 

 the following intermuscular septum. 



