144 WILLIAM F. ALLEN 



ganglion of this nerve and the following ganglion, and they are 

 almost the distance of a segment behind the exit of the cephalic 

 root fibers. The two caudal rootlets soon unite in a common 

 stem, which continues cephalad in the neural canal about on a 

 level with the ventral surface of the spinal cord, to the region 

 of the spinal ganglion, where it receives a common stem 

 formed from the miion of the two cephalic rootlets. This 

 common trunk is designated as the caudal root. It continues 

 forward in the neural canal to the exit of the cephalic root, and 

 then leaves the neural canal through a foramen immediately 

 behind the foramen for the cephalic root. Outside the mem- 

 branous neural arch the two roots unite in the formation of a 

 trunk, which passes dorsad a short distance to separate into a 

 dorsal and a ventral motor ramus. The dorsal motor ramus 

 (R.jD.M.) pursues a dorsal course along the inner surface of the 

 myotomes, to which it sends numerous branches. At first its 

 course is considerably cephalad of the corresponding sensory 

 ramus, but when near the level of the roof of the neural arch it 

 is approached by the dorsal sensory ramus, and at this point 

 terminates in a cephalic and a caudal branch, which soon disap- 

 pear in the muscle. In figure 6 the cephalic branch may have 

 the appearance of joining the dorsal sensory ramus; this, how- 

 ever, is not the case, for the two are widely separated by con- 

 nective tissue. The motor ramus is the more lateral and trav- 

 erses the inner surface of the myotomes; while the dorsal sen- 

 sory ramus follows closely the outer surface of the neural arch. 

 Since the myotomes (D.Myo.) extend some distance above the 

 neural arch, it might be expected that the dorsal motor ramus 

 of the first spinal nerve would extend farther dorsad than is 

 shown in figure 6. The course of the ventral motor ramus 

 (R.V.M.) is in general lateral, caudal and ventral. After 

 cutting through a median portion of the myotome it unites with 

 fibers of the ventral sensory ramus, opposite the notochord and 

 a little lateral of the combined glossopharyngeal-vagus trunk, 

 in forming a mixed ramus ventralis {R.V.). From this point 

 on the course of the ramus ventralis is along the inner margin 

 of the intermuscular septum of the myotomes. Since the further 



