146 WILLIAM F. ALLEN 



first motor rootlet of the second spinal nerve is but little greater 

 than the distance between two rootlets of the first spinal nerve. 

 In the caudal region the position and arrangement of these 

 rootlets was shown in figure 1 of a previous paper. 



A reconstruction of a spinal nerve taken from the region of the 

 caudal end of the retractor mandibulae muscles (fig. 5) presents 

 a number of differences from the spinal nerves of the more 

 cephalic region described above. 



It will be seen that the dorsal root (D.R.) does not at first 

 pursue a caudal course within the neural arch, but immediately 

 passes inward to enter the latero-dorsal surface of the spinal 

 cord. The fibers for this root arise from the inner central sur- 

 face of the spinal ganglion from a dorsal and a ventral bundle 

 (see previous paper, fig. 71). These bundles soon unite, form- 

 ing the dorsal root, which immediately enters its foramen in 

 the neural arch. This foramen is situated about on a level with 

 the central axis of the spinal cord. The dorsal sensory ramus 

 (R.D.S.) does not follow the contour of the neural arch so closely 

 as the more cephalic rami do in reconstruction 6. Instead, it 

 cuts obliquely dorsad through that portion of the myotomes 

 which is situated above the roof of the neural arch, and approxi- 

 mates the inner surface of the myotomes more closely than the 

 median dorsal septum. Throughout its entire course the dorsal 

 sensory ramus is well separated from the corresponding motor 

 ramus or any of its branches. The point of union between the 

 ventral motor and the ventral sensory ramus is considerably 

 more distal in this nerve than is the case of the more cephalic 

 nerves. As was noted for the more cephalic nerves, the ventral 

 sensory ramus is situated behind the ventral motor ramus, and 

 both appear to gradually approach each other, join, and form a 

 mixed ramus ventralis. Near the ventral border of the myotome 

 there is a conspicuous ganglion cell in the ramus ventralis. This 

 cell, which is shown in section in figure 11, is seen to be no dif- 

 ferent in size or structure from any of the spinal ganglion cells. 



Several variations in the arrangement of the motor compo- 

 nents are to be recorded for this nerve. It is clear from figure 

 5 that all of the motor rootlets, some eight in number, lie cephalad 



