SPINAL NERVES IN VERTEBRATES 145 



distribution of the ramus ventralis is identical to the ramus 

 ventralis of the spinal nerve in reconstruction 5, this nerve was 

 not reconstructed further than the level of the dorsal aorta in 

 figure 6. 



Several facts remain to be recorded in connection with the 

 components of the second spinal nerve in figure 6. They are 

 in the main either differences from the condition described for 

 the first spinal nerve or additional points of detail. First, the 

 second spinal ganglion is much larger than the first. It extends 

 dorsally nearly to the apex of the neural arch and ventrally to 

 the notochord. In this region the myotomes follow very 

 closely the outer surface of the neural arch and the myotomes of 

 each side are attached above the neural arch to a median dorsal 

 connective tissue septum; so that there is a wall of muscle above 

 the neural arch of considerable thickness. It is between this 

 wall of muscle and the dorsal median septum that the second 

 dorsal sensory ramus in figure 6 passes to reach the skin. Upon 

 arriving at the dorsal surface of the myotomes it bends laterally 

 finally to pass ventrad along the inner surface of the sldn as the 

 superior cutaneous nerve of Cole. No peripheral ganglion cells 

 were found along the course of this nerve outside the spinal 

 ganglion. The dorsal motor ramus of the second spinal nerve 

 in figiu'e 6 does not arise as a branch from the common trunk 

 formed from the union of the cephalic and caudal roots, but 

 rather branches off from the cephalic root outside the neural 

 arch shortly before it joins the caudal root to form the ventral 

 motor ramus. The dorsal motor ramus of the second nerve 

 can be traced to a much higher level on the myotome, but 

 crosses the sensory ramus much closer to the ganglion. Through- 

 out, it is lateral to the sensory ramus and there is no exchange of 

 fibers. The caudal motor root takes origin from more than four 

 rootlets. Attention should be called to the number and the 

 distance apart of the motor rootlets in this region. There may 

 be from five to eight taking part in the formation of the two 

 motor roots of a spinal nerve and their extent is nearly equal 

 to the length of a segment. In fact, in figure 6 the distance 



between the last motor rootlet of the first spinal nerve and the 

 ? 



THE JOURNAL OF COMPARATIVE NEUROLOGY, VOL. 28, NO. 1 



