SPINAL NERVES IN VERTEBRATES 143 



One of the first spinal nerves is shown from a graphic recon- 

 struction (fig. 6) to have one sensory and two motor roots. 

 The dorsal or sensory root fibers penetrate immediately the 

 membranous neural arch, the point of entrance being indicated 

 by dotted lines. Within the neural canal their course is at first 

 caudad for some little distance, then bending medially they 

 terminate in the latero-dorsal surface of the spinal cord, the 

 space from the arrow to the dotted line (D.R.) representing the 

 area of the cord where the fibers enter. The spinal ganglion 

 (Sp.G.), which is much smaller than the one behind, is decidedly 

 compressed, not more than two cells thick. From the outside 

 the ganglion is somewhat spindle-shaped the two poles being 

 dorsal and ventral in position, and the distance between them 

 is little more than the dorso-ventral diameter of the spinal cord. 

 From the poles peripheral fibers leave to form the dorsal and 

 ventral sensory rami {R.D.S. and R.V.S.). In this nerve the 

 ventral sensory ramus is but little larger than the dorsal. Each 

 follows the inner surface of the intermuscular septum of the 

 myotomes, which bends, both dorsally and ventrally, slightly 

 cephalad at first and then caudad. Throughout its course the 

 dorsal sensory ramus is entirely separated from the dorsal motor 

 ramus, but the ventral sensory ramus is joined by the correspond- 

 ing motor ramus opposite the notochord to form a mixed ramus 

 ventralis {R.V.). The extreme peripheral course of the dorsal 

 sensory ramus of the first spinal nerve is not shown in figure 6; 

 it is, however, very similar to the dorsal sensory ramus of the 

 succeeding nerve, which ^^dll be described later. 



As pre\'iously stated, the first spinal nerve in figure 6 has two 

 motor roots. They are more difficult to isolate than the corre- 

 sponding roots of a more caudal region. The more cephalic 

 root {V.R. (1) ) leaves the latero-ventral surface of the spinal 

 cord almost opposite its foramen in the neral arch, which wdll 

 be seen to be some little distance cephalad of the spinal ganglion 

 for this nerve. In contrast, the caudal root may be said to 

 take origin from four rootlets. The two last leave the spinal 

 cord almost directly below the entrance of the sensory root 

 fibers, which appear in figure 6 to be midway between the spinal 



