THE NERVOUS SYSTEM 



181 



structures in the sides and ventral parts of the body; and (3) a 

 ramus communicans, penetrating the coelomic cavity, but 

 remaining retroperitoneal, just lateral to the attachment of the 

 mesentery, where it forms connections with the autonomic 

 nervous system (Fig. 117). The rami communicantes may be 

 double, in which case one is a white ramus and the other gray. 



Fig. 117. — Diagram showing relations between spinal cord, spinal nerve and 

 ganglia of autonomic nervous system, c.g., chain ganglia; g, spinal ganglion; 

 g.m., gray matter of spinal cord; g.r., gray ramus communicans; m.r., motor 

 root of spinal nerve; pe.g., peripheral ganglion; pr.g., prevertebral ganglion; 

 r.d., ramus dorsalis; R.v., ramus ventralis; s.r., sensory root of spinal nerve; 

 w.m., white matter of spinal cord; w.r., white ramus communicans; broken lines 

 represent postganglionic fibers. 



This difference is due to the fact that the fibers of the white ramus 

 are medullated and those of the gray are nonmedullated. Both 

 kinds of rami are not present in all spinal nerves. Thus in Man 

 each spinal nerve has a gray ramus and nearly all except the 

 cervical spinal nerves have, in addition, a white ramus 

 communicans. 



The functional significance of the dorsal and ventral roots of 

 the spinal nerves of a mammal can be demonstrated by experi- 



