1018 



THE NERVE SYSTEM 



resulting from the union of the roots, in the intervertebral foramina; and, passing 

 dorsad, divide into internal or medial and external or lateral branches, which are 

 distributed to the muscles and integument behind the spine. The posterior 



^ ..INTERNAL 



BRANCH 

 EXTERNAL BRANCH 



POSTERIOR PRIMARY 

 DIVISION 



ANTERIOR PRIMARY 

 DIVISION 



RAMUS COMMUMCANS 



FIG. 750. Plan of the constitution of a spinal nerve. (W. Keiller, in Gerrish's Text-book of Anatomy.) 



primary divisions of the spinal nerves form two small plexuses, the posterior 

 cervical plexus and the posterior sacral plexus. The first cervical, the fourth and 

 fifth sacral, and the coccygeal nerves do not divide into external and internal 

 branches. 



Spinal nerve 



\ Sympathetic 

 \ ganglion. 



'X 



Cell of Dogiel 

 Spinal ganglion 



Sympathetic 

 ganglion 



FIG. 751. Scheme showing structure of a typical spinal nerve. 



The anterior or ventral primary divisions (rami anteriores) (Fig. 750) of the 

 spinal nerves supply the parts of the body ventrad of the spine, including the limbs. 

 They are for the most part larger than the posterior primary divisions. Each 

 division, soon after its origin, receives a slender filament from the sympathetic 

 which is called the gray ramus communicans. In the thoracic region the anterior 



