126 



INTRODUCTION TO NEUROLOGY 



Each spinal nerve distributes efferent (motor) fibers to the 

 muscles and afferent (sensory) fibers to the skin and deep tissues 

 of its appropriate segment of the body, and through its connec- 

 tions with the sympathetic nervous system it may effect various 

 visceral connections (Figs. 55 and 56). The efferent fibers 

 leave the cord through the ventral roots of the spinal nerves, 

 these fibers arising from cells within the gray matter of the cord, 

 and the afferent fibers enter through the dorsal roots, these 



Dorsal root 



Lateral column 



Ventral column . . , _ , . , 



Preganglionic fiber 



Ramus communicans 

 Sympathetic ganglion 

 Postganglionic fiber 



Ventral root 



Visceral muscle 



Mucous membrane 



Fig. 56. Diagram illustrating the composition of a typical spinal nerve 

 in the thoracic region. The somatic sensory system is indicated by broken 

 lines, the visceral sensory by dotted lines, the somatic efferent by heavy 

 continuous lines, the visceral efferent by lighter continuous lines. (Compare 

 Figs. 1 and 55.) 



fibers arising from cell bodies of the spinal ganglia (see Fig. 1, 

 p. 25, and Figs. 55, 56). The fibers of the spinal nerves are chssi- 

 fied in accordance with the same physiological criteria as their 

 end-organs (see pp. 79-94, and compare the cranial nerves, pp. 

 143-150) into somatic afferent (or sensory), visceral afferent 

 (or sensory), somatic efferent (or motor), and visceral efferent 

 (or motor) systems (Fig. 56). 



In the spinal cord the originally wide cavity of the embryonic 

 neural tube (see p. 116) is reduced to a slender central canal and 



