826 THE NERVOUS SYSTEM 



2. The Cerebrospinal System 



The cerebrospinal system of nerves is composed of the cranial and spinal 

 nerves. Two sets of neurons enter into the composition of the cranial and 

 spinal nerves, viz.: 



( 1 ) afferent neurons, whose fibers receive stimuli from certain receptor or- 

 gans and convey the impulses to the central nervous system, and 



(2) efferent neurons, with fibers which convey the impulses from the cen- 

 tral nervous system to the peripheral areas. The central nervous system 

 with its multitudes of association neurons thus acts to correlate the 

 incoming impulses from afferent neurons and to shunt them into the 

 correct outgoing pathways through the fibers of the efferent neurons 

 (see figure 358A). 



Most of the afferent or sensory neurons are located in ganglia outside of 

 the central nerve tube, within the dorsal root ganglia of the spinal nerves and 

 in the ganglia of the cranial nerves in close association with the brain (fig. 

 356B). On the other hand, the cell bodies of the somatic efferent or motor 

 fibers are found within the gray matter of the central nerve tube, and the 

 cell bodies of the visceral efferent or motor fibers are located within the gray 

 matter of the central nerve tube and also in peripheral (autonomic) ganglia. 



3. General Structure and Function of the Spinal Nerves 



In each of the spinal nerves the nerve fibers are of four functional varieties, 

 namely, visceral sensory (afferent); visceral motor (efferent); somatic sensory 



(afferent); and somatic motor (efferent). The visceral components are dis- 

 tributed to the glands, smooth muscles, etc., of the viscera located within the 

 thoracic and abdominal cavities, together with the blood vessels of the general 

 body areas. The somatic components innervate the body wall tissues includ- 

 ing the skin and its appendages. A spinal nerve and its component fibers in 

 the trunk region is shown in figure 358A, and figure 358B shows this dis- 

 tribution in the region of the brachial plexus. 



A typical spinal nerve is composed of the following general parts: 



( 1 ) The dorsal or sensory root with its ganglion, and 



(2) the ventral or motor root. 



(3) Each spinal nerve divides into 



(4) a dorsal ramus, and 



(5) a ventral ramus. The ventral ramus may divide into 



(6) a lateral branch and 



(7) a ventral branch. Connecting with the spinal nerve also are 



(8) the gray and white rami of the autonomic nervous system. 



As the peripheral nerve fibers grow distad they become grouped together 

 to form peripheral nerves. Each nerve in consequence is an association of 



