Nervous System and Sense Organs 203 



block of gray matter. Between and around the gray matter is the 

 white matter. This arrangement of nerve cell bodies and processes is 

 the opposite of that of the brain in which the gray matter is on the 

 outside. 



The principal functions of the cord are to relay messages to and 

 from the brain and also to act as a reflex center. 



DORSAL FISSURE 



VENTRAL 

 NERVE 

 ROOT 



VENTRAL FISSURE 



Fig. 63. — Cross section of spinal cord showing origin of spinal nerves. 



The Peripheral Nervous System. — The nerves which enter and 

 leave the central nervous system compose the peripheral nervous sys- 

 tem. By means of these nerves and the connecting nerve tracts, the 

 various stimuli of the environment are relayed to the proper places 

 and needed responses are made. The nerve cell processes can carry 

 impulses in but one direction: those carrying the impulses toward 

 the central nervous system are afferent or sensory neurons; those car- 

 rying impulses away are efferent or motor neurons (Fig. 58). These 

 processes are grouped into the familiar nerves, of which there are 

 three possible types : the afferent or sensory nerves composed of den- 

 drites ; the efferent or motor nerves made up of axons ; and the mixed 

 nerves composed of both dendrites and axons. 



The nerves making up the peripheral system are fundamentally 

 segmental in the manner in which they leave the brain and spinal 

 cord. This segmental arrangement is more apparent in the spinal 

 nerves which are arranged in pairs. Each spinal nerve is derived from 

 two roots (Fig. 63), the dorsal being sensory, the ventral, motor. The 

 ventral root arises directly from the ventral portion of the gray mat- 

 ter of the cord while the dorsal root has its origin in a ganglion out- 



