RAT, A REPRESENTATIVE MAMMAL 137 



different levels of the cord with each other and the brain. Each of 

 the spinal nerves, which were mentioned above, joins the gray matter 

 of the cord by two roots, a ventral root which enters the ventral 

 column and a dorsal ix)ot which enters the dorsal column of gray 

 matter. The dorsal root carries the afferent nerve fibers to the cord, 

 while the ventral root carries the efferent fibers. A spinal ganglion 

 containing cell bodies of afferent fibers is associated with each dorsal 

 root. 



The trunk and limbs receive man}^ impulses produced by environ- 

 mental stimuli, as well as entertaining most of the activities of locomo- 

 tion, digestion, circulation, excretion, respiration, reproduction, etc. 

 All of these must be kept properly related and coordinated in order 

 that the organism be kept alive and carrying on normal activities. This 

 is done largely by the spinal cord and associated autonomic portion of 

 the system. Certain impulses are transmitted by the cord to the brain 

 for association and reaction there. In general, it may be said that 

 the cord functions as a center with some power of integration, par- 

 ticularly for reflexes of the trunk and limbs, as well as a pathway 

 to the brain. 



Peripheral Nerves. — Twelve pairs of cranial nerves leave the brain 

 of the rat and thirty-four pairs of spinal nerves are given off by the 

 spinal cord. These make connections with the sense organs (recep- 

 tors), motor end plates in muscles (effectors), as well as the auto- 

 nomic portion of the system. The nerves are composed of bundles of 

 nerve fibers, and each fiber is the extended process of a neuron. These 

 processes are axons (neurites) and dendrites. All portions of the 

 head, trunk, and limbs are supplied with nerve fibers, which are in 

 communication with the central nervous system. 



The paired cranial nerves all pass from the skull through foramina, 

 and all except one are distributed to the head and neck. They are 

 divided according to function into motor, sensory, or mixed (having 

 both motor and sensory fibers). The olfactory (I), optic (II), and 

 auditory (VIII) are the only cranial nerves which are wholly sensory. 

 The oculomotor (III), trochlear (IV), abducent (VI), spinal acces- 

 sory (XI), and hypoglossal (XII) are motor. The trigemiyial (V), 

 facial (VII), glossopharyngeal (IX), vagus (X) are mixed. The last 

 two named are mainly motor, having only a small number of sensory 

 fibers. Some of the sensory roots of these nerves bear ganglia (aggre- 

 gations of nerve cell bodies). The more important of these is the 

 Gasserian (semilunar) ganglion of the trigeminus. 



