THE NERVOUS SYSTEM 435 



muscles, giving these parts some degree of sensation. In all 

 sensory nerves the impulse necessarily travels from the termi- 

 nus to the central organ, and these nerves are consequently 

 designated as centripetal or afferent. The other type of nerve, 

 the motor, supplies the muscles and furnishes them with the 

 impulse to contract. In these nerves the current runs from 

 the center to the terminus, and they are thus centrifugal, or 

 efferent. The nerves that regulate the action of other organs, 



Dorsal 



VENTRAL 



Ventral 



FIG. 122. Diagram of a typical spinal nerve. 



such as the secretion of glands, are a subdivision of the motor 

 class. 



Nerves issue from the central organ, whether brain or cord, 

 in bundles called roots, each of which contains mainly one 

 type of nerve fiber. The roots are hence called either motor 

 or sensory, but since a given part must usually be supplied with 

 both motion and sensation, two roots, one of each sort, become 

 associated together, and blend their fibers within a single ex- 

 ternal sheath, thus forming a mixed nerve. This is true of all 

 of the spinal nerves and of some of the cranial pairs, but 

 others of this latter class arise from single roots and retain 

 their simple character. Each metamere possesses typically a 

 pair of sensory and a pair of motor roots, the sensory situated 

 dorsally, the motor ventrally, thus forming two longitudinal 



