68 HUMAN PHYSIOLOGY. 



nically known as stimuli, and for the liberation of energy capable of exciting 

 the nerve-fiber to activity. 



Relation of Spinal Nerves to the Spinal Cord. The nerves in con- 

 nection with the spinal cord are thirty-one in number, on each side and have 

 two roots of origin, an anterior and a posterior, which arise from the anterior 

 and posterior surfaces of the cord respectively. They are more properly 

 termed ventral and dorsal roots. The dorsal roots present, near their entrance 

 into the cord, an enlargement termed a ganglion. Beyond the spinal canal 

 these two roots unite to form the ordinary spinal nerve. Some of the nerves 

 in connection with the base of the brain also present a ganglionic enlargement, 

 and may, therefore, be regarded physiologically as dorsal nerves, while 

 others may be regarded as ventral nerves. 



Experimentally, it has been determined that the anterior or ventral roots 

 contain all the efferent fibers, the posterior or dorsal roots all the afferent 

 fibers. The proofs in support of this view are as follows: 



Stimulation of the ventral roots produces: 



1. Convulsive movements of muscles. 



2. The formation of a secretion of glands. 



3. Changes in the caliber of blood-vessels. 



4. Inhibition of the rhythmic -activity of certain organs. 

 Divisions of these roots is followed by: 



1. Loss of muscular movement (paralysis of motion). 



2. Cessation of secretion. 



3. Cessation of vascular changes. 

 Stimulation of the dorsal roots causes: 



1. Reflex activities. 



2. Conscious sensations. 



3. Inhibition of the rhythmic activity of certain organs. 

 Division of these roots is followed by: 



1. Loss of reflex activities, and 



2. Loss of sensation in all parts to which they are distributed. 



The ventral roots are, therefore, efferent in function, transmitting nerve 

 impulses from the nerve centers to the periphery. The dorsal roots are 

 afferent in function, transmitting nerve impulses from the general periphery 

 to the nerve centers. 



Development and Nutrition of Nerves. The efferent nerve-fibers, 



which constitute some of the cranial nerves and all the ventral roots of the 

 spinal nerves, have their origin in cells located in the gray matter beneath the 

 aqueduct of Sylvius, beneath the floor of the fourth ventricle and in the 

 anterior horns of the gray matter of the spinal cord. These cells are the modi- 



