PHYSIOLOGY OF NERVE TISSUE. 77 



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 modified descendants of inde- 

 pendent, oval, pear-shaped cells the neuroblasts which migrate from 

 the medullary tube. As they approach the surface of the cord their 

 axons are directed toward the ventral surface, which eventually they 

 pierce. Emerging from the cord, the axons continue to grow, and 

 become invested with the myelin sheath and neurilemma, thus con- 

 stituting the ventral roots. 



The afferent nerve-fibers, which constitute some of the cranial 

 nerves and all the dorsal roots of the spinal nerves, develop outside 

 of the central nervous system and only subsequently become connected 

 with it. (See Fig. 9.) At the time of the closure of the medullary 

 tube a band or ridge of epithelial tissue develops near the dorsal 

 surface, which, becoming segmented, moves outward and forms the 

 rudimentary spinal ganglia. The cells in this situation develop two 

 axons, one from each end of 'the cell, which pass in opposite direc- 

 tions, one toward the spinal cord, the other toward the periphery. 

 In the adult condition the two axons shift their position, unite, and 

 form a T-shaped process, after which a division into two branches 



