GENERAL PHYSIOLOGY OF NERVE-TISSUE. 



95 



into several long narrow branches which wind themselves in a spiral manner 

 around the contained muscle-fiber and terminate in small oval-shaped discs. 

 Similar endings have been observed in the tendons of muscles. 



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 

 ventral horns of the gray matter of the spinal cord. These cells are the 

 modified descendants of independent, oval, pear-shaped cells the neuro- 

 blasts 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 

 constituting the ventral roots. (Fig. 48.) 



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 

 nerve system and only subsequently 

 become connected with it. (See Fig. 

 48.) At the time of the closure of 

 the medullary tube a band or ridge of 

 epithelial tissue develops near the dor-, 

 sal surface, which, becoming seg- 

 mented, moves outward and forms 

 the rudimentary spinal ganglia. The 

 cells in this situation develop ^. 



axons, one from each end of the cell, RoOTS ._ (Edinger, after His.) 

 which pass in opposite directions, 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 again takes place. In 

 the ganglia of all the sensori-cranial and sensori-spinal nerves the cells 

 have this histologic peculiarity. 



The efferent fibers are therefore to be regarded as outgrowths from the 

 nerve-cells in the ventral horns of the gray matter, and serve to bring the 

 cells into anatomic and physiologic relationship directly with the skeletal 

 muscles and indirectly, through the intermediation of ganglia (see sym- 

 pathetic nerve system), with visceral muscles, blood-vessels, and glands. 



The afferent fibers are to be regarded as outgrowths from the cells of the 

 dorsal nerve ganglia, and serve to bring the skin, mucous membrane, and 

 certain visceral structures into relation with specialized centers in the central 

 nerve system. 



Nerve Degeneration. If any one of the cranial or spinal nerves be di- 

 vided in any portion of its course, the part in connection with the periphery in 

 a short time exhibits certain structural changes, to which the term degenera- 

 tion is applied. The portion in connection with the brain or cord retains its 



