GENERAL PHYSIOLOGY OF NERVE-TISSUE 99 



divided between the ganglion and the cord, degeneration takes place only 

 in the central portion of the root. From these facts it is evident that the 

 trophic centers for the ventral and dorsal roots lie in the spinal cord and spinal 

 nerve ganglia, respectively, or, in other words, in the cells of which they are 

 an integral part. The structural changes which nerves undergo after separa- 

 tion from their centers are degenerative in character, and the process is 

 usually spoken of, after its discoverer, as the Wallerian degeneration. 



When the nerve-cells 'from which the nerve-fibers arise, whether efferent 

 or afferent, undergo degeneration from any cause whatever, the nerve-fiber 

 becomes involved in the degenerative process and when it is completed the 

 structures to which they are distributed, especially the muscles, undergo an 

 atrophic or fatty degeneration, with a change or loss of their irritability. 

 This is, apparently, not to be attributed merely to inactivity, but rather to a 

 loss of nerve influences, inasmuch as inactivity merely leads to atrophy and 

 not to degeneration. 



Reunion and Regeneration. When a nerve-trunk is divided there is 

 a loss of function of the parts to which it is distributed, and usually involves 

 both motion and sensation. This, however, is not necessarily permanent, 

 for after a variable period of time it not infrequently happens that the func- 

 tions are restored because of a reunion of the separated ends and a regenera- 

 tion of the peripheral portion. A histologic study of the nerve-fibers after 

 separation from the nerve-cells shows that coincidently with the degenerative 

 process there occurs a regenerative process, consisting in a multiplication of 

 the nuclei lying just beneath the neurilemma and an accumulation around 

 them of a granular protoplasm which in due time completely fill the neuri- 

 lemma. At this stage the fiber is known as a band-fiber. If now the physical 

 conditions are such as to permit of a reunion of the nerve, this takes place, 

 and under the nutritive influence of the cell the axis-cylinder grows into the 

 band-fiber and the protoplasm becomes transformed into myelin as in the 

 original fiber. The axis-cylinder continues to grow and extend itself 

 forward until it reaches its ultimate termination. 



CLASSIFICATION OF NERVES 



The Efferent Nerves. The efferent nerves may be classified, in accord- 

 ance with their distribution and the characteristic forms of activity to which 

 they give rise, into several groups, as follows : 



1. Skeletal-muscle or motor nerves, those which convey nerve energy or nerve 



impulses directly to skeletal-muscles and excite them to activity. 



2. Gland or secretor nerves, those which convey nerve impulses to glands by 



way of ganglia and influence in one direction or another the degree of 

 their activity. Those which cause the formation and discharge of the 

 secretion peculiar to the gland, are known as secreto-motor, while those 

 which decrease or inhibit the secretion are known as secreto-inhibitor 

 nerves. 



3. Vascular or vaso-motor nerves, those which convey nerve impulses to the 



muscle-fibers of the blood-vessels and change in one direction or the 

 other the degree of their natural contraction. Those which increase the 

 contraction are known as vaso-constrictors or vaso-augmentors; those 

 which decrease the contraction are known as vaso-dilatators or vaso- 



