GENERAL PHYSIOLOGY OF NERVE-TISSUE. 113 



posterior root be divided on the peripheral side of the ganglion,*de- 

 generation takes place only in the peripheral portion of the nerve. 

 (See Fig. 48.) If the root be 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 

 separation from their centers are degenerative in character, and the 



FIG. 48. DEGENERATION or SPINAL NERVES AND NERVE-ROOTS AFTER SECTION. 

 A. Section of nerve-trunk beyond the ganglion. B. Section of anterior root. 

 C. Section of posterior root. D. Excision of ganglion, a. Anterior root. p. 

 Posterior root. g. Ganglion. (Dalton.} 



process is usually spoken of, after its discoverer, as the Wallerian 

 degeneration. 



When the degeneration of the efferent nerves is completed, the 

 structures to which they are distributed, especially the muscles, un- 

 dergo an atrophic or fatty degeneration, with a change or loss of their 

 irritability. This is, apparently, not to be attributed merely to in- 

 activity, but rather to a loss of nerve influences, inasmuch as inactivity 

 merely leads to atrophy and not to degeneration. 



CLASSIFICATION OF NERVES. 



The efferent nerves may be classified, in accordance with the 

 characteristic form of activity to which they give rise, into several 

 groups, as follows : \. 



1. Muscle or motor nerves, those which convey nerve energy or nerve 



impulses to muscles and excite them to activity. 



2. Gland or secretory nerves, those which convey nerve impulses to 



glands, and cause the formation and discharge of the secretion 

 peculiar to the gland. 



3. Vascular or vase-motor nerves, those which convey nerve impulses 



to blood-vessels", and cause, either by stimulation or inhibition of 



