GENERAL PHYSIOLOGY OF NERVE-TISSUE. 97 



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 conincidently 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 neuril- 

 emma. 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 may be classified, in accordance with the character- 

 istic 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 cause the formation and discharge of the secretion 

 peculiar to the gland. 



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- 

 inhibitors. The nerves which pass to that specialized part of the 

 vascular apparatus, the heart, transmit nerve impulses which on the 

 one hand accelerate its rate or augment its force, and on the other hand 

 inhibit or retard its rate and diminish its force. For this reason they 

 are termed cardiac nerves, one set of which is known as cardio-accelera- 

 tor and cardio-augmentor, the other as cardio-inhibitor nerves. 



4. Visceral or viscero-motor nerves, those which transmit nerve impulses to 



the muscle walls of the viscera and change in one direction or another 

 the degree of their contraction. Those which increase or augment the 

 contraction are known as viscero-augmentor, while those which decrease 

 or inhibit the contraction, are known as viscero-inhibitor nerves. 



5. Hair bulb or pilo-motor nerves, those which transmit nerve impulses to the 



muscle-fibers which cause an erection of the hairs. 

 Of the foregoing nerves the skeletal-muscle or motor nerves alone pass 

 directly to the muscle. The gland, the vascular and the visceral nerves, all 

 terminate at a variable distance from the peripheral organ around a local 

 sympathetic ganglion, which in turn is connected with the peripheral organ. 

 The former are termed pre-ganglionic. The latter post-ganglionic fibers. 

 (See Fig. 13.) 



