164 PHYSIOLOGY OF NERVE. [CH. XV. 



one or more of the spinal nerves, and so reaches the muscles of 

 the hand which are thrown into contraction. Such nerves are 

 called motor, but all efferent nerves are not motor ; some cause 

 secretion instead of movement, and others may cause a stoppage 

 of movement, etc. A list of the classes of efferent nerves is as 

 follows : 



a. Motor. 



b. Accelerator. 



c. Inhibitory. 



d. Secretory. 



e. Electrical. 



f. Trophic. 



a. Motor nerves. Some of these go to voluntary muscles ; others 

 to involuntary muscles, such as the vaso-motor nerves which 

 supply the muscular tissue in the walls of arteries. 



b. Accelerator nerves are those which produce an increase in 

 the rate of rhythmical action. An instance of these is seen 

 in the sympathetic nerves that supply the heart. 



c. Inhibitory nerves are those which cause a slowing in the rate 

 of rhythmical action, or it may be its complete cessation. 

 Inhibitory nerves are found supplying many kinds of in- 

 voluntary muscle ; a very typical instance is found in the 

 inhibitory fibres of the heart which are contained within the 

 trunk of the vagus nerve.* 



d. Secretory nerves are found supplying many secreting glands, 

 such as the salivary glands, pancreas, gastric glands, and 

 sweat glands. The impulse which travels down a secretory 

 nerve causes a formation of the secretion in the gland it 

 supplies. 



e. Electrical nerves are found in the few fishes which possess 

 electrical organs. The impulse which travels down these 

 nerves causes the electrical organ to be thrown into 

 activity. 



/. Trophic nerves are those which control the nutrition of the 

 part they supply. 



2. Afferent or centripetal nerves are those which conduct 

 impulses in the reverse direction, namely from all parts of the 

 body to the central nervous system. When one feels pain in the 

 finger, the nerves of the finger are stimulated, an impulse travels 



* The question has been much debated whether voluntary muscle is pro- 

 vided with inhibitory nerves ; they do, however, appear to be present in 

 certain nerves supplying the muscles of the claws of lobsters and similar 

 crustaceans. 



