162 PHYSIOLOGY OF NERVE foil. XV. 



1. 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. Inhibitor!/ 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 

 involuntary 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 stimulates 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 

 up the nerves to the spinal cord, and then to the brain. The mental 

 process set up in the brain is called a sensation ; the sensation, how- 

 ever, is referred to the end of the nerve where the impulse started, 

 and the sensation of pain does not appear to occur in the brain, but 

 in the finger. This is an instance of a sensory nerve ; and the terms 

 afferent and sensory may often be used synonymously. The nerves 

 of sensation may be grouped as follows : 



a. The nerves of special sense ; that is, of sight, hearing, taste, 



smell, and touch. 



b. The nerves of general sensibility ; that is, of a vague kind of 



sensation not referable to any of the special senses just 

 enumerated ; as an instance, we may take the vague feelings 

 of comfort or discomfort in the interior of the body. 



c. Nerves of pain. It is a moot point whether these are anatomi- 



cally distinct from the others, for any excessive stimulation 

 of a sensory nerve whether of the special or general kind 

 will cause pain. 



The words "sensory" and "afferent," however, are not quite 

 synonymous. Just as we may have efferent impulses leaving the 



* The question has been much debated whether voluntary muscle is provided 

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

 supplying the muscles of the claws of lobsters and similar crustaceans. 



