446 STUDIES IN ADVANCED PHYSIOLOGY. 



First. Sensory fibers which, when stimulated, produce 

 sensations of which we are conscious. The best illustra- 

 tions of these sensory fibers are the nerves of the special 

 senses. 



Second. Reflex sensory fibers which when excited carry 

 sensations to the brain, which, however, do not usually 

 reach consciousness, but in the lower brain centers give rise 

 to motor impulses without the intervention of the will. The 

 best illustration of these possibly is the narrowing of the 

 pupil when subjected to a strong light. Some of these reflex 

 sensory fibers may occasionally come within the reach of 

 consciousness. Many of them, however, are entirely outside 

 of it. Such are the reflex sensory fibers, for instance, whicli 

 carry the sensations from the stomach, when food reaches 

 it, to the brain, to be there translated into impulses leading 

 to the active secretion of the glands of the stomach or to 

 the contraction of its walls. 



Third. Inhibitory sensory fibers; fibers whicli when 

 stimulated carry sensations to nervous centers which seem 

 to inhibit their action. So, for instance, the biting of the 

 lips may succeed in preventing a spasm of sneezing. 



The three classes so far mentioned are included iinder 

 the usual term of sensory fibers. Of the efferent nerve 

 fibers physiologists usually make the following classes: 



Fourth.. Motor fibers proper, those which innervate the 

 muscles and produce their contractions. Under this head 

 are included not only the nerves which run to the skeletal 

 muscles, but also the nerves which run to some of the in- 

 voluntary muscles, especially the vaso-motor fibers which 

 run to the muscular coats of the blood-vessels. 



Fifth. Secretory fibers. These are fibers distributed to 

 the cells which constitute the various glands of the body and 

 which govern their secretion. 



Sixth. Inhibitory nerves whicli inhibit muscular action, 

 the clearest example of which is the inhibitory nerve of the 



