80 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



to the peripheral tissues, and the afferent fibers, which carry their 

 impulses inwardly, that is, from the peripheral tissues to the 

 nerve centers. Under normal conditions the afferent fibers are 

 stimulated only at their endings in the peripheral tissues, in the 

 skin, the mucous membranes, the sense organs, etc., while the 

 efferent fibers are stimulated only at their central origin, that 

 is, through the nerve cells from which they spring. The difference 

 in the direction of conduction depends, therefore, on the anatomical 

 fact that the efferent fibers have a stimulating mechanism at their 

 central ends only, while the afferent fibers are adapted only for 

 stimulation at their peripheral ends. 



Classification of Nerve Fibers. In addition to this funda- 

 mental separation we may subdivide peripheral nerve fibers into 

 smaller groups, making use of either anatomical or physiological 

 differences upon which to base a classification. For the purpose 

 here in view a classification that is physiological as far as possible 

 seems preferable. In the first place, experimental physiology has 

 shown that the effect of the impulse conveyed by nerve fibers may 

 be either exciting or inhibiting. That is, the tissue or the cell 

 to which the impulse is carried may be thereby stimulated to ac- 

 tivity, in which case the effect is excitatory, or, on the contrary, 

 it may, if already in activity, be reduced to a condition of rest or 

 lessened activity; the effect in this case is inhibitory. Many 

 physiologists believe that one and the same nerve fiber may carry 

 excitatory or inhibitory impulses, but in some cases at least we 

 have positive proof that these functions are discharged by separate 

 fibers. We may subdivide both the afferent and the efferent sys- 

 tems into excitatory and inhibitory fibers. Each of these sub- 

 groups again falls into smaller divisions according to the kind of 

 activity it excites or inhibits. In the efferent system, for instance, 

 the excitatory fibers may cause contraction or motion if they ter- 

 minate in muscular tissue, or secretion if they terminate in glandu- 

 lar tissue. For convenience of description each of the groups in 

 turn may be further classified according to the kind of muscle in 

 which it ends or the kind of glandular tissue. In the motor group 

 we speak of vasomotor fibers in reference to those that end in the 

 plain muscle of the walls of the blood-vessels; visceromotor fibers, 

 those ending in the muscular tissue of the abdominal and thoracic 

 viscera; pilomotor fibers, those ending in the muscles attached to 

 the hair follicles. The classification that is suggested in tabular 

 form below depends, therefore, on three principles: first, the direc- 

 tion in which the impulse travels normally; second, whether this 

 impulse excites or inhibits; third, the kind of action excited or 

 inhibited, which in turn depends upon the kind of tissue in which 

 the fibers end. 



