82 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 

 lo 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. We may 

 subdivide both the afferent and the efferent systems into excita- 

 tory and inhibitory fibers. Each of these subgroups 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 terminate in muscular 

 tissue, or secretion if they terminate in glandular tissue. For con- 

 venience 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 vaso- 

 motor 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 folHcles. 

 The classification that is suggested in tabular form below depends, 

 therefore, on three principles: first, the direction in which the im- 

 pulse 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. 



