THE PHENOMENON OF CONDUCTION. 75 



nerve fibers found in a nerve may be entirely independent in their 

 physiological activity. In the vagus nerve, for instance, we have 

 nerve fibers running side by side, some of which supply the heart, 

 some the muscles of the larynx, some the muscles of the stomach 

 or intestines, some the glands of the stomach or pancreas, and so 

 on. Nerves are, therefore, anatomical units simply, containing 

 groups of fibers which have very different activities and which may 

 function entirely independently of one another. 



Afferent and Efferent Nerve Fibers. The older physiologists 

 believed that one and the same nerve or nerve fiber might conduct 

 sensory impulses toward the central nervous system or motor im- 

 pulses from the central nervous system to the periphery. Bell and 

 Magendie succeeded in establishing the great truth that a nerve 

 fiber cannot be both motor and sensory. Since their time it has 

 been recognized that we must divide the nerve fibers connected 

 with the central nervous system into two great groups : the efferent 

 fibers, which carry impulses outwardly from the nervous system 

 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- 



