80 HAND-BOOK OF PHYSIOLOGY. 



ment which, for medium-sized and large nerves, is the rule rather than 

 the exception. 



Conduction in Nerves. A nerve when removed from the body will 

 be found to conduct electrical impressions in either direction equally well, 

 and microscopic examination fails to discover the slightest essential differ- 

 ence between motor and sensory nerve-fibres. The question, therefore, 

 naturally arises, whether the conduction of a stimulus in the living body, 

 in one direction only, is not rather apparent than real, the difference in 

 the result being due to the different connections of the two kinds of 

 nerve-fibres respectively at their extremities. In other words, when the 

 stimulation of a nerve-fibre causes pain, the result is due to its central 

 extremity being in connection with structures which alone can give rise 

 to the sensation, while its peripheral extremity, although the stimulus is 

 equally conducted to it, has no connection with a structure which can 

 respond to the irritation in any manner sensible to the observer. So, 

 when motion is the result of a like irritation, it is because the peripheral 

 extremity of the nerve-fibre is in connection with muscles which will re- 

 spond by contracting, while it f central extremity, although equally 

 stimulated, has no means of showing the fact by any evident result. 



That this is the true explanation is made highly probable, not merely 

 by the absense of any structural differences in the two kinds of nerve -fibre, 

 but also by the fact, proved by direct experiment, that if a centripetal 

 nerve (gustatory) be divided, and its central portion be made to unite with 

 the distal portion of a divided motor nerve (hypoglossal) the effect of irri- 

 tating the former after the parts have healed, is to excite contraction in 

 the muscles supplied by the latter. (Philippeaux and Vulpian. ) 



Classification of Nerve-Fibres. 1. Centripetal, afferent, or, 2. 

 Centrifugal, afferent, or motor. 3. Intercentral. 



Centripetal or afferent, and centrifugal or efferent, are frequently em- 

 ployed in connection with nerve-fibres in lieu of the corresponding terms 

 sensory and motor, because the result of stimulating a nerve of the former 

 kind is not always the production of pain or other form of sensation, nor 

 is motion the invariable result of stimulating the latter. 



Conduction in centripetal nerves may cause (a) pain, or some other kind 

 of sensation; or (b) reflex action; or (c) inhibition, or restraint of action. 



Conduction in centrifugal nerves, may cause (a) contraction of muscle 

 (p. 25, Vol. II.), (motor nerves); (b) it may influence nutrition (trophic 

 nerves); or (c) may influence secretion (secretory nerves). 



The term intercentral is applied to those nerve-fibres which connect 

 more or less distinct nerve-centres, and may, therefore, be said to have no 

 peripheral distribution, in the ordinary sense of the term. 



It is a law of action in all nerve-fibres, and corresponds with the con- 

 tinuity and simplicity of their course, that an impression made on any 



