458 HANDBOOK OF PHYSIOLOGY. 



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. 



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. Nerve- 

 fibres then are either (a) Centripetal or afferent, (b) Centrifugal or 

 efferent, or (c) Intercentral. 



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

 kind of sensation; (b) special sensation; or (c) reflex action of some kind; 

 or (d) inhibition, or restraint of action. 



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

 (motor nerves) ; (b) it may influence nutrition (trophic nerves) ; or (c) 

 may influence secretion (secretory nerves); or (d) inhibit, augment, or 

 stop any other efferent action. 



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 

 fibre, is simply and uninterruptedly transmitted alongjt, without being 

 imparted or diffused to any of the fibres lying near it. In other words, 

 all nerve-fibres are mere conductors of impressions. Their adaptation to 

 this purpose is, perhaps, due to the contents of each fibre being com- 

 pletely isolated from those of adjacent fibres by the membrane or sheath 

 in which each is inclosed, and which acts, it may be supposed, just as 

 silk, or other non-conductors of electricity do, which, when covering a 

 wire, prevent the electric condition of the wire from being conducted 

 into the surrounding medium. 



Velocity of Nerve-force. The change which a stimulus sets up in a 

 nerve, of the exact nature of which we are unacquainted, appears to 

 travel along a nerve-fibre in both directions in the form of a wave with 

 considerable velocity. Helmholtz and Baxt have estimated the average 

 rate of conduction in human motor nerves at 111 feet (nearly 29 metres) 

 per second; this result agreeing very closely with that previously ob- 

 tained. It is probably rather under than over the average velocity. 

 Rutherford's observations agree with those of Von Wittich, that the 

 rate of transmission in sensory nerves is about 140 feet per second. 

 Various conditions modify the rate of transmission, of which temperature 

 is one of the most important, a very low or a very high temperature 

 diminishing it; fatigue of the nerve acting in the same direction, but 

 increase of the stimulus up to a certain point increasing it, as does also 

 the Icathelectrotonic condition of the nerve. 



Conduction in Sensory .A^rves. Centripetal nerves appear able to 

 convey impressions only from the parts in which they are distributed, 

 towards the nerve-centre from which they arise, or to which they tend. 

 Thus, when a sensitive nerve is divided, and irritation is applied to the 



