CHAP. XI.] ENDOWMENTS OF NERVE-FIBRES. 231 



is induced in the particles of the nerve by the action of the 

 stimulus, which is capable of exciting an analogous change in other 

 particles, whether muscular or nervous ; whence results the pecu- 

 liar effect of the nerve's influence. 



Thus, if a nerve be distributed among muscular fibres in the 

 manner described at a former page, it will be capable of exciting 

 muscular contraction, and is properly a muscular or motor nerve; 

 and it is so connected, at its origin, with the nervous centre, that a 

 change there, whether induced by mental or by physical influence, 

 may be readily communicated to it. When a nerve is distributed 

 upon an expanded surface, as upon the skin or mucous membrane, 

 or is otherwise favourably disposed for the reception of any physical 

 stimulus from without, it will propagate the change induced by such 

 stimulus to the nervous centre ; and this change in the centre may 

 produce an impression upon the mind, giving rise to a sensation; or 

 it may affect a motor nerve connected with the excited one or 

 arising from the nervous centre adjacent to it, and thus may in- 

 directly excite muscular movement. When a nerve is capable of 

 acting in the former way, it is called a nerve of sensation, or sensi- 

 tive ; when in the latter, it is an excitor of a motor nerve. 



It is not necessary to suppose any intrinsic difference of structure 

 in the nerves which are thus capable of producing effects so mani- 

 festly different. The action of a nerve depends upon the nature of 

 its central and peripheral connexions. It cannot be motor, unless it 

 be intimately connected with muscles ; nor sentient, if its relation 

 to the nervous centre be not such as will enable it to affect the sen- 

 sorium commune. The terms efferent and afferent are only so far 

 applicable to certain nerves, as they refer to the direction in which 

 such nerves appear to propagate the change produced in them, or 

 to the position at which the effects of the stimulation become 

 manifest, that direction having reference to the point at which the 

 stimulus is destined to act. In a motor nerve, the ordinary stimu- 

 lus acts from the nervous centre ; but a mechanical or electrical 

 stimulus affecting such a nerve at any part of its course will 

 cause contraction of the muscles supplied by it below the point of 

 irritation. In the sensitive or excitor nerves, the usual situation 

 from which the stimulus acts is at their peripheral distribution; 

 but at whatever point a sentient nerve be stimulated, a sensation 

 will be produced, which will be referred to those parts, and to 

 those only, to which the fibres irritated are distributed ; and wher- 

 ever the stimulus be applied to an excitor nerve, it will, with 

 equal effect, rouse its corresponding motor nerve to action. There 



