194 THE HUMAN BODY. 



trunks and studied by aid of a galvanometer: in all respects 

 these currents correspond to those of muscle, except that they 

 are feebler. A perfectly fresh uninjured nerve at rest is 

 isolectric, and so is a completely dead nerve. A dying por- 

 tion of a nerve is negative to a more normal portion, and in 

 consequence, if electrodes be placed, one on the centre and 

 the other on the cut end of a freshly-removed portion of nerve, 

 a current will be found passing through the connecting wire 

 from the central portion of the piece of nerve towards the 

 peripheral. A region of nerve in activity, that is transmit- 

 ting a nervous impulse, is electro-negative to a region at rest, 

 other things being equal; accordingly, an act ion -cur rent or 

 negative variation can be demonstrated on nerves as on mus- 

 cles; the electrical change starting from the point of stimu- 

 lation and travelling along the trunk, to be found at a distant 

 part at a time when it has gone from the place of its first ap- 

 pearance. The account of the, similar phenomena in muscle 

 (Chap. X) may be consulted for a fuller statement. 



The Stimuli of Nerve-Fibres. Nerve-fibres, like mus- 

 cular fibres, possess no automatic! ty; acted upon by certain 

 external forces or stinmli\ihey propagate a change, which is 

 known as ,a nervous impure, from the point acted jupon to 

 their ends; but they do not generate, nervous impulses when 

 left entirely to themselves. Formally, in the living Body 

 the stimulus acts on a nerve-fibre at one of its ends, being 

 either some change in a nerve-centre with which the fibre is 

 connected (efferent nerves) or some change in an organ at- 

 tached to the outer end of the nerve (afferent fibres). Ex- 

 periment shows, however, that a nerve can be stimulated in 

 any part ot its course; that it is irritable all through its ex- 

 tent. If, for example, the sciatic of a frog be exposed in the 

 thigh and divided, it will be found that electric shocks ap- 

 plied at the point of division to the outer half of the nerve 

 stimulate the motor fibres in it, and cause the muscular fibres 

 of the leg to contract: and similarly such shocks applied to 

 the cut end of the central half irritate the afferent fibres in 

 it, as shown by the signs of feeling exhibited by the animal. 

 In ourselves, too, we often have the opportunity of observing 

 that the sensory fibres can be stimulated in their course at 

 some distance from their ends. A blow at the back of the 

 elbow, at the point commonly known as the "funny bone "or 

 the " crazy bone," compresses the ulnar nerve there against the 



