194 THE HUMAN BODY. 



along the fibre. If the muscle were cut away from the end 

 of the nerve we could still detect that a nervous impulse 

 had traveled from the point of stimulation to that where 

 the fibres were divided, by tracking the negative variation. 

 Now if we examine the part of the nerve on the central 

 side of the stimulated point we find that a negative varia- 

 tion (and hence we conclude a nervous impulse) travels that 

 way too; it starts at the same moment as the efferent nega- 

 tive variation and travels in the same way, but the impulse 

 of which it is a sign produces no more effect than the efferent 

 impulse would after the muscle had been cut away; for it 

 does not reach any muscular fibre, or sensory or reflex centre, 

 which it can arouse to activity. (4) The following experi- 

 ment has been put forward as even more conclusive. If 

 the tail of a rat be amputated close to the body of the ani- 

 mal and then be transplanted to the back of the rat, and 

 its tip be there stitched beneath the skin, it will, in many 

 cases, continue to live in this new position, although it is 

 "upside down." In such circumstances, it has been found 

 that after a time the transplanted tail is sensitive when 

 pinched. It has been argued that this phenomenon proves 

 one of two things: either that originally afferent (or sen- 

 sory) nerve-fibres of the tail which normally carried im- 

 pulses from its tip now convey them from its stump to the 

 tip imbedded in and grown into the wound; or that efferent 

 (or motor) nerve-fibres which formerly conveyed motor or 

 efferent impulses down the tail now carry them up, and 

 transmit them to sensory fibres in the skin or subcutaneous 

 tissue with which they have become anatomically connected. 

 If this were a correct statement of the facts it would be of 

 great force. But we know now that the axis cylinder of 

 every nerve-fibre is but the prolonged branch of a nerve- 

 cell; and that such branches when divided grow rapidly 

 again towards the periphery along the connective-tissue 

 paths marked out by the sheaths of the peripheral portions 

 of divided nerves. It is more probable that the afferent 

 cutaneous nerve-fibres, cut in making the skin incision and 

 in the implantation of the tail-tip, send outgrowths from 

 their cut ends into the inverted tail and become its sensory 



