CHAP, i.] SENSORY NERVES. 487 



generation and transmission of nervous impulses, may be taken 

 as indicating not so much that the afferent and efferent fibres 

 are themselves acted upon in a different way by heat or by the 

 constant current as that the molecular disturbances generated in 

 both cases have different effects according as they impinge upon a 

 central or a peripheral mechanism. We can readily imagine that 

 molecular disturbances which would be impotent to stir the 

 sluggish muscular substance to a contraction, and thus so to speak 

 be lost upon the muscle, might produce a very great effect on the 

 more sensitive and mobile material of the central nervous system. 

 We may for the present therefore conclude that there is no distinct 

 proof of an absolute difference between afferent and efferent fibres, 

 but we must at the same time be cautious not to consider the 

 grosser phenomena, presented by a muscle-nerve preparation, as 

 a satisfactory test of all the changes which may take place in a 

 nerve-fibre. The necessity of this caution will be almost im- 

 mediately illustrated from another point of view. 



The apparent identity in function between afferent and efferent 

 fibres, taken into consideration with the facts just mentioned con- 

 cerning the regeneration of nerves, suggests the inquiry whether by 

 a change of the peripheral or central organs a motor nerve can be 

 converted into a sensory nerve, or vice versa. Experiments made 

 with a view of obtaining a functional union between purely motor 

 and sensory nerves have, in the hands of most observers, failed. 

 And though an apparent union between the central portion of a, 

 divided lingual (sensory) nerve and the peripheral portion of a 

 divided hypoglossal (motor) nerve has been accomplished, with the 

 result that stimulation of the lingual trunk produced movements in 

 the tongue, the case breaks down upon examination. In the first 

 place though the nerves appeared to have united, there was no 

 actual union between the lingual and hypoglossal fibres, but 

 degeneration of the latter, and a growth downward of the former ; 

 in the second place the movements of the tongue when the lingual 

 trunk was stimulated appear to have been brought about by stimu- 

 lation not of the sensory, true lingual, fibres, but of motor (chorda 

 tympani) fibres running in the lingual trunk. 



We have already seen (p. 106) that a sensory nerve in its 

 simplest form may be regarded as a strand of eminently irritable 

 protoplasm, forming a link between a superficial cell which alone is 

 subject to extrinsic stimuli, and a central (reflex or automatic) cell 

 which receives stimuli, chiefly in the form of nervous impulses pro- 

 ceeding from the former along the connecting strand. In the 

 earliest stages of the developement of a sensory nervous system, the 

 superficial sensory cell is susceptible of stimuli of all kinds, pro- 

 vided they are sufficiently strong; and probably all the impulses 

 which it transmits to the central cell resemble each other very 

 closely, differing only in degree. It is obvious however that the 

 economy would gain by a further division of labour, by a differen-* 



