2(14 PHYSIOLOGY OF MUSCLES AND NERVES. 



the presence or ab-ence of this medullary sheath does 

 not correspond with differences in tin- physiological 

 actions of nerves. Nor are the small differences in 

 diameter of the separate nerve-fibres of greater import- 

 ance. Nor do experimental tests bring any differen. < -s 

 to light. The bearing of nerves to irritants does not 

 vary: the electromotive effects are the same in all. Jn 

 all these points we need simplv refer to the previous 

 chapter, for the explanations there given are equally 

 true of all kinds of nerve-fibres. 



If, therefore, all kinds of nerve-fibres are alike, we 

 can only explain the difference in their action as due 

 to their connection with terminal organs of various 

 form. We have already made use of this principle 

 in explanation of the difference between motor and 

 secretory nerves, and \ve must now endeavour to ex- 

 tend it to all other nerves. 



3. While the motor and secretory nerves have their 

 terminal organs in the periphery of the body, the sensi- 

 tive or sensory nerves act on apparatus which are situ- 

 ated in the central organs of the nervous svstem. An 

 irritant which affects a motor nerve, to become appa- 

 rent, must propagate itself toward the peripherv, till it 

 reaches the muscle situated there; an irritant, on tin- 

 other hand, which affects a sensory nerve, must be pro- 

 pagated toward the centre before it seta tree anvaction. 

 Nerves of tin' former kind are therefore called n'ntrif'n- 



ifill, tho>e of the latter <> III I' I IK till. We have, l|o\V(.\er. 



already found that this does not depend n a difference 

 in the nerve itself, but that each ner\ e-libre, when it is 

 alfeded at any point in its course, transmits the e.\- 

 ci ten lent in both directions ; and we therefore presumed 

 that the faet that action takes place only at one end 



