264 niYSlOLOGY OF MUSCLES AND NERVES. 



the presence or absence of this medullary sheath does 

 not correspond with differences in the 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 differences 

 to light. The bearing of nerves to irritants does not 

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

 all these points we need simply 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 we 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 system. An 

 irritant which affects a motor nerve, to become appa- 

 rent, must propagate itself toward the periphery, till it 

 reaches the muscle situated there ; an irritant, on the 

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

 pagated toward the centre before it sets free any action. 

 Nerves of the former kind are therefore called centrifu- 

 gal, those of the latter centripetal. We have, however, 

 already found that this does not depend on a difference 

 in the nerve itself, but that each nerve-fibre, when it is 

 affected at any point in its course, transmits the ex- 

 citement in both directions ; and we therefore presumed 

 that the fact that action takes place only at one end 



