XI AFFERENT AND EFFERENT NERVES 477 



they do not give rise to movements. The pneuinogastric 

 when it stops the beat of the heart cannot be called a 

 motor nerve, and yet is then acting as an efferent nerve. 

 Similarly the nerves which cause the cells of a gland 

 such as the salivary glands, sweat glands, &c., to 

 commence secreting are not motor nerves but are 

 strictly efferent as regards the direction in which they 

 convey their impulses. It will, of course, be understood, 

 as pointed out above, that the use of these words does 

 not imply that when a nerve is irritated in the middle of 

 its length, the impulses set up by that irritation travel 

 only away from the central organ if the nerve be efferent, 

 and towards it, if it be afierent. On the contrary, we 

 have evidence that in both cases the impulses travel both 

 ways. All that is meant is this, that the afferent nerve 

 from the disposition of its two ends, in tlie skin, or other 

 peripheral organs on the one hand, and in the central 

 organ on the other, is of use only when impulses are 

 travelling along it towards the central organ, and similarly 

 the efferent nerve is of use only when impulses are 

 travelling along it, away from the central organ. 



There is no difference in structure, in chemical or in 

 physical character, between afferent and efferent nerves. 

 The impulse which travels along them requires a certain 

 time for its propagation, and is vastly slower than many 

 other movements — even slower than sound. (See p. 481.) 

 We know but little of the nature of a iiervous impulse. 

 We know that it may be started in a nerve by various 

 artificial means such as by pinching or knocking the 

 nerve, or by suddenly warming or cooling it, and, most 

 readily, by stimulating the nerve electi-ically. And we 

 suppose that by any of these means there is set up in that 

 bit of nerve to which any one of the above "stimuli" 

 is applied, a disturbance, which is then propagated in suc- 

 cession from one particle (or molecule) of the axis cylinder 

 to the next, so that it ultimately reaches a point in the 

 nerve remote from that in which it was started. In this 



