486 AFFERENT AND EFFERENT NERVE FIBRES. [BOOK in. 



gr. when the stimulus Is applied to the toe, will give the time 

 required for the sensory impulse to pass along a piece of sensory 

 nerve as long as the difference of length between the above two 

 nerves ; from which the velocity can be calculated. Observations 

 carried on in this way have led to most discordant results, varying 

 from 26 metres to 94 metres, or even more, per sec. The difference 

 here is far too great to allow any value to be attached to an 

 average. When it is remembered how complex are all the central 

 nervous operations in these instances, as compared with the changes 

 going on in a muscle during the latent period of its contraction, 

 and how these central operations might vary according as one or 

 other spot of skin was stimulated, quite independently of the 

 length of nerve between the centre and the spot stimulated, these 

 discrepancies will not . be wondered at ; and it may fairly be 

 concluded that the velocity of a sensory impulse does not materially 

 differ from that of a motor impulse. 



There are, however, certain phenomena which might at first 

 sight be interpreted as indicating that afferent and efferent nerve- 

 fibres behave differently towards stimuli. We have already (p. 93) 

 stated that according to most observers, when an ordinary motor 

 nerve, such as a nerve supplying a muscle, is heated, no indications 

 of the generation of nervous impulses, no contractions of the 

 muscle for instance, are observed. The heat does not act as a 

 stimulus; it may increase the irritability of the nerve for the 

 time being, but apparently cannot originate the explosive dis- 

 charge which we call an impulse. We have also seen that during 

 the passage of a constant current along the nerve of a muscle- 

 nerve preparation no contractions are visible, no impulses, save 

 in certain particular cases, are generated, so long as the current 

 is not suddenly varied in strength. But it has been found 

 that when afferent nerve-fibres, such as those in the central 

 stump of the divided sciatic or in the central stump of the 

 vagus, are heated to 45 or 50 events occur, clearly proving that 

 impulses are generated in the afferent fibres by the elevation of 

 temperature. In the case of the sciatic the animal shews sign 

 of pain, the blood-pressure is affected, &c. ; and in the case of the 

 vagus the heart is slowed by reflex inhibitory impulses passing 

 down the other, intact, vagus, though heating the peripheral 

 instead of the central stump of the divided vagus has no effect 

 whatever on the heart. Similarly when the same nerves or other 

 nerves containing afferent fibres are submitted to the action of the 

 constant current, there are like evidences of the continued genera- 

 tion of nervous impulses during the whole time of the passage of 

 the current, even though it be kept as uniform in strength as 

 possible. On the other hand many chemical substances which 

 act as powerful stimuli to motor nerves are ineffectual towards 

 afferent fibres, These results, however, until the contrary is 

 proved by further inquiries into the phenomena attending the 



