204: THE HUMAN BODY. 



elude that the new internal molecular arrangement in a nerve- 

 fibre which constitutes its active as compared with its resting 

 state, is one which changes also the electrical properties of the 

 fibre. Now it is found that the negative variation travels along 

 the nerve (in the frog) at the rate of 28 metres (92.00 feet) in 

 a second, and takes .0007 second to pass by a given point : 

 accordingly at any one moment it extends over about 18 mm. 

 (0.720 inch) of the nerve-fibre. Moreover, when first reach- 

 ing a point it is very feeble, then rises to a maximum,, and 

 gradually fades away again. Taking it as an indication of 

 what is going on in the nerve, we may assume that the nerv- 

 ous impulse is a progressive molecular change of a wavelike 

 character, rising from a minimum to a maximum, then grad- 

 ually ceasing, and. about 18 millimetres in wave-length. 



A nervous impulse does not appear to exhaust a fibre when 

 transmitted along it. As a ray of light traversing the ether 

 sets up a transient change in it but does not in any way use it 

 up or leave it less fit to transmit a succeeding ray, so it is 

 with the nervous impulse in its transmission. It is true that 

 when a motor nerve attached to a muscle is continuously 

 stimulated the muscular contractions cease after a certain 

 time, though the muscle still responds to electrical stimula- 

 tion directly applied, and it has been argued that we thus get 

 evidence of the exhaustion of the nerve; but it must be 

 borne in mind that an electrical shock directly applied is un- 

 doubtedly a much more powerful stimulus to the muscle than 

 any nervous impulse, and the muscle may have been so 

 fatigued by its previous work as to have become irresponsive 

 to stimulation through its nerve, though still reacting to the 

 grosser excitation. And we have direct evidence that stimu- 

 lation of a nerve may be continued for a very long time with 

 out causing loss of activity. As an instance, we may take the 

 nerve already mentioned which stops the beat of the heart : 

 when it is stimulated continuously for a few seconds the heart 

 breaks beyond its control and begins to beat again, though the 

 stimulation of the nerve be kept up. This, however, is due to 

 fatigue of the endings of the nerve in the heart, and not of 

 the nerve fibres, as may be proved in this way: the nerve 

 (pneumo-gastric) being carefully exposed in the neck is arti- 

 ficially cooled in one region to below the temperature at 

 which it can conduct a nervous impulse; it is then stimu- 

 lated at a point nearer the head than the cooled portion: the 



