72 CHANGES DURING A NERVOUS IMPULSE. [BOOK i. 



on the one hand and the two forms of actual energy on the other. 

 "We have reason to think that the proportion between heat and 

 work varies considerably under different circumstances, the work 

 sometimes rising as high as one-fourth, sometimes possibly sink- 

 ing as low as one twenty-fourth of the total energy, and obser- 

 vations seem to shew that the greater the resistance which the 

 muscle has to overcome, the larger the proportion of the total 

 energy expended which goes out as work done. The muscle in 

 fact seems to be so far self-regulating, that the more work it has to 

 do, the greater, within certain limits, is the economy with which 

 it works. 



Lastly it must be remembered that the giving out of heat by 

 the muscle is not confined to the occasions when it is actually con- 

 tracting. When, at a later period, we treat of the heat of the body 

 generally, evidence will be brought forward that the muscles even 

 when at rest are giving rise to heat, so that the heat given out at 

 a contraction is not some wholly new phenomenon, but a temporary 

 exaggeration of what is going on continually, at a more feeble 

 rate. 



The Changes in a Nerve during the passage of a Nervous 



Impulse. 



The change in the form of a muscle during its contraction is a 

 thing which can be seen and felt; but the changes in a nerve 

 during its activity are invisible and impalpable. We stimulate one 

 end of a nerve, and since we see this followed by a contraction of 

 the muscle attached to the other end, we know that some changes 

 or other, constituting a nervous impulse, have been propagated 

 along the nerve ; but these are changes which we cannot see. Nor 

 have we satisfactory evidence of any chemical events or of any pro- 

 duction of heat, accompanying a nervous impulse. We may fairly 

 suppose that some chemical changes form the basis of a nervous 

 impulse, and that these changes set free a certain amount of heat ; 

 but these if they occur are too slight to be recognized satisfactorily 

 by the means at present at our disposal. In fact, beyond the 

 terminal results, such as a muscular contraction in the case of 

 a nerve going to a muscle, or some affection of the central 

 nervous system in the case of a nerve still in connection with 

 its nervous centre, there is one event and one event only which 

 we are able to recognize as the objective token of a nervous 

 impulse, and that is the so-called negative variation of the nerve- 

 current. For a piece of nerve removed from the body exhibits 

 nearly the same electric phenomena as a piece of muscle. It has 

 an equator which is electrically positive as compared to its two cut 

 ends. In fact the diagram Fig. 12, and the description which it 

 was used on p. 59 to illustrate, may be applied to nerve as well as 



