SIGNIFICANCE OF PULSE RATE BUCHANAN. 503 



would, with the aid of the central nervous system, that is to say 

 reflexly, have done one or other or both things, in some species more 

 the one, in some more the other. 



Is it also by means of the central nervous system that the muscles, 

 put into play either voluntarily or involuntarily to produce the extra 

 amount of heat and taking up more oxygen from the blood, ask the 

 heart to make good the loss ? It is well known that muscular action 

 is accompanied by acceleration of the heart, and that acceleration of 

 the heart may be brought about by the intervention of nerves. But 

 to answer the question we have to know a good deal more than this, 

 and, in the first place, whether either reflexly by the excitation of the 

 afferent nerves of the muscle or by the excitation of motor cells of 

 the cortex such acceleration can be produced, also whether poikolo- 

 thermic vertebrates differ from homoeothermic ones in this respect. 

 That it can be produced in one or other of these ways in one species 

 of homoeothermic vertebrate, namely man, is shown, I think con- 

 clusively, by the results obtained from experiments which, by the 

 kindness of several Oxford undergraduates in serving as subjects for 

 them, I have been able to make. Having recorded the frequency of 

 the beat with the subject sitting quietly with one hand and one foot 

 in basins of salt water connected with the terminals of the capillary 

 electrometer, it was then again recorded when, instead of being at 

 rest, he clenched the fist that was free, or made some other definite 

 muscular action, on hearing a signal given automatically just as the 

 plate began to pass behind the capillary electrometer and with the 

 exact moment at which it was given recorded on the plate. The 

 reaction time of the subject to the particular sound had been first 

 ascertained with the same instrument, in a way which need not be 

 here described, to enable us to tell the moment at which the mus- 

 cular action began to be made, and to see in how long or how short a 

 time after it the acceleration of the heart took place. We have of 

 course to take our chance as to when in a cardiac cycle the signal is 

 given, but by taking a sufficient number of records we are likely to 

 meet with it in all phases of the cycle. The amount of the accelera- 

 tion with such a slight action as clenching a fist is very different in 

 different people, but if it is marked at all we have no difficulty in 

 ascertaining that it occurs so promptly that if the muscle begins to 

 contract only at the end of a systole, the immediately ensuing diastole 

 of the same cardiac cycle is considerably shortened and that of the 

 following cycles still more so. Thus in a man whose heart when at 

 rest was beating very regularly 73 times a minute, the period of the 

 cycle being therefore 0.82 second, the period became 0.67 second 

 when the fist was clenched at the end of the systole, and the next 

 ones were 0.57 or 0.56 second, the frequency being thus temporarily 

 raised to over 100 per minute. That the stimulus should be anything 



