THE PULSE-RATE IN VERTEBRATE ANIMALS 79 



the subject to the particular sound had been first ascertained 

 with the same instrument, in a way which need not be here de- 

 scribed, to enable us to tell the moment at which the muscular 

 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 the phases of the cycle. The 

 amount of the acceleration 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 o"82 sec, the period became 

 067 sec. when the fist was clenched at the end of the systole, 

 and the next ones were 0*57 or o'56 sec, raising the frequency 

 to over 100 per minute. That any stimulus involving mechanical 

 movements of the blood could produce an effect so promptly is 

 hardly conceivable. The shortening of the cycle in cases of 

 such slight action is due to shortening of the diastole only, and 

 MacWilliam's researches (12) on cats have shown us that it is 

 the vagus nerve which principally, if not solely, affects the 

 duration of diastole, and that stimulation of the peripheral end 

 of this nerve produces an immediate effect, whereas that of the 

 accelerator nerve to the heart (the sympathetic) takes some few 

 seconds to produce one. We can therefore not only say from 

 the promptitude with which the heart accelerates when a volun- 

 tary action is made that it is due to nerve action, but also that it 

 is the vagus nerve which conveys the impulse to the heart and 

 therefore that the afferent nerve, whether the sensory nerve of 

 a muscle or an axon from a cortex cell, acts on the vagus 

 centre in such a way as to suspend its tonic action, Bowen, in 

 a paper (15) discovered after these experiments had been made, 

 has shown that even so small an action as gently tapping 

 a key, the subject being at rest with his arm supported on a 

 table, produces a prompt acceleration of the heart. His method 

 of recording does not show so well as that described above 

 how prompt it is, but he saw that it was enough to indicate 



