THE PLANT AS A MACHINE 3 



sudden contractile movement is followed by relaxation. By 

 a series of these, the blood is forced in a pulsating manner 

 through the arteries, and we perceive the pulsatory movement 

 at the wrist. Physicians by feeling such pulse-throbbings 

 are able to pronounce on the condition of a patient. Or 

 the movements may be recorded by means of a lever- 

 arrangement, in which the short arm of the lever rests on 

 the throbbing pulse. Its longer arm, provided with a tracing 

 point, records these pulsatory movements on a travelling 

 band of paper which is moved by clock-work at a uniform 

 rate. It is on this principle that the instruments known as 

 sphygmographs are constructed, and the response-records, or 

 sphygmograms, reveal the physiological condition of the 

 individual at the time (figs. 1 and 3). 



Fig. 1. Record of (a) Healthy Adult and (b) Senile Human Pulse 

 (Broadbent) 



The heart-record, however, has been still more directly 

 obtained, in the case of the lower animals, by attaching one 

 end of the lever to the apex of the heart itself. Each con- 

 traction and subsequent recovery is now recorded, in the 

 manner which has been indicated. If we know the rate at 

 which the recording surface is travelling, or if we make time- 

 marks at regular intervals, we are able to determine the 

 frequency of pulsation. The record also gives us the amplitude 

 of each pulse. 



If now these records are to furnish reliable indications 

 of the internal condition of the living machine, then, any 

 circumstance which affects this internal condition must reveal 

 itself in the external record. And this is found to be 

 the case. For example, the effects of age are seen in 

 the accompanying record (fig. 1) ; and that of poison by 



b 2 



