THE BLOODVESSELS 



79 



rate drops, and in old age the pulsations are not only reduced in 

 number but weaker. 



The rigid condition of the arterial wall alters the shape and 

 nature of the pulse tracing in old age. 



Between size of body and pulse-rate there is a distinct con- 

 nection ; it varies inversely as the height — in the elephant 27 

 beats a minute, in the mouse 670. 



The heart-rate is rapidly responsive to all outside influence, 

 such as excitement or fear. A harsh word, fear, or timiditv will 



Fig. 34. — Plethysmograph for Arm. 



F, Float attached by A to lever, which records variations of level of the water 

 in B, and therefore variations in the volume of the arm in the glass vessel C. 

 Or the plethysmograph may be connected to a recording tambour. The 

 tubulure at the upper part of C is closed when the tracing is being taken. 



Fig. 35. — Plethysmograph Tracing from Arm (Stewart). 

 The tracing was taken by means of a tambour connected with the plethysmograph. 



cause the pulse of a nervous animal to register nearly double the 

 number of beats of the heart. To sickness or injury the pulse 

 is instantly responsive, and is one of the cardinal aids both in 

 diagnosis and prognosis. Variations of pulse-rate follow as the 

 result of work, so that a marked increase in the number of beats 

 occurs ; this means a larger amount of blood in circulation through 

 tissues in~a state of activity, and which consequently are in 

 urgent need both of repair and flushing. 



