BLOOD PRESSURE 



127 



blood pressure in the renal or femoral artery is very little less than that 

 in the aorta. 



If we examine the contour of the tracing which the pressure draws, we 

 shall find that it exhibits two types of wave, small and large ; and if we 

 observe the animal while the tracing is being taken, we shall find that 





Fig. 22. The arterial blood pressure recorded with a mercury manometer (lower tracing), 

 along with a tracing of the. respiratory movements of the thorax. Note that the beginning of 

 respiration occurs distinctly before the rise in blood pressure. 



the former are caused by the heartbeats and the latter by the respira- 

 tions an observation which immediately raises the question as to the 

 trustworthiness of the method, for it will be asked, How can it be that 

 the heartbeat produces an effect on blood pressure which is less than 

 that of the respirations? Obviously the tracing must be faulty in re- 

 gard to the relative significance of the waves. 



