SEC. 2. THE MAIN FACTS OF THE CIRCULATION. 



95. Before we attempt to study in detail the working of 

 these several parts of the mechanism, it will be well, even at the 

 risk of some future repetition, to take a brief survey of some 

 of the salient features. 



At each beat of the heart, which in man is repeated about 72 

 times a minute, the contraction or systole of the ventricles drives 

 a quantity of blood with very great force into the aorta (and the 

 same quantity of blood with less force into the pulmonary artery) ; 

 the actual amount varies from time to time, but 180 c.c. (4 to 6 oz.) 

 may be taken as a rather high estimate. The discharge of blood 

 from the ventricle into the aorta is very rapid, and the time 

 taken up by it is, as we shall see, less than the time which inter- 

 venes between it and the next discharge of the next beat. So 

 that the flow from the heart into the arteries is most distinctly 

 intermittent, sudden, rapid discharges alternating with relatively 

 longer intervals, during which the arteries receive no blood from 

 the heart. 



At each beat of the heart just as much blood flows, as we shall 

 see, from the veins into the right auricle as escapes from the left 

 ventricle into the aorta ; but, as we shall also see, this inflow is 

 much slower, takes a longer time, than the discharge from the 

 ventricle. 



When the finger is placed on an artery in the living body, a 

 sense of resistance is felt, and this resistance seems to be increased 

 at intervals, corresponding to the heart beats, the artery at each 

 heart beat being felt to rise up or expand under the finger, 

 constituting what we shall study hereafter as the pulse. In certain 

 arteries this pulse may be seen by the eye. When the finger is 

 similarly placed on a corresponding vein, very little resistance is 

 felt, and under ordinary circumstances no pulse can be perceived 

 by the touch or by the eye. 



When an artery is severed, the flow of blood from the proximal 

 cut end, that on the heart side, is not equable, but comes in jets, 



