72 PHYSIOLOGY OF THE CIRCULATION. 



that blood will at the same time add to its backward 

 pressure, and thus impede the flow of fresh blood 

 from the ventricle. 



The condition which, in a healthy state of the 

 body, regulates the efflux of blood from the arterial 

 system, is evidently the contractility of the small 

 arteries and capillaries a force influenced by the 

 nervous system, and known to be of varying ac- 

 tivity ; while the influx of blood is still more 

 distinctly referrible to the contractions of the ven- 

 tricle. 



Now if this cavity of the heart invariably emptied 

 itself at each stroke, we might readily estimate the 

 rate of influx of the arterial blood by ascertaining 

 the number of its pulsations ; but, from the previous 

 reasoning, it appears more probable that the ventricle 

 does not always expel the whole of its contents at 

 each contraction, and that the extent to which it 

 does so, at any one time, mainly depends upon the 

 relative quantity of blood then contained within the 

 aorta. For when the quantity of this detained blood, 

 and the consequent distension of the aorta, are very 

 much diminished, as after loss of blood, then the op- 

 position to the influx of fresh fluid from the ventricle 

 is so much reduced, that it may readily empty its 

 contents into the aorta; but if the heart's action be 

 suddenly increased at a time when the quantity of 

 blood detained in the aorta is already very con- 

 sideral >le, each contraction will but add to the resist- 

 ance opposing the entrance of fresh fluid into the arte- 

 rial system, and will thus prevent the ventricle from 

 expelling the whole of its content* at each pulsation. 



