244 THE HUMAN BODY. 



but little. In the small arteries and capillaries the resistance 

 overcome and left behind is (on account of the great internal 

 friction due to their small calibre) very great, and conse- 

 quently the fall of pressure between the medium-sized arteries 

 and the veins is rapid and considerable. 



Modifications of Arterial Pressure by Changes in the 

 Bate of the Heart's Beat. A little consideration will make 

 it clear that the pressure prevailing at any timu in a given 

 artery depends on two things the rate at which the vessel 

 is filled, i.e., upon the amount of work done by the heart; 

 and the ease or difficulty with which it is emptied, that is, 

 upon the resistance in front. A third factor has to be taken 

 into account in some cases; namely, that when the muscular 

 coats of the small arteries contract the local capacity of the 

 vascular system is diminished, and has to be compensated for 

 by greater distention elsewhere, and vice versa. This would 

 of itself of course bring about changes in the pressure ex- 

 erted on the contained liquid, but for the present it may be 

 left out of consideration. Returning to the system of elastic 

 tubes with a pump represented in Fig. 98, let us suppose the 

 pump to be driving as before 10,800 cub. cent. (360 oz.) per 

 minute into the tubes B, and that these latter are so dis- 

 tended that they drive out just that quantity in the same 

 time. Under such conditions the pressure at any given 

 point in B will remain constant, apart from the small varia- 

 tions dependent upon each stroke of the pump. Now, how- 

 ever, let the latter, while still sending in 180 cub. cent. 

 (6 oz.) at each stroke, work 80 instead of 60 times a minute 

 and so send in that time 180 X 80 = 14,400 cub. cent. 

 (480 oz.) instead of the former quantity. This will lead to 

 an accumulation in B, since its squeeze is only sufficient, 

 against the resistance opposed to it, to send out 10,800 cub. 

 cent. (360 oz.) in a minute. B consequently will become 

 more stretched and the pressure in it will rise. As this 

 takes place, however, it will press more powerfully on its 

 contents until at last its distention is such that its elastic 

 reaction is able to force out in a minute through the small 

 tubes D, 14,400 cub. cent. (480 oz.) Thenceforth, so long 

 as the pump beats with the same force and at the same rate 

 and the peripheral resistanee remains the same, the mean 

 pressure in B will neither rise nor fall B sending into A in 

 a minute as much as c takes from it, and we would have a 



