238 THE HUMAN BODY. 



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

 lead to an accumulation in B, since its squeeze is only suf- 

 ficient, against the resistance opposed to it, to send out 10,- 

 800 cab. 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 squeeze more powerfully 

 on its contents until at last its distension is such that its 

 elasticity 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 resistance 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 steady condition of things with a higher mean pressure 

 in B than before. 



On the other hand if .the pump begins to w r ork more 

 slowly while the resistance remains the same, it is clear that 

 the mean pressure in B will fall. If, for example, the pump 

 works only forty times a minute and so sends in that time 

 180 X 40 = 7200 cub. cent. (240 oz.) into B, which is so 

 stretched that it is squeezing out 10,800 cub. cent. (360 

 oz.) in that time, it is clear that B will gradually empty 

 itself and its walls become less stretched and the pressure 

 in it fall. As this takes place, however, it will force less 

 liquid in a minute through the small tubes, until at last a 

 pressure is reached at which the squeeze of B only sends 

 out 7200 cub. cent. (240 oz.) in. a minute; and then the 

 fall of pressure will cease and a steady one will be main- 

 tained, but lower than before. 



Applying the same reasoning to the vascular system we 

 see that (when the peripheral resistance remains unaltered), 

 if the heart's force remains the same but its rate increases, 

 arterial pressure will rise to a new level, while a slowing of 

 the heart's beat will bring about a fall of pressure. 



Modifications of Arterial Pressure Dependent on 

 Changes in the Force of the Heart's Beat. Returning 

 again to Fig. 87; suppose that while the rate of the pump 

 remains the same, its power alters so that each time it 

 sends 200 cub. cent. (6.6 oz.) instead of 180 (6 oz.) and so in 



