316 EXPERIMENTAL INVESTIGATION OF THE ACTION OF MEDICINES. 



Blood' Pressure. 



The blood-pressure depends on two tilings — 1, the activity 

 with which the heart pumps the blood into one end of the 

 arterial system ; 2, the rate at which it flows out at the other 

 end into the veins. The rate is regulated by the small arteries 

 and capillaries, which dilate and contract so as to quicken or 

 slow it. The power of contraction is denied to the capillaries 

 by many physiologists ; but Strieker has, 1 think, conclusively 

 shown that they do possess it. 



The rapidity with which the blood flows through them does 

 not depend entirely on the width of the capillaries, but also on 

 the pressure in the arteries which is forcing the blood into 

 them. The higher this is, the more rapidly does the blood flow ; 

 and in proportion as it diminishes does the current become 

 slower. From this circumstance we can judge of the force of 

 the heart-beats from the form of the curve which we obtain 

 with the sphygmoscope. When the heart contracts with great 

 force, it drives the blood out of the ventricle into the arteries so 

 quickly that there is no time for much to escape from the capil- 

 laries while the systole lasts, and so the tension rises high. 

 This increased tension makes the blood run quickly out of the 

 capillaries, and we have a fall of pressure, rapid at first, but 

 gradually becoming slower as the tension diminishes. This is 

 shown in Fig. 139. When the heart contracts less forcibly, it 



Fig. 139. 



sends in the blood more slowly, and there is time for a greater 

 quantity to escape by the capillaries during the systole ; and 

 the tension does not rise so high. From the tension being 

 lower, the outflow of blood is not so quick, and the pressure 

 therefore sinks more gradually than in the former case. This 

 is represented in Fig. 140, Both of these figures were obtained 



