322 



CIRCULATORY SYSTEM. 



Large arteries (e. #., carotid) -f 140 mm. of mercury. 



Medium arteries (e. g., radial) -i- 110 



Capillaries + 15 to -f 20 



Small veins of arm +9 



Portal vein . . ' . + 10 



Inferior vena cava +3 



Large veins of neck from to 8 



The Sphygmometer (Fig. 168). The sphygmometer is an 

 instrument for ascertaining the blood-pressure in the human 

 subject. It consists of a rubber bag, containing a colored fluid, 

 connected with a graduated glass -tube, the top of which is 

 expanded into a bulb and closed with a stopcock. The instru- 

 ment is so attached to the body that the rubber bag is on the 

 artery whose blood-pressure is to be ascertained. The bag being 

 pressed down upon the artery, the fluid rises in the tube, and the 

 air in the bulb, being compressed, acts as an elastic spring. The 

 top of the fluid is watched carefully, and when its pulsation is 

 greatest, which is known as the maximal pulsation, its height is 

 read off on the scale ; this is so graduated as to correspond to 

 millimeters of mercury pressure, and represents the arterial press- 

 ure. By this instrument the radial pressure of an adult has 



FIG. 168. Hill and Barnard's sphygmometer. 



been found to be from 110 to 120 mm. of mercury. The instru- 

 ment can also be used to obtain venous pressure. 



Stewart states that the blood-pressure in the radial artery of a 

 healthy man may average 150 mm. of mercury. In the anterior 

 tibial artery of a boy whose leg was to be amputated, it was found 

 to vary from 100 mm. to 160 mm. according to the position of 

 the body and other circumstances. The pressure in the pulmo- 

 nary artery is about one-third that in the aorta. The mean press- 



