THE CIRCULATION OF THE BLOOD 331 



narily not far from 170 millimeters of mercury. The pressure in 

 the veins diminishes from 3 or 4 millimeters of mercury in the 

 large veins of the front leg to zero at the entrance to the auricle 

 (see p. 325). 



Blood-Pressure in Man. In man it is necessary to determine 

 blood-pressures by methods that do not involve operative pro- 

 cedure. Various devices are in use for this purpose. Most of 

 them depend on the fact that bodily tissues, being for the most 

 part liquid, are virtually incompressible and so transmit through- 

 out their extent pressures applied to them. For determining 

 arterial pressures the upper arm is inclosed in a cuff of hollow 

 rubber tubing so arranged that its inflation presses from all sides 

 on the arm. The cuff is inflated until its pressure on the arm is 

 just sufficient to squeeze shut the brachial artery. By means of 

 a manometer attached to the cuff the amount of pressure applied 

 can be determined. The differences between the various forms of 

 instruments depend chiefly on their methods for determining 

 exactly when the artery is occluded. These instruments do not 

 give mean blood-pressures, as does the mercury manometer, but 

 maximum (systolic) and minimum (diastolic) pressures. It is 

 found that in man the systolic pressure averages from 110 to 

 120 mms. of mercury, and the diastolic about 65 mms. of mercury. 



Determinations of capillary and venous pressures in man can 

 be made more easily than determinations of arterial pressure 

 because there are superficial capillaries and veins whose occlusion 

 can be observed directly; in capillaries by whitening of the skin, 

 in veins by the disappearance of the vein-ridge along it. The 

 basis of the method is the same as for arterial pressure, namely, 

 determination of the pressure necessary to occlude the vessel. 

 Capillary pressures measured by this method average about 

 30 mms. of mercury; venous pressures 10 mms. or less. 



The Rate of the Blood-Flow. As the vascular system be- 

 comes more capacious from the aorta to the capillaries the rate 

 of flow in it becomes proportionately slower, and as the total 

 area of the channels diminishes again from the capillaries to the 

 venae cavse, so does the rate of flow quicken, just as a river current 

 slackens when it spreads out, and flows faster when it is confined 

 to a narrower channel ; a fact taken advantage of in the construc- 

 tion of Eads' jetties at the mouth of the Mississippi, the object 



