CIRCULATION OF THE BLOOD. 



151 



mm.) of mercury, in the dog 4 to 7 inches (100 to 175 mm.), in the 

 horse 5 to 8 inches (150 to 200 mm.), and in man about the same. 



To measure the absolute amount of this pressure in any artery, it is 

 necessary merely to multiply the area of its transverse section by the 

 height of the column of mercury which is already known to be supported 

 by the blood- pressure in any part of the arterial system. The weight 

 of a column of mercury thus found will represent the pressure of the 

 blood. Calculated in this way, the blood-pressure in the human aorta is 



FIG. 136. A form of Pick's Spring Kymograph, a, tube to be connected with artery: c, hollow 

 spring, the movement of which moves 6, the writing lever; e, screw to regulate height or 6; d, out- 

 side protective spring; gr, screw to fix on the upright of the support. 



equal to 4 Ib. 4 oz. avoirdupois; that in the aorta of the horse being 

 11 Ib. 9 oz.; and that in the radial artery at the human wrist only 4 drs. 

 Supposing the muscular power of the right ventricle to be only one- 

 half that of the left, the blood-pressure in the pulmonary artery will be 

 only 2 Ib. 2 oz. avoirdupois. The amounts above stated represent the 

 arterial tension at the time of the ventricular contraction. 



The blood-pressure is greatest in the left ventricle and at the begin- 

 ning of the aorta, and decreases toward the capillaries. It is greatest in 

 the arteries at the period of the ventricular systole, and is least in the 

 auricles, during diastole, when the pressure there and in the great veins 

 becomes, as we have seen, negative. The mean arterial pressure equals 

 the average of the pressures in all the arteries. The pressure in the 

 veins is never more than one-tenth of the pressure in the corresponding 



