14:6 HANDBOOK OF PHYSIOLOGY. 



manometer, it has been found that the pressure of blood in the carotid 

 of a rabbit is capable of supporting a column of 2 to 3 inches (50 to 90 

 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 the pressure is esti- 

 mated to be 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 



FIG. 135. A form of Fick'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 of 6; d, out- 

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



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 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 be- 

 ginning of the aorta, and decreases to ward 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 



