THE CIRCULATION OF THE BLOOD. 



ported 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 Ibs. 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 to the time of the ventricular contraction. 



Fig. 177. Fick^s Kymograph, improved by Hering (after McKendrick). a, Hollow spring filled 

 with alcohol, bearing lever arrangement 6, d, c, to which is attached the marker e; the rod c passes 

 downward into the tube/, containing castor oil, which offers resistance to the oscillations of c; gr, 

 syringe for filling the leaden tube b with saturated sulphate of sodium solution, and to apply suffi- 

 cient pressure as to prevent the blood from passing into the tube h at i, the canula inserted into 

 the vessel; I, abscissa-marker, which can be applied to the moving surface by turning the screw m; 

 k, screw for adjusting the whole apparatus to the moving surface; o, screw for elevating or de- 

 pressing by a rack and pinion movement the Kymograph; n, screw for adjusting the position of 

 the tube/. 



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. The blood-pressure 

 gradually lessens then as we proceed from the arteries near the heart to 

 those more remote, and again from these to the capillaries, and thence 

 along the veins to the right auricle. The blood-pressure in the 

 veins is nowhere very great, but is greatest in the small veins, while in 

 the large veins toward the heart the pressure becomes negative, or, in 

 other words, when a vein is put in connection with a mercurial man- 



