I 
TIIK CIRCtTLATION OF TliF BLOOD. 585 
the arteriey, and therefore is carried into one little cavity of the heart, 
and at one pulsation is driven by the pulniouary artery into the lungs, 
where, coming; in contact with the air through their membranes, it ab- 
sorbs oxygen from the air breathed, which changes its colour from dark 
to brig'ljt red. The blood, thus prepared for supporting life, is taken 
back by the pulmonary veins into the other side of tlie heart, which 
communicates with the aorta, and by one pulsation is sent to all parts 
of the body, returning again, as before, through the veins ; and this 
course takes place at every pulsation of that great and beautiful ma- 
chine — the heart. 
Experiments to ascertain the Force of the Blood in the Heart and 
Arteries. 
Physiologists and anatomists have from time to time attempted 
to make estimates of the force of the blood in the heart and arteries : 
but have differed as widely from each other, as they have from tlie 
truth, for want of sufficient data. This set the ingenious Dr. Hales 
upon making various experiments to ascertain the force of the blood 
in the veins and arteries of several animals, if, according to Dr. 
K-pil’s estimate, the left verticle of a man’s heart throws out in each 
systole an ounce, or 1.638 cubic inches of the blood, and the area of 
the orifice of the aorta be 0.4187, then dividing the former by this, 
the quotient 3.9 is the length of the cylinder of blood which is formed 
in passing through the aorta in each systole of the ventricle ; and in 
the 75 pulses of a minute, a cylinder of 292.5 inches in length will 
pass ; this is at the rate of 1462 feet in an hour. But the systole 
of the heart being performed in one-third of the time, the velocity of 
the blood in that instant will be thrice as much, viz. at the rate of 
4386 feet in an hour, or 73 feet in a minute’. And if the ventricle 
throws out one ounce in a pulse, then in the 75 pulses of a minute, 
the quantity of blood will be equal to 4lb. 11 oz. hence in thirty-f)ur 
minutes, a quantity equal to the weight of a middle-sized man, viz. 
159lb. will pass through the heart. 
But if, with Dr. Harvey and others, w'e suppose 2 ounces of blood, 
that is, 3.276 cubic inches, to be thrown out at. each svstole of the 
ventricle, then the velocity of the blood in entering the orifice of the 
aorta will be double the former, viz. at the rate of 146 feet in a minute, 
and the quantity of blood will be equal to the weight of a man’s body 
in half the time, viz. 17 minutes. If we suppose, what is probable, that 
the blood will rise 7-}-i feet high in a tube fixed to the carotid artery 
of a man, and that the inward area of the left ventricle of his heart is 
equal to 15 square inches, these multiplied in 7 + g feet give 1350 
cubic inches of blood, which presses on that ventricle, when it first 
begins to contract, with a weight equal to 15 5lbs. 
What Dr. Hales thus calculated from supposition, with rc> 2 :ard to 
mankind, he actually experimented upon horses, 'dogs, fallow deer, &c. 
so that by fixing tubes in orifices opened in their veins and arteries ; 
by observing the several heights to which the blood rose in those tubes 
as they lay on the ground ; and by measuring the capacities of the 
ventricles in Ihe heart, and orifices of the arteries— he obtained the 
most satisfactory results. 
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