OF ANIMAL BODIES. 
93 
them any fluid which they may at that time contain: by 
the relaxation of the same fibres, the cavities are in their 
turn dilated, and, of course, prepared to admit every fluid 
which may be poured into them. Into these cavities are 
inserted the great trunks, both of the arteries which carry 
out the blood, and of the veins which bring it back. This 
is a general account of the apparatus: and the simplest idea 
of its action is, that, by each contraction, a portion of blood 
is forced by a syringe into the arteries; and, at each 
dilatation, an equal portion is received from the veins. This 
produces, at each pulse, a motion, and change in the mass 
of blood, to the amount of what the cavity contains, which, 
in a full-grown human heart, I understand, is about an 
ounce, or two table-spoons full. How quickly these changes 
succeed one another, and by this succession how sufficient 
they are to support a stream or circulation throughout the 
system, may be understood by the following computation, 
abridged from Keill’s Anatomy, p. 117. ed. 3: “Each ven¬ 
tricle will at least contain one ounce, of blood. The heart 
contracts four thousand times in one hour; from which it 
follows, that there pass through the heart, every hour, 
four thousand ounces, or three hundred and fifty pounds of 
blood. Now the whole mass of blood is said to be about 
twenty-five pounds; so that a quantity of blood, equal to the 
whole mass of blood, passes through the heart fourteen times 
m one hour; which is about once every four minutes.” 
Consider what an affair this is, when we come to very large 
animals. The aorta of a whale is larger in the bore than 
the main pipe of the water-works at London bridge; and 
the water roaring in its passage through that pipe is in¬ 
ferior in impetus and velocity, to the blood gushing from 
the whale’s heart. Hear Dr. Hunter’s account of the dis¬ 
section of a whale:—“ The aorta measured a foot diame¬ 
ter. Ten or fifteen gallons of blood are thrown out of the 
heart at a stroke with an immense velocity, through a tube 
of a foot diameter. The whole idea fills the mind with won¬ 
der.”* 
The account which we have here stated, of the injec¬ 
tion of blood into the arteries by the contraction, and of 
the corresponding reception of it from the veins by the di¬ 
latation of the cavities of the heart, and of the circulation 
being thereby maintained through the blood-vessels of the 
body, is true, but imperfect. The heart performs this of¬ 
fice, but it is in conjunction with another of equal curiosi- 
* Dr. Hunter’s account of the dissection of a whale. Phil. Trans. 
