This also contracts, and forces the blood through a valve into the left ventricle. This ventricle then contracts in its turn, and the blood passes through another valve into the great aorta, to go the round of the circulation and return in the manner just described. Many interesting experiments have been made to estimate the quantity of blood in an animal. "The weight of a dog," says Mr. Percival, "being ascertained to be seventy-nine pounds, a puncture was made with the lancet into the jugular vein, from which the blood was collected. The vein having ceased to bleed, the carotid artery of the same side was divided, but no blood came from it; in a few seconds afterwards, the animal was dead. The weight of the carcass was now found to be seventy-three and a half pounds; consequently it had sustained a loss of five and a half pounds--precisely the measure of the blood drawn. It appears from this experiment, that an animal will lose about one fifteenth part of its weight of blood before it dies; though a less quantity may so far debilitate the vital powers, as to be, though less suddenly, equally fatal. In the human subject, the quantity of blood has been computed at about one eighth part of the weight of the body; and as such an opinion has been broached from the results of experiments on quadrupeds, we may fairly take that to be about the proportion of it in the horse; so that if we estimate the weight of a horse to be thirteen hundred and forty-four pounds, the whole quantity of blood will amount to eighty-four quarts, or one hundred and sixty-eight pounds; of which about forty-five quarts, or ninety pounds, will commonly flow from the jugular vein prior to death; though the loss of a much less quantity will deprive the animal of life." REMARKS ON BLOOD-LETTING.