2. _The Greater Circulation_.--From the left auricle, the blood is forced past the bicuspid valve to the left ventricle; thence it is driven through the semilunar valves into the great aorta, the main trunk of the arterial system. Passing through the arteries, capillaries, and veins, it returns through the venæ cavæ, ascending and descending, gathers again in the right auricle, and so completes the "grand round" of the body. Both these circulations are going on constantly, as the two auricles contract, and the two ventricles expand simultaneously, and _vice versa_.
THE VELOCITY OF THE BLOOD varies so much in different parts of the body, and is influenced by so many circumstances, that it can not be calculated with any degree of accuracy. It has been estimated that a portion of the blood will make the tour of the body in about twenty-three seconds (FLINT), and that the entire mass passes through the heart in from one to two minutes. [Footnote: The total amount of blood in an adult of average weight is about eighteen pounds. Dividing this by five ounces, the quantity discharged by the left ventricle at each systole, gives fifty- eight pulsations as the number necessary to transmit all the blood in the body. This, however, is an extremely unreliable basis of calculation, as the rapidity of the blood is itself so variable. Chauvreau has shown by experiments with his instrument that, corresponding to the first dilation of the vessels, the blood moves with immense rapidity; following this, the current suddenly becomes nearly arrested; this is succeeded by a second acceleration in the current, not quite so rapid as the first; and after this there is a gradual decline in the rapidity to the time of the next pulsation.] (See p. 314.)
DISTRIBUTION AND REGULATION OF THE HEAT OF THE BODY.--1. _Distribution_.--The natural temperature is not far from 98°. [Footnote: The average temperature is, however, easily departed from. Through some trivial cause the cooling agencies may be interfered with, and then, the heating processes getting the superiority, a high temperature or fever comes on. Or the reverse may ensue. In Asiatic cholera, the constitution of the blood is so changed that its disks can no longer carry oxygen into the system, the heat-making processes are put a stop to, and, the temperature declining, the body becomes of a marble coldness, characteristic of that terrible disease.--DRAPER.] This is maintained, as we have already seen, by the action of the oxygen within us. Each capillary tube is a tiny stove, where oxygen is combining with the tissues of the body (see note, p. 107). Every contraction of a muscle develops heat, the latent heat being set free by the breaking up of the tissue. The warmth so produced is distributed by the circulation of the blood. Thus the arteries, veins, and capillaries form a series of hot- water pipes, through which the heated liquid is forced by a pump--the heart--while the heat is kept up, not by a central furnace and boiler, but by a multitude of little fires placed here and there along its course.