_A._ I thank you, Isabel, for giving me an opportunity to lend you temporary relief.--"The disposition of the blood-vessels, as far as regards the supply of the body, is like that of the water-pipes in a city, viz. large and main trunks branching off by smaller pipes (and these again by still narrower tubes) in every direction and towards every part in which the fluid which they convey can be wanted. So far, the water-pipes which serve a town may represent the vessels which carry the blood from the heart. But there is another thing necessary to the blood, which is not wanted for the water; and that is, the carrying of it back again to its source. For this office, a reversed system of vessels is prepared, which, uniting at their extremities with the extremities of the first system, collects the divided and subdivided streamlets, first by capillary ramifications into larger branches, secondly by these branches into trunks; and thus returns the blood (almost exactly inverting the order in which it went out) to the fountain whence its motion proceeded. The body, therefore, contains two systems of blood-vessels, arteries and veins. "The next thing to be considered is the engine which works this machinery, viz., the _heart_. There is provided in the central part of the body a hollow muscle invested with spiral fibres, running in both directions, the layers intersecting one another. By the contraction of these fibres, the sides of the muscular cavity are necessarily squeezed together, so as to force out from 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. As soon as the blood is received by the heart from the veins of the body, and _before_ that is sent out again into its arteries, it is carried, by the force of the contraction of the heart, and by means of a separate and supplementary artery, to the lungs, and made to enter the vessels of the lungs, from which, after it has undergone the action, whatever it may be, of that viscus, it is brought back, by a large vein, once more to the heart, in order, when thus concocted and prepared, to be thence distributed anew into the system. This assigns to the heart a double office. The pulmonary circulation is a system within a system; and one action of the heart is the origin of both. For this complicated function four cavities become necessary, and four are accordingly provided; two called ventricles, which _send out_ the blood, viz., one into the lungs in the first instance, the other into the mass, after it has returned from the lungs; two others also, called auricles, which receive the blood from the veins, viz. one as it comes from the body; the other, as the same blood comes a second time