other? Cut away the auricles and pour water into the ventricles. Then squeeze the heart gently, and notice the flaps which rise to close the openings into the ventricles. Could blood pass from the ventricles to the auricles? Why not? Lay open the bases of the vessels P.A. and Ao. leading from the right and left ventricles respectively, and notice the pockets of thin membrane which are attached there. Open them out gently with the point of a pencil. Put the heart under the tap and let water trickle down the vessels towards the ventricles; notice how the pockets stand out as they fill with water. What would be the effect of blood trying to pass from P.A. or Ao. back into their respective ventricles? The circulation of the blood.—The blood of such an animal as the rabbit is contained in a system of closed tubes called blood-vessels, through which the fluid is continually flowing. The regular flow of the blood in one direction is maintained by the action of the heart, a four-chambered organ which is situated in the chest, between the lungs. The heart is muscular, and, like other muscles (p. 225), has the power of “contracting” in definite directions. The contraction of the walls of the chambers of the heart lessens their capacity, and therefore drives the blood out of them, the [Pg 237] direction of flow being determined by valves. The vessels which carry blood to the heart are called veins; those which transmit the blood which is pumped out of the heart are called arteries. The arteries branch into smaller and smaller tubes which supply the various organs of the body, and the small arterial branches divide again and again in the organs until their finest ramifications form a close-meshed network of vessels with excessively thin walls, through which diffusion can readily take place between the tissues and the blood. These finest blood-vessels are called capillaries. They can be studied in the transparent web of the frog’s foot with the aid of a low power of the microscope. The blood can then be seen flowing, at a speed which varies with the size of the vessel, its course being rendered obvious by the tiny oval particles (red corpuscles) which are suspended in it. The smallest capillaries are so thin-walled that they appear to be merely channels in the substance of the web, and the corpuscles creep along in single file. But these channels unite to form larger vessels with obvious walls, and these unite again and again until a main vein is formed in which the blood, with its suspended corpuscles, rushes along in a swift torrent towards the heart. The heart.—The beginner will find the sheep’s heart more convenient for examination than the rabbit’s, on account of its larger size; but apart from some difference in the arrangement of the great blood-vessels opening into them, the two hearts are broadly similar.