46. THE CIRCULATION OF THE BLOOD.
1. Evidence of the circulation.—(a) The arterial pulse.—Feel your pulse at the wrist and count the number of beats per minute.
(b) The beats of the heart.—Similarly count how many times your heart beats per minute. Put your ear over the heart of a friend, and listen to the sounds of the heart. How many different sounds can you distinguish?
(c) The valves of the veins.—Press your thumb on your arm inside the elbow, and then move it down the arm towards the wrist. Notice the small knots which rise under the skin between the point of pressure and the wrist.
2. The structure of a sheep’s heart.[12]—Procure from the butcher a sheep’s heart with, if possible, the lungs still attached. If the thin transparent bag which naturally surrounds the heart has been left on, carefully cut it away. Make out the main external features of the heart before cutting into it. The shape is conical, the apex of the cone being posterior, the base (where the blood-vessels are attached) anterior. The ventral face is rounded; the dorsal face is flatter. Compare [Fig. 164]. The line of fat (3) marks the line of division between two chambers, the right (R.V.) and left (L.V.) ventricles. Feel that the right ventricle yields to pressure more readily than the left. Two more chambers, the right and left auricles, are situated at the basal (thick) end. R.A. and L.A. are flaps of the right and left auricles respectively. Identify the great vessels SVC and IVC which discharge into the right auricle. Cut them open to see the entrance. Then lay open the right auricle. Pass your finger down and notice that the right auricle communicates freely with the right ventricle. Observe that you can from the right ventricle pass your finger into the tube P.A.; make out that this goes to the lungs. Lay open the left auricle, and see that it receives vessels from the lungs. Pass your finger from the left auricle into the left ventricle, and notice that this latter chamber leads into the Aorta, Ao. (A´o´. a branch of Ao.). Notice that the walls of the blood-vessels in connection with the two auricles are collapsed, while those (P.A. and Ao.) leading from the two ventricles are more elastic and remain open. Which of the two auricles has the thicker wall? Which of the two ventricles has the thicker wall? Can you pass your finger from one auricle to the other? From one ventricle to the 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 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.
The heart ([Fig. 164]) consists of four chambers. Two of these, the auricles, are receiving-chambers, and are placed at the thick, anterior end of the heart. Into the right auricle open the great veins (SVC, IVC,) which bring blood from all parts of the body except the lungs; the left auricle receives only blood from the lungs. Each auricle opens into a more posterior chamber called a ventricle, the right auricle opening into the right ventricle and the left auricle into the left ventricle. The ventricles pump blood into the arteries. The blood from the right ventricle is sent into the artery (P.A.) which supplies the capillaries of the lungs; while the blood of the left ventricle is forced into the aorta (Ao.), a great artery which branches and supplies with blood all parts of the body except the lungs. The two auricles contract simultaneously, forcing their contents into the flaccid ventricles. Then both the filled ventricles contract at once, and pump blood into the great arteries, flaps of membrane between the auricles and ventricles preventing a backward flow into the auricles. Similarly the bases of the great arteries are provided with membranous pockets which readily admit the blood from the ventricles when these contract, but entirely prevent a return of blood to the ventricles. The appearance of these four sets of valves, as seen from above, is shown in [Fig. 165]. After the contraction of the two ventricles there is a short rest, then the auricles contract again and the whole process is repeated.
