I. COURSE OF THE BLOOD IN ALAN.
The organs of circulation consist of the heart, arteries, veins, and capillary vessels. We refer the reader to the articles on these different organs for all details relative to their anatomical structure.
In man and warm-blooded animals there are two passages through the interior of the heart, through each of which a stream of blood is propelled at the same time, so that the heart is alternately receiving and giving out a certain quantity of blood upon each side.
The two auricles serve as receiving cavities for the blood which is constantly flowing into the heart from the veins or those vessels which have the office of returning blood to the centre of the circulation. By the contraction of the muscular parietes of the auricles, the blood is propelled from theSe cavities into the ventricles, which, in their turn, contract with force and thus propel their contents into the arteries,- or those vessels which serve to transmit blood outwards from the centre of the circulatory organs. The auricles and ventricles of the opposite sides acting simultaneously, and the size of these cavities on the right and left sides of the heart beim. nearly equal, the quantity of blood which is made to pass through each of them at one and the same time must also be nearly equal.
The cavities on the left side of the heart are adapted to propel the blood into those arteries which are subservient to the nutrition of the body, while those on the right side of the heart send the blood to the lungs for the purposes of respiration. The construction of the heart and the connection of its parts with the arte ries and veins are such that the whole of that blood which has served the purposes of nu trition, and the other uses for which the bloo.d is destined throughout the body, on being re turned to the heart, is directed by the cavities on the right side of that organ to the lungs, and made to pass through them before returning to the left side of the heart to repeat its course through the nutritive vessels of the body.
. In all those animals in which there exists a disposition of the heart and bloodvessels such as that described, the circulation is said to be double, because the blood is moved in two circles at once, and the respiration is said to be complete, because the whole of that blood which has passed through the nutritive vessels of the body is subjected to the respiratory action of air in the lungs.
The blood returned from the lungs of a bright red colour, or arterial blood, on being expelled from the left ventricle (fig. 312, by the muscular contraction of that cavity, passes .into the aorta or great artery of the system (A), and is distributed in various pro portions to all parts of the body by the branches of the aortic trunk (a) and their in finitely minute ramifications. The smallest arteries lead, by an intermediate set of minute tubes to which the name of capillary vessels is given, into the systemic veins (v), all of which (the veins of the intestinal canal excepted) join ing gradually together into larger and fewer branches, form at last the great trunks of the superior and inferior venm cav ( 0), which carry back to the centre of the circulation the whole of the blood that had passed from the left ventricle into the aorta.
In passing from the arteries to the veins through the capillary vessels, the properties Of the arterial blood are changed ; its colour is altered from bright scarlet to dark purple, it expends some of its substance in the nou-' rishment of the textures, and a considerable quantity of its thinner part transudes through the Small vessels, constituting the lymph that is taken up by the absorbent vessels. The venous or dark blood, as it approaches the heart upon its return, has its composition fur ther changed by its admixture with the chyle or imperfectly _formed blood, which is the pro duct of digestion,_ and which is poured along with the lymph from the thoracic duct into the great veins of the head and superior ex- . tremities.