The heart is inclosed in the pericardium and placed behind the sternum and costal cartilages on the border end or base, by which it is attached, being directed upwards, backwards, and to the right, and extending from the level of the fourth to that of the eighth dorsal vertebra, the apex downwards, forwards, and to the left. In size, it is about five inches long, three and a half in its greatest width, and two in its extreme thickness from the anterior to the posterior surface. The weight is from nine to ten ounces. The circulation of the blood.-- The body is divided into two chief cavities, the chest or thorax, and abdomen, by a curved muscular partition called the diaphragm or midriff. The chest is almost entirely filled with lungs and heart, the latter being fitted in, so to speak, between the two lungs, nearer to the front than the back of the chest, and partly overlapped by them. In the living body the heart and lungs are in constant rhythmic movement, the result of which is an unceasing stream of air through the trachea alternately into and out of the lungs, and an unceasing stream of blood into and out of the heart. The blood is conveyed away from the heart by the arteries and returned to it by the veins; the arteries and veins being continuous with each other, at one end by means of the heart, and at the other by a fine network of vessels called capillaries. The blood, therefore, in its passage from the heart passes first into the arteries, then into the capillaries, and lastly into the veins, by which it is conveyed back again to the heart--thus completing a revolution, or circulation. There are two circulations by which all the blood must pass--the one a shorter circuit from the heart to the lungs and back again, which is called the pulmonic; the other the larger circuit, from the heart to all parts of the body and back again, which is called the systemic; and a subordinate stream of blood, that has been collected by the blood-vessels of the intestines, passes by means of the portal vein through the liver, and is called the portal circulation. The principal force provided for constantly moving the blood on this course, is that of the muscular substance of the heart; other assistant forces are (2) those of the elastic walls of the arteries, (3) the pressure of the muscles among which some of the veins run, (4) the movements of the walls of the chest in respiration, and (5) probably to some extent the interchange of relations between the blood and the tissues which ensues in the capillary system during the nutritive processes. The right direction of the blood's course is determined and maintained by the valves of the heart. The heart is divided into two chief chambers or cavities--right and left. Each of these chambers is again divided into an upper and lower portion called respectively auricle and ventricle, which freely communicate with each other.