When blood has been drawn from an animal, and it assumes a cupped or hollow form, if serum, or buffy coat, remains on its surface, it denotes an impoverished state; but if the whole, when coagulated, be of one uniform mass, it indicates a healthy state of that fluid. The blood of a young animal, provided it be in health, coagulates into a firm mass, while that of an old or debilitated one is generally less dense, and more easily separated. The power that propels the blood through the different blood-vessels is a mechanico-vital power, and is accomplished through the involuntary contractions and relaxations of the heart; from certain parts of which arteries arise, in other parts veins terminate. (See Plate.) The heart is invested with a strong membranous sac, called _pericardium_, which adheres to the tendinous centre of the diaphragm, and to the great vessels at its superior portion. The heart is lubricated by a serous fluid, secreted within the pericardium, for the purpose of guarding against friction. When an excess of fluid accumulates within the sac, it is termed dropsy of the heart. The heart is divided into four cavities, viz., two auricles, named from their resemblance to an ear, and two ventricles, (as seen at _a_, _b_,) forming the body. The left ventricle is smaller than the right, yet its walls are much thicker and stronger than those of the latter: it is from this part that the large trunk of the arteries proceed, called the _great aorta_. The right cavity, or ventricle, is the receptacle for blood returned by the venous structure after having gone the rounds of the circulation; the veins terminating, as they approach the heart, in a single vessel, called _vena cava_, (see plate, _o_, _q_, ascending and descending portion.) The auricle on the left side of the heart receives the blood that has been distributed through the lungs for purification. Where the veins terminate in auricles, there are valves placed, to prevent the blood from returning. For example, the blood proceeds out of the heart along the aorta; the valve opens upwards; the blood also moves upwards, and raises the valve, and passes through; the pressure from above effectually closes the passage. The valves of the heart are composed of elastic cartilage, which admits of free motion. They sometimes, however, become ossified. The heart and its appendages are, like other parts of the system, subject to various diseases, which are frequently very little understood, yet often fatal. Now, the blood, having passed through the veins and vena cava, flows into the right auricle; and this, when distended, contracts, and forces its contents into the right ventricle, which, contracting in its turn, propels the blood into the pulmonary arteries, whose numerous ramifications bring it in contact with the air-cells of the lungs. It then, being deprived of its carbon, assumes a crimson color. Having passed through its proper vessels, it accumulates in the left auricle.