The liquor sanguinis is pale and clear, and consists of water, fibrin, albumen, fatty compounds, extracts, odoriferous and saline matters. The serum is a thin, transparent liquid, of a pale-straw or yellow color, consisting of the liquor sanguinis deprived of fibrin. It contains nearly 90 per cent of water, is always slightly alkaline, and coagulates when heated, owing to the large quantity of albumen it contains. Fibrin is a white, stringy elastic substance, which, when the blood is in circulation, is in solution, and cannot be distinguished from the other constituents of the plasma. HEART.--The heart is the principal organ of circulation; it weighs about six and one-half pounds in the average horse and acts as a force pump to force the blood through the arteries. It is composed of strong muscular tissue, which acts involuntarily, and is situated between the lungs, which are divided by what is known as the mediastinum. This is a division between the lungs made up of two folds, the heart being between them. The bottom or apex, of the heart is downward and rests just above the breast-bone; the upper part, or base is directed upward and to the left side, the left lung having a depression on its inner surface for the heart to work in. There is a covering or sack around the heart which helps to protect and support it in its place. It is attached above to the back-bone, and below to the bones of the breast. This sack is made up of fibrous tissue and is of a whitish appearance; inner surface is smooth, and supplied with numerous small glands which secrete an oily substance called serous fluid. This lubricates the outer surface of the heart and the inner surface of the sack so that in action it does not irritate the walls or surfaces. The cavity of the heart is divided into two parts, the right and left sides; each of these parts is again subdivided. The upper cavity is called auricle and the lower cavity ventricle; thus there are the right and left ventricle and right and left auricle. The right auricle communicates with the right ventricle by an opening in the septum or partition on the right side of the heart. This opening is guarded by a valve to keep the blood from flowing back into the auricle. The left auricle communicates with the left ventricle, same as on the right side. The right side of the heart is sometimes called the venous side and contains only venous or impure blood. The left side is sometimes called the arterial side. It contains pure blood only. This side of the heart is very much stronger and thicker than the right side. [Illustration: PLATE V. THE HEART AND THE CHIEF BLOOD VESSELS OF THE HORSE--AFTER MEGNIN.] EXPLANATION OF PLATE V BLOOD-VASCULAR SYSTEM OF THE HORSE