The mammary glands constitute an organ termed the _udder_, which is composed of two symmetrical halves, placed one against the other. Each half is again divided into two distinct glands, each with its own teat, so that the udder consists of four mammae and four teats; behind this there may be two small rudimentary teats. In the center of each quarter, just at the base of the teat, is a large cavity, the general receptacle of all the milk ducts. From this cavity, which is sometimes large enough to contain a quart, proceeds down the center of the teat one defined canal from which the milk is drawn. In the small ruminants as the sheep and goat there are two mammae and two teats, constructed like those of the cow. CHAPTER V. ANGIOLOGY. Blood-Vascular System of the Horse. Under this heading we describe the organs of circulation, by the action of which certain fluids are propelled through the body. It is customary to divide this branch of the subject into two sections, considering respectively the blood-vascular and lymphatic systems. BLOOD-VASCULAR SYSTEM.--This involves the consideration of the blood, a fluid which supplies nutriment to the tissues and receives effete material from them; the heart, a muscular organ which, by its contraction, initiates the motion of the blood; the arteries, a series of tubes which convey the blood from the heart to all parts of the body; the veins, tubes which return that fluid to the heart; and the capillaries, minute tubes joining the small arteries and veins. BLOOD.--Blood is a fluid tissue, which nourishes all living structures, being the medium by which nutritive material is conveyed to, and effete or waste material conveyed away from the solid tissues. It is an opaque, thickish, clammy liquid, with a peculiar odor, sickly saline taste, and alkaline reaction. Its color varies in different parts of the same animal, that in the arteries being bright red or scarlet, while the blood in the veins is of a dark purplish hue. When examined microscopically, the blood is found to consist of minute corpuscles, and a clear, transparent, yellow fluid, the liquor sanguinis, or plasma, in which the corpuscles float. The corpuscles are of two kinds, the red and the white or colorless; the former, by far more numerous, vary in proportion. Red corpuscles vary in shape, but in all mammals (animals that suckle their young) are more or less flat, the families excepted where they are oval, as in birds, reptiles, and fish, which are also nucleated. Their average diameter in the horse, ox or sheep is about 1/4000th part of an inch, their average thickness being about one-fourth of this. Each surface is depressed towards its center, hence the corpuscle is appropriately described as a bi-concaved disc. The white corpuscles are larger than the red, round in shape, and nucleated.