CAPILLARIES.--The small arteries terminate in a system of minute vessels--the capillaries--which are interposed between the termination of the arteries and the commencement of the veins, forming plexuses (network) which vary much in arrangement. Their average diameter is about 2/1000ths of an inch, varying in different construction of the organs, smallest in the brain and mucous membrane of the intestines, larger in the skin, in glands, and the interior of bones. All arteries do not terminate in capillaries, an exception being in erectile tissue of the penis, where arteries end in cells or cavities placed at the origin of the veins. As the blood passes slowly through these capillaries, the nourishment is absorbed from it through their very thin walls to supply the tissues of the body. When the blood passes through this capillary network it again enters into large vessels called the veins, which carry it on its way back to the heart. COURSE OF THE BLOOD.--We have seen that the heart is divided into a right or venous, and a left or arterial portion. The blood is pumped by the heart to all parts of the body, through the arteries, passing through the capillary system, where it parts with its nourishment, is collected and returned to the heart by the veins, is again pumped by the heart to the lungs, where it is purified and returned to the heart to again commence the circuit as before. LYMPHATIC SYSTEM.--The lymphatic or absorbent system is closely connected with the blood-vascular system, and is made up of very fine minute tubes and glands. These convey from the tissues of the body a clear fluid known as lymph, and pours it into the blood of the veins as it is on its way back to the heart. These glands are found all through the body; for instance, there is a large group inside the thigh or stifle joint of the horse, and another large group inside the shoulder. It is important to note these, as they sometimes become inflamed and the leg is swollen. They are then the seat of the disease called weed in the leg, or lymphangitis. [Illustration: PLATE VI. _1. Brain._ _2. Spinal cord._ _3. Brachial plexus._ _4. Sacrolumbar plexus._ _5. Pneumogastric._ _6. Sciatic._ _7. Sympathetic System._ _8. Solar plexus._ NERVOUS SYSTEM OF A HORSE--AFTER MEGNIN.] CHAPTER VI. NEUROLOGY. This illustration shows where the brain, spinal canal and the principal nerves of the horse are located. STUDY OF THE NERVES.--The nervous system is a very important set of organs controlling the motion of the various members of the body and supplying the different senses of feeling, seeing, hearing, smelling and tasting. The two principal organs of the nervous system are the brain and spinal cord.