Some of them remove necrotic tissue or other detritus, and therefore function as scavengers. The lymph consists of plasma and white blood cells. The lymph vessels are the parts of the vascular system which transport lymph.
The heart receives blood laden with carbon dioxide (venous blood) from the systemic veins. It pumps this blood to the lungs through the pulmonary arteries. Carbon dioxide is surrendered here and oxygen enters the blood from the air chambers of the lungs. The oxygenated blood then returns to the heart through the pulmonary veins, and is propelled to the body through the systemic arteries. The branches of the arteries terminate in vessels of microscopic size, the capillaries, in which food and oxygen are surrendered to the cells of the muscles, glands, etc., and carbon dioxide enters the blood. The blood from the capillaries ultimately drains into the systemic veins and flows back to the heart. The internal structure of the heart and other details of the circulation will be discussed later.
The arteries and veins in any part of the body may often be dissected simultaneously. This is the logical procedure, since the arteries carrying blood to a part and the veins conducting blood away from it often lie close together. Considerable care must be exercised in dissecting out blood vessels, particularly veins, since they are frequently broken easily. It is better, as a rule, to locate a vessel at one place, then to follow it from that locality into the adjoining tissues just as a workman, having exposed a buried water pipe which he desires to uncover completely, follows it from the point at which it was first located. Every attempt to find a vessel anew at a place where it is concealed by other tissues unnecessarily increases the chance of breaking it accidentally.
The arteries should be injected with a colored fluid immediately after killing the rat. This fluid will force the
