THE VASCULAR SYSTEM 51 



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 (ve- 

 nous blood) from the systemic veins. It pumps this blood 

 to the lungs through the pulmonary arteries. Carbon diox- 

 ide 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 micro- 

 scopic 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 



