114 THE HORSE IN HEALTH AND DISEASE 



of which are directed toward the heart to prevent the blood from 

 flowing back. Veins are satellites of the arteries, and conduct the 

 blood back to the heart from the tissues. The anterior vena cava 

 returns the blood from the head, neck, and thoracic limbs to the 

 heart. The posterior vena cava is the largest vein of the body, 

 and conveys almost all the blood from the abdomen, pelvis, and 

 pelvic limbs. Among its principal branches are the hepatic vein 

 and the common iliac veins, which are formed by the confluence 

 of the veins from the posterior parts of the body. 



The capillaries are microscopic in size. In many cases they 

 are not much larger in diameter than a red blood-corpuscle. 

 They have extremely thin walls, often consisting of only one layer 

 of cells. If all the capillary vessels were cut across and put to- 

 gether, the sectional area would be about seven hundred times that 

 of the aorta. They function as connecting tubes between the 

 arteries and veins. It is through the capillary walls that the inter- 

 change of gases between the blood and the tissues takes place. 



The lymphatics consist of a number of well-defined groups of 

 lymph-glands and connecting vessels which are closely related to 

 the vessels carrying the red blood. Numerous very small radicals 

 unite to form a great trunk, which commences in a reservoir known 

 as the receptaculum chyli, located in the right sublumbar region. 

 The thoracic duct leads forward from this and empties into the 

 anterior vena cava, at the junction of the jugular veins near the 

 base of the heart (see Fig. 24). 



Lymph-glands are found in connection with every organ and 

 group of muscles. They act as filters for the lymph, and are very 

 important structures in maintaining the health. The bronchial 

 lymph-glands are located at the bifurcation of the bronchi, and are 

 usually discolored from the large amounts of very minute particles 

 of carbon and dust which they have caught. In certain diseases, 

 like tuberculosis, these glands early become infected with micro- 

 organisms and greatly enlarge. 



THE PHYSIOLOGY OF CIRCULATION 

 The circulation of the blood was not discovered until 1621, when 

 Harvey showed how the blood flows and the heart functions. 

 Before that time it was thought that the blood ebbed and flowed 

 like the tide. In Fig. 39 is diagrammatically shown the course 

 taken by the blood as it circulates through the system. 



