DISEASES OF HEART, BLOOD VESSELS, AND LYMPHATICS. 71 



traction of the ventricles follows ; the auriculo- ventricular valves are 

 forced up by the pressure of the blood and close the auriculo- ventric- 

 ular openings and prevent the return of blood into the auricles; the 

 contraction of the ventricles forces the blood from the right ventricle 

 into the lungs through the pulmonary artery and its branches, and 

 from the left ventricle into the aorta and all parts of the body through 

 the arteries. After the contraction of the ventricles the heart is again 

 in momentary repose and being filled with blood, while the valves in 

 the aorta and pulmonary artery close to prevent the return of blood 

 into the ventricles. (See PI. VII. ) 



The average weight of the heart of an ox is said to be from 3-£ to 5 

 pounds, but, of course, the weight must be very variable in different 

 animals, owing to the many breeds and sizes of cattle. 



The vessels that convey the blood from the heart to all parts of 

 the body are called arteries; the vessels which return the blood to 

 the heart are called veins. Between the ultimate ramifications of the 

 arteries and the beginning of the veins there is an intermediate system 

 of very minute vessels called capillaries, which connect, the arterial 

 with the venous system of the circulation. The walls of the arteries 

 are possessed of a certain amount of rigidity, sufficient to keep the 

 tubes open when they are empty. 



The blood leaves the left ventricle through a single vessel, the com- 

 mon aorta, which divides into the anterior and posterior aortas, which 

 in turn give off the large arteries. 



The arteries divide and subdivide (like the branches of a tree), 

 become smaller and smaller, and ultimately ramify into every part of 

 the body, terminating in a network of very small tubes called capil- 

 laries, which can only be recognized by the aid of a microscope. The 

 capillaries terminate in veins. 



The veins take the blood from the capillaries in all parts of the body. 

 They begin in very small tubes, which unite to become larger in size 

 and less in number as they approach the heart. 



In its course an artery is usually accompanied by a vein and in many 

 situations by a nerve. The more important arteries are placed deep 

 within the body; but in those cases where they are superficial they 

 are generally found where least exposed to injury, as, for example, on 

 the inner side of the legs. Arteries are less numerous than veins, and 

 the total capacity of the arteries is much less than that of the veins. 

 A great number of veins are in the tissue immediately beneath the 

 skin, and these are not generally accompanied by arteries. 



The blood throughout its course, in the heart, arteries, capillaries, 

 and veins, is inclosed within these vessels. There is no opening into 

 the course of the blood, except where the large lymphatics empty into 

 the venous blood. 



All the arteries, except the pulmonary artery and its branches, 

 carry bright-red blood, and all the veins, except the pulmonary veins, 



