72 DISEASES OF CATTLE. 



filled; contraction of the ventricles follows; the aiiriculo-ventricular 

 valves are forced up by the pressure of the blood and close the aiiric- 

 ulo-ventricular 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 ven- 

 tricles the heart is again in momentary repose and being filled with 

 blood, while the valves in the aorta and pulmonary aitery close to 

 prevent the return of blood into the ventricles. (See PL 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 sys- 

 tem of very minute vessels called capillaries, which connect the 

 arterial wuth 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 wdien 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 Aeins 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 exam- 

 ple, 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. 



