116 DISEASES OF CATTLE, SHEEP, GOATS, ETC. 



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 

 common 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 capillaries, 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 ex- 

 ample, 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 ar- 

 teries. The blood throughout its course, in the heart, arteries, capil- 

 laries, and veins, is inclosed within these vessels. There is no open- 

 ing 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, carry dark-red blood. The impure dark-red blood is collected 

 from the capillary vessels and carried to the right auricle by the 

 veins; it passes down into the right ventricle and thence into the 

 pulmonary artery, and through its branches to the capillaries of the 

 lungs, where the carbonic-acid gas and other impurities are given 

 up to the air in the air cells of the lungs (through the thin walls 

 between the capillaries and the air cells), and where it also absorbs 

 from the air the oxygen gas necessary to sustain life, which changes 

 it to the bright-red, pure blood. It passes from the capillaries to the 

 branches of the pulmonary veins, which convey it to the left auricle 

 of the heart; it then passes through the auriculo-ventricular open- 

 ing into the left ventricle, the contraction of which forces it through 

 the common aorta into the posterior and anterior aortas, and 

 through all the arteries of the body into the capillaries, where it parts 

 with the oxygen and nutritive elements and where it absorbs car- 

 bonic-acid gas and becomes dark colored. 



The branches of certain arteries in different parts unite again 

 after subdividing. This reuniting is called anastomosing, and as- 

 sures a quota of blood to a part if one of the anastomosing arteries 

 should be tied in case of hemorrhage, or should be destroyed by 

 accident or operation. 



BLOOD. 



The various kinds of food, after being digested in the alimen- 



