72 DISEASES OF CATTLE. 



veins; the auriculo- ventricular orifices being Tvidely open, the ven- 

 tricles also receive blood ; the auricles contract and the ventricles are 

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

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

 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, thence through 

 the arteries to all parts of the body. 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 PL VII.) 



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

 pounds ; but, of course, owing to the many breeds and sizes of cattle, 

 it must vary in different animals. 



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

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

 heart are called veins. The arteries divide and subdivide (like the 

 branches of a tree), become smaller and smaller, and ultimately 

 ramify into every part of the body. Between the ultimate ramifica- 

 tions of the arteries and the beginning of the veins there is an inters 

 mediate system of very minute vessels called capillaries, which con- 

 nect 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, consisting of the anterior and posterior aortas, which 

 give off the large arteries. 



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 with a vein and in 

 many situations with a nerve. The more important arteries are 

 placed deep within the body; when they are superficial, however, 

 they are generally found where least exposed to injury, as, for exam- 

 ple, on the inner side of the legs. Arteries are le&s numerous than 

 veins, and their total capacity is much less than that of the veins 

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

 skin and do not generally accompany arteries. 



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

 and veins, is inclosed within these vessels. Except where the large 

 lymphatics empty into the venous blood, there is no opening into the 

 course of the blood. 



All the arteries except the pulmonary and its branches carry 

 bright-red blood, and all the veins, except the puhnonary veins. 



