80 DISEASES OF CATTLE. 



blood and close the uuriculo-ventricular openings, and prevent the re- 

 turn 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 pre- 

 vent the return of blood into the ventricles. (See Plate vu.) 



The heart is the most irritable muscle in the body; it has no rest 

 from the time the first few cells are formed that go to make up the 

 organ until its action is stopped by death, except during the repose or 

 momentary pause between the beats. 



The average weight of the heart of an ox is said to be from 3 to 4 

 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 ar- 

 teries 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. 



ARTERIES. 



The walls of the arteries are possessed of a certain amount of rigid- 

 ity sufficient to keep the tubes open when they are empty; this fact 

 led the ancients to believe that they contained air and hence their 

 name arteries. The walls of the arteries are composed of three coats 

 or tunics. The internal coat may be considered a prolongation of the 

 endocardium from the left ventricle. The middle coat is composed of 

 yellow elastic tissue and muscular tissue; the amount of each tissue 

 varies according to the size of the vessel. The muscular tissue being 

 contractile is required in some parts more than in others. In the aorta 

 and large arteries near the heart the elastic tissue predominates; in the 

 vessels of intermediate size the elastic and muscular tissues average 

 about equal proportions, while in the smallest arteries the muscular 

 tissue predominates and even comprises the entire middle coat. The 

 external coat is composed of a layer of thin but remarkably strong 

 fibrous tissue and a small proportion of elastic tissue. 



A ligature may be tied sufficiently tight around an artery to com- 

 pletely rupture the middle and internal coats without severing the ex- 

 ternal coat; and owing to the fact that the union is not complete between 

 the middle and external coats, the ruptured coats retract within the 

 external one, assisting to a great extent in surgery. 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 ofl' 

 the large arteries. 



