390 NAVIN ox THE HORSE. 



THE BLOOD-VESSELS. 



These are the heart, arteries, 'veins, and capillaries. They 

 form the channel through which the blood circulates. 



The heart is a pear-shaped body of considerable size, situated 

 mostly in the left side of the chest. Its larger end is forward, 

 while its pointed end is directed backward and toward the left 

 side. The heart is a large, muscular organ, having four cavi- 

 ties, or chambers, called the right and left ventricles, and the 

 right and left auricles. The ventricles have thick, strong, mus- 

 cular walls ; the left the stronger of the two. The auricles 

 have thin walls. The heart is surrounded by a tough, elastic 

 membrane, called the pericardium, which is attached to the di- 

 aphragm and the large blood-vessels in front. 



Two great blood-vessels pass out from the heart. Both are 

 arteries, one called the aorta, and the other the pulmonary ar- 

 tery. These arise from the ventricles. Three blood-vessels re- 

 turn to the right auricle, and two to the left. The first of these is 

 the vena cava. The other two are the pulmonary veins. There 

 is an opening between the right auricle and right ventricle, and 

 also between the left auricle and left ventricle ; but the two 

 sides of the heart are separated by a perfect partition. The 

 opening between the auricle and ventricle is closed by valves — 

 between the right auricle and ventricle, called the tricuspid 

 valves ; between the left auricle and ventricle, the mitral 

 valves. The opening of the large arteries leading from the 

 ventricles are closed by three valves, called the semilunar 

 (Jialf-moon) valves. The blood can pass through the valves in 

 the proper direction, but can not flow back through them. 



The blood, loaded with impurities, is brought irom all parts 

 of the body, by the veins and from the twf> vena cava, is poured 

 into the right auricle, which contracts and forces the blood 

 through the tricuspid valves into the right ventricle. This 

 then contracts and forces it through the semilunar valves intc 



