36 THE ARMY HORSE. 



Circuit of the Blood. 



The heart, from the action of its invohmtary muscles, may be 

 likened to a force pump. The blood from the veins, venous or 

 impure blood, entering the right auricle of the heart, is pumped 

 into the right ventricle and thence through the pulmonary 

 artery (lung artery) into the lungs. 



In the lungs the pulmonary artery branches into small 

 arteries and then into capillaries which surround the air cells. 

 Here the blood gives olf carbonic acid gas and receives its 

 purifying supply of oxygen. The purified blood passes from 

 the capillaries into the small veins, which unite in the pulmo- 

 nary veins leading back to the left auricle. 



The arterial, pure, or bright-red blood is tlien pumped into 

 the left ventricle and thence into the arteries, small arteries, 

 and capillaries. In these last vessels it gives up the oxygen 

 supply to the tissues and receives the impure carbonic acid gas, 

 which causes it to change color. The dark impure blood is 

 then collected through the small veins into the larger veins and 

 thence into the right auricle from which it started. This round 

 or circuit, Avhich is constantly going on, gives rise to the name 

 circulation. 



It has been stated that arteries convey the blood away from 

 the heart and that veins return it. In supplying the body, 

 arteries carry pure blood and veins carry impure blood. 'Wlien, 

 how^ever, the impure blood is sent to the lungs for purification, 

 it is conveyed in an arter}^ and the pure blood returns in a vein. 

 These two important exceptions must be carefully noted. 



Arterial Ramification. 



The large artery given off from the left ventricle of the heart 

 is the common aorta., which passes upward and forward for 

 2 or 3 inches and divides into the anterior aorta and the 

 posterior aorta., supplying, respectively, the fore and hind 

 portions of the bod3\ 



The anterior aorta is very short (1 or 2 inches), passes 

 upward and forward under the trachea and between the lungs, 

 and divides into the right and left brachial arteries., each sup- 

 plying blood to one of the fore limbs and its neighboring 

 muscles. 



The right hradiial artery gives off a large branch, called the 

 common cartoid. There is no corresponding branch of the left 



