32 THE ARMY HORSE IN ACCIDENT AND DISEASE. 



Arteries are hollow structures or tubes, conveying the blood av/ay 

 from the heart, and veins are similar structures, bringing it back to 

 the heart. The walls of the tubes are thicker in arteries than in 

 veins. Veins have valves; arteries have none. Veins as well as 

 arteries branch off and diminish in size as they extend from the 

 heart. 



The smallest arteries are connected with the smallest veins by 

 minute vessels called capillaries, which are to be found in the tissue 

 throughout the body. They are too small to be seen with the 

 naked eye. 



Blood. 



The blood is a fluid which is the medium by which nutritive mate- 

 rial is conveyed to all tissues of the body. It is an opaque, thickish, 

 clammy fluid, with a peculiar odor and sickly, suline (salty) taste. 

 Its color varies in different parts of the same animal, that in the 

 arteries being a bright red or scarlet, while that in the veins is a dark 

 purple. 



Blood is composed of red blood corpuscles, or cells, and white 

 blood corpuscles, floating in a watery liquid called serum, which 

 contains the nutrient material absorbed by digestion, and certain 

 salts. 



The red cells convey the oxygen, and their presence in countless 

 numbers gives the bright-red color to the fluid. The white cor- 

 puscles act as a protection to all parts of the body in case of disease 

 or injury; they assist in the repair of injured tissue and destroy or 

 check invading germs. Blood cells can be seen only with the aid 

 of the microscope. 



Circuit of the hlood. 



The heart, from the action of its involuntary 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 off 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 pulmonary veins leading back to the 

 left auricle. 



The arterial, pure, or bright-red blood is then 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 



