34 THE ARMY HORSE. 



CIRCULATION. 



(Plate VII.) 



The organs which convey the blood throughout the body are 

 the hearty arteries^ capillaries^ and oeins. 



The heart is a hollow organ, made iq) of involuntary muscles; 

 it is situated between the lungs, in the thoracic cavity, and 

 averages about 6| pounds in weight. It is divided into two 

 parts, right and left, separated by a muscular wall. Each 

 part contains two cavities, one above the other, th« upper 

 called the auricle and the lower the ventricle. The cavities 

 are connected by openings, which are guarded by valves to 

 prevent a back flow of the blood. 



Ai'teries are hollow structures or tubes, conveying the blood 

 away from the heart, and vei7is 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 

 material is conveyed to the solid tissues. It is an opaque, 

 thickish, clammy fluid, with a peculiar odor and sickly, saline 

 (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 jjresence in 

 countless numbers gives the bright-red color to the fluid. The 

 Avhite corpuscles 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. 



