THE ANATOMY OF THE HORSE 49 



break up into very small ones which run into the capillary 

 network of the body. These are numerous, very small ves- 

 sels about 1/1000 of an inch in diameter. Their walls are 

 very thin and cannot be distinguished except under a micro- 

 scope. They are found in all parts of the body. As the 

 blood passes slowly through these small vessels, the nourish- 

 ment is absorbed from it through the very thin walls to 

 supply the tissues of the body. When the blood passes 

 through this capillary network it again enters into larger 

 vessels called the veins, which carry it on its way back to 

 the heart. 



II. THE BLOOD. 



The blood is that which carries nourishment to all parts 

 of the body, and also carries away all the waste material of 

 the body, where it is thrown off in the form of urine, which 

 is secreted from the blood by the kidneys. We find the waste 

 material also thrown off in sweat or perspiration through 

 the skin, and also by the lungs. The blood varies in color 

 in the different parts of the body — in the arteries it is a bright 

 red, while in the veins it is a dark red color. 



III. — COURSE OF THE BLOOD. 



Briefly speaking, the blood is pumped by the heart to 

 all parts of the body, passes through the capillary system 

 where it parts with its nourishment, is collected and returned 

 to the heart by the veins, is again pumped by the heart to 

 the lungs where it is purified and returned to the heart to 

 again commence the circuit as before. 



To trace the blood through the heart it is convenient 

 to commence at the right auricle into which chamber the 

 blood is carried by the anterior and posterior vena cavas. 

 These empty the impure blood of the body into the right 

 auricle of the heart. It is there guarded by two small valves 

 at the mouth of each vein, while the right auricle contracts, 

 forcing the blood down through the hole in the septum into 

 the right ventricle. It is here guarded by a valve to keep 

 the blood from flowing back, while the right ventricle con- 

 tracts and forces the blood up into the pulmonary artery, 

 which passes only two or three inches above the heart and 

 divides into two branches, one to the right lung and the other 

 to the left. These again divide into other small arteries, 

 which flow into what is known as the capillary network. 



