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 of¥ in the form of urine, which 
is secreted from the blood by the kidneys. We find the waSv'Q 
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, 
whkh flow into what is known as the capillary network. 
