50 
THE VETERINARY SCIENCE 
This network is situated around the air cells of the lungs, 
where, by a process, the blood gives off carbonic acid gas, 
which is breathed out along with the air. The blood takes in 
the oxygen from the pure air, which changes the color of the 
blood from a black red to* a bright red. This is a point of 
importance to note as it shows that stables should be well 
ventilated so that animals can have plenty of pure air. The 
blood now flows into the pulmonary veins, which carry the 
pure blood back to the heart to be emptied into the left 
auricle. Here these veins are guarded with valves to prevent 
the blood from flowing back while the left auricle contracts 
to force the blood down through the opening mentioned 
before into the left ventricle. This opening is also guarded 
by valves to prevent it from flowing back while the left 
ventricle contracts, with great force, to drive the blood up 
into the common aorta, the largest artery in the body. This 
artery passes above the heart two or three inches, and, just 
below the backbone. It breaks into branches, one passing 
forward and supplying the parts of the body in front of the 
heart, the other branch passing backward under the spinal 
bones, supplying the parts of the body behind the heart. 
The branch which goes forward runs just a few inches 
under the spine where it breaks into other branches, some 
going to supply the shoulder and front leg; the other 
branches being two large ones, pass on each side of the 
neck under the jugular vein. They are called carotid arteries. 
These give off small branches, as they pass up the side of 
the neck, to feed the muscles and other parts thereof, while 
just below the butt of the ear, this artery breaks into three 
large branches, which go to supply the brain and different 
parts of the head. The branch which runs backward from 
the heart is a very long, large artery, passing just below the 
spine, between the kidneys, breaking up about six inches 
behind them into four large branches — two on the left and 
two on the right side. That on the left goes to supply the 
left hip and organs in the pelvic or hip ^cavity, while the 
other passes down the left leg to supply it with blood; that 
on the right side helps to supply the right hip and pelvic 
cavity, while the other passes down the right leg. This large 
branch, in passing back along the spine, gives off small 
branches — one to the liver, one to the spleen, one to the 
stomach, branches to the large and small bowels, and one to 
each kidney. 
