CHAPTER V. 
CIRCULATORY SYSTEM. 
HIS is an important system because of it being the 
means by which the various parts of the body are 
fed or nourished. 
I. ORGANS OF THE CIRCULATORY SYSTEM. 
The principal organs to consider are the heart, arteries, 
capillary vessels, veins, and the very important fluid they 
carry, called the blood. 
1. The Heart. — The heart is the principal organ of circu- 
lation ; it weighs about six and one-half pounds in the aver- 
age horse and acts as a force pump to force the blood 
through the vessels already ' named. It is made of strong 
muscular tissue, which acts involuntarily, and is situated be- 
tween the lungs, which are divided by what is known as the 
mediastinum. This is a division between the lungs made up 
of two folds, the heart being between them. The bottom 
end,- or apex, of the heart is downward and rests just above 
the breast-bone; the base, or upper part, is directed upward 
and to the left side, the left lung having a hollow on its 
inside for the heart to work in. There is a covering or sack 
around the heart which helps to protect and support it in 
its place. It is attached above to the back-bone and below 
to th6 bones of the sternum, or breast-bone. This sack is 
made up of fibrous tissue and is of a whitish appearance ; 
inside it is smooth, and supplied with numerous small glands 
which secrete an oily substance called serous fluid. This 
lubricates the outer wall of the heart and the inner wall of 
the sack so that in action it does not irritate the walls. The 
cavity in the heart is divided into two parts, the right and 
left sides ; each of these parts is again subdivided. The upper 
cavity is called auricle and the lower cavity ventricle ; thus 
there are the right and left ventricle and right and left 
auricle. The right auricle communicates with the right ven- 
tricle by an opening in the septum, or partition in the right 
side of the heart. This opening is guarded by a valve to 
keep the blood from flowing back into the auricle. The left 
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