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 the 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 



