22 THE VETERINARY SCIENCE. 



CHAPTER V. 



CIRCULATORY SYSTEM. 



THIS is an important system to understand on account of it 

 being the means by which the various parts of the body 

 are fed or nourished. The principal points to consider 

 are the heart, arteries, capillary vessels, veins, and the very 

 important fluid they carry, called the blood. Considering the 

 heart, we find this the main organ of circulation ; it weighs about 

 six and one-half pounds in the average horse ; it acts as a force 

 pump to force the blood through the vessels already named. It 

 is made of strong muscular tissue, which acts involuntary, and is 

 situated between the lungs, which are divided by what is known 

 as the mediastinum, which is a division between the lungs and is 

 made up of two folds, the heart being between these. The bottom 

 end, or apex, of the heart is downwards and rests just above the 

 breast-bone; the base, or upper part, being directed upwards and 

 to the left side, the left lung having a hollow on its inside for the 

 Iieart to work in. There is a covering or sack around the heart 

 which helps to protect and support it in its place, attached above 

 to 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 appear- 

 ance ; inside it is smooth, and has numerous small glands which 

 secrete an oily substance called serous fluid, which 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 one of these 

 parts are again divided into an upper cavity called auricle and a 

 lower cavity called ventricle ; thus we have the right and left 

 ventricle and right and left auricle. The right auricle communi- 

 cates with the right ventricle through 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 auricle communicates with the left ventricle, same as on 

 the right side. The right side of the heart is sometimes called the 

 venous ; this side only deals with the impure blood. The left side 

 is sometimes called arterial side, and only deals with pure blood ; 

 this side of the heart is very much stronger and thicker than the 

 riirht side. 



