142 THE HORSE 



The lungs are suspended in the chest through the 

 medium of blood-vessels and connective tissue. Like 

 other structures they are commonly liable to disease. 



The cavity of the chest is separated from that of the 

 belly by a single muscle known as the midriff, or the 

 diaphragm, through which there are several openings for 

 the passage of blood-vessels, the gullet, etc. 



The diaphragm is the chief muscle of inspiration. 



As previously stated the heart is enclosed in a sac — the 

 pericardium. It is a hollow muscular organ with a right 

 and a left compartment below, namely, the ventricles, 

 and a right and a left compartment above, the auricles, 

 the divisions between being formed of muscle in each case, 

 but the right ventricle is separated from the right auricle 

 by a valve, a remark which applies also to the left auricle 

 and ventricle. These valves are of extremely delicate 

 construction and their integrity is absolutely indispensable 

 for the maintenance of perfect health. Unfortunately, 

 however, they are often the seat of disease, more especi- 

 ally in that disastrous trouble chronic rheumatism. 



Impairment of the valves leads to their inefficiency, 

 and the result is that the circulation of the blood becomes 

 interfered with. As stated elsewhere in this chapter the 

 heart is composed of a special kind of muscle which 

 confers upon it its powers of contraction and expansion 

 for the distribution of blood throughout the body and 

 its subsequent return to the heart after having been 

 purified in the lungs. 



The animal body has two circulations, namely, the 

 pulmonary, or lesser, which is confined to the right side 

 of the body, and a left, or systemic one, for the general 

 supply of the whole body. In order to facilitate this 

 distribution of blood throughout the body the left side 

 of the heart is a great deal thicker than the right, hence 

 its power of contractihty is much greater than that of 

 the right. 



