22 THE PERFECT HOKSE. 



ipurified only by coming in contact, in passing through 

 the lungs, with atmospheric air. When quiet, the blood 

 in an animal's body moves through the circulating chan- 

 nels slowly, and respiration is performed easily : but in 

 exertion the circulation is quickened ; the blood is pumped 

 in and shot out of the heart with great rapidity ; the 

 breathing becomes labored, and a fearful pressure is put 

 upon the lung substance ; the multitudinous air-cells are 

 dilated, and exposed to a strain which nothing but the 

 strongest possible texture can withstand. In addition to 

 this, the reader must bear in mind that the blood that is 

 brought back to the heart after having gone the rounds 

 of the system is in an impure condition, and can only be 

 purified by the oxygen taken with every breath into the 

 lungs ; so that the lungs and heart work, as it were, in 

 unison, and are mutually dependent one upon the other. 

 Every ounce of blood circulated by the heart must re- 

 ceive a certain amount of air from the air-cells of the 

 lungs ; and, as the rapidity of the circulation is gauged 

 by the degree of exertion put forth, it follows that the 

 capacity of the heart and lungs decides, in a great meas- 

 ure, the amount of exertion which the horse can put forth. 

 To illustrate : The faster he goes, the greater the number 

 of heart-beats and the amount of air required ; so that 

 the capacity of the heart and lungs really decides (the 

 proper temperament and muscular strength being grant- 

 ed) the speed of the horse. Hence the necessity of 

 paying special attention to the shape and size of the 

 chest, in which the heart and lungs are placed. 



