THE CHEST 63 



essential to life . . . cannot be defective from excess of development. 

 If disproportion seems to exist in the trunk compared to the limbs, then 

 the latter are not properly constructed to support the former. But 

 generally, although excess may not be objectionable, the same cannot be 

 said of deficiency of development. 



" Our machine, having a weak chest and small abdomen, will be without 

 energy, without wind, and capable of very little exertion. Such an animal 

 will be a poor feeder and will not last long." 



With reference to the general dimensions of the trunk, it need only 

 be said here that ample depth and width throughout are of the first 

 importance to continued soundness and endurance. Undue length becomes 

 a defect in so far as it can only be acquired by such an increase in 

 the length of the back as to place the muscles of the latter at a distinct 

 mechanical disadvantage in raising the forehand. Moreover, as pointed 

 out by Captain Hayes, " The longer it is, the further removed will the 

 fore- and hind-limbs be from each other, and the less able will the animal 

 be to carry weight ", or to move it. It is a point of conformation equally 

 objectionable in the heavier and in the lighter types of horses, since the 

 solidity and strength of the back, so necessary in draught, is diminished 

 by undue length, and the action of the hind-limbs is transmitted with 

 less force along the spine. 



THE CHEST 



The cavity termed the thorax or chest has for its bony base the 

 dorsal vertebrae above, the sternum or breast-bone below, and the ribs 

 on either side. The spaces between the latter are filled in by the 

 intercostal muscles. In front it is limited by the neck, and behind by 

 the diaphragm — a broad, flat muscle which intervenes between it and the 

 abdominal cavity. 



The size and mobility of the chest walls, and consequently the volume 

 and breathing capacity of the lungs, are mainly due to the length and 

 disposition of the ribs, especially those situated behind the shoulder, where, 

 unlike those in front, the lower extremities have no fixed attachment 

 to the sternum below, and are greatly increased in length Iiy the addition 

 of tapering rods of elastic cartilage (fig. 49). The movements of the 

 ribs in ordinary breathing tend to increase the size of the chest during 

 inspiration and to diminish it during expiration, but the manner in which 

 these changes are eff'ected is not, as commonly supposed by some people, 

 by the rising and falling of the ribs at right angles with the long axis 

 of the body. It is found, on the contrary, that when the chest expands 



