120 THE EXTERIOR OF THE HORSE. 



retain it throughout their whole life. Under the influence of the 

 work to which the animal has been submitted, and poor and innutri- 

 tions alimentation, the double back often becomes single and the mus- 

 cular organization in general loses its volume and its high degree of 

 development. 



Conversely, the crest of the back becomes prominent and stands 

 out in relief over the surrounding muscles in subjects emaciated from 

 the ravages of age, or in those of a certain natural conformation whose 

 chest is narrow, the back slightly convex, the withers high, and the 

 muscles less voluminous. In the latter form the region is predisposed 

 to become sharp, and is more exposed to wounds from the saddle or 

 the harness. 



Length. The length of the back is in relation with the depth of 

 the chest and the rapidity of the gait. Its measurement, which requires 

 some judgment, is usually combined with that of the loins. The typical 

 back is regarded as equal in length to the distance between the dorsal 

 angle of the scapula and the external angle of the ilium or the haunch. 

 The determination of how much of this distance belongs to the loins 

 is of little practical utility, but it should be as small as possible. A 

 long back implies a corresponding length of the thoracic cavity whose 

 superior wall it forms. We know that it is indispensable to seek, for 

 any kind of work, those anatomical dispositions which imply the de- 

 velopment of the respiratory apparatus. Such development requires 

 a proper separation of the anterior from the posterior members ; and 

 this separation is in relation with rapidity of locomotion, in that it is 

 the expression of the length of the muscles which pass from the trunk 

 to the superior part of the limbs, as the psoas, great dorsal, and pectoral ; 

 also of those which occupy the costo-vertebral gutters, whose function is 

 the extension and propulsion of the vertebral column. Again, it en- 

 larges the base of support and leaves a sufficiently large area underneath 

 the trunk, circumscribed by the four feet, for the movements of the pos- 

 terior limbs. The latter are thus less liable to come in contact with 

 each other, and so to produce that disagreeable sound called forging. 



These advantages, however, are often diminished by the following 

 associated imperfections, which the shortness of the loins cannot always 

 rectify. 



The dorsal column being long, tends to alter its direction under 

 the impetus received from the posterior extremities during the move- 

 ments of progression. A certain part of this impulsive force is there- 

 fore necessarily lost at the expense of propulsion. Further, from the 

 augmented flexibility and diminished solidity of the vertebral column, 



