DISEASES. 699 



muscular slings which attach them to the trunk, but it is partly 

 diminished in the scapulo-humeral joint, which closes, notwith- 

 standing the resistance of the muscles implanted on its apex. The 

 remaining force is transmitted to the vertical column, represented 

 by the union of the radius, the carjDus and the metacarpus. Reach- 

 ing the digital region, this force is there decomposed. Part of it, 

 passing on the phalanx, loses itself and disaj^pears in front of the 

 horny box of the foot, the other being thrown uj)on the flexor 

 tendons, and finally upon the perforans, which distributes it to 

 the posterior parts of the foot, and to the navicular bone. It must 

 be observed that in this complex action of decomposition of the 

 shock, the os sesamoid, though pushed from before backward by 

 the OS coronse is, however, supported by the resistance of the per- 

 forans tendon. Consequently, both the bone and the tendon are 

 pressing upon each other, when the feet are placed on the ground, 

 throwing the body forward by the impulse of the hinder parts, 

 and thus press powerfully against each other. 



When this pressure takes place in an animal going full speed, 

 and a good and high stepper, it may commence by becoming 

 merely a slight confusion, but, if often repeated, the result may 

 be some lesion upon the corresponding surface of the bone and 

 of the tendon, or of the synovial which facilitates their move- 

 ments. But the energy of action in the animal cannot be con- 

 sidered the only producing cause of these lesions, as a vice of 

 conformation in the foot, a want of elasticity in its posterior parts 

 where the resisting power is diminished, may also produce it. 

 The disease, then, is observed in animals whose plantar cushion, 

 covered by a small, dry and atrophied frog, is itself badly devel- 

 oped, from being compressed between the bars, which are more 

 vertical, or the heels, which are more contracted; all these be- 

 ing conditions which diminish the flexibiUty of the back of the 

 foot. 



Two principal causes, then, co-operate in the genesis of navi- 

 cular disease, and are almost always present in animals thus 

 affected. On the one hand, it will appear among well-bred ani- 

 mals, especially those of English breeds, those from Hanover, 

 Mecklenburg and Normandy, which will be more affected. Loiset 

 and Lafosse, however, have seen it in common breeds, in animals 

 with flat feet and soft horns. Lafosse says he has seen it in mules. 

 But besides this influence of the breed, there is the effect of what 



