DISEASES OF THE HOESE. 409 



becomes involved subsequently b}^ an extension of the inflaraniatory 

 process. (Plate XXXII, fig. 5.) 



The Thoroughbred horse is more commonly affected than any other, 

 yet no class or breed of horses is entirely exempt. The mule, how- 

 ever, seems rarely, if ever, to suffer from it. For reasons which will 

 appear when considering the causes of the disease, the hind feet are 

 not liable to be affected. Usually but one fore foot suffers from the 

 disease, but if both should be attacked the trouble has become chronic 

 in the first before the second shows signs of the disease. 



Causes. — To comprehend full}" how navicular disease ma}' be caused 

 by conditions and usages common to nearly all animals, it is necessary 

 to recall the peculiar anatomy of the parts involved in the process and 

 the functions which they perform in locomotion. 



It must be remembered that the fore legs largely support the weight 

 of the body when the animal is at rest, and that the faster he moves 

 the greater is the shock which the fore feet must receive as the body 

 is thrown forward by the propelling force of the hind legs. This 

 shock could not be withstood by the tissues of ^ the fore feet and legs 

 were it not that it is largely dissipated by the"' elastic muscles which 

 bind the shoulder to the body, the ease with which the arm closes on 

 the shoulder blade, and the spring of the fetlock joint. But even these 

 means are not sufficient within themselves to protect the foot from 

 injury; so nature has further supplemented them by placing the coflin 

 joint on the hind part of the coffinbone instead of directly on top of 

 it, whereby a large part of the shock of locomotion is dispersed before 

 it can reach the vertical column represented by the cannon, knee, and 

 arm bones. A still further provision is made by placing a soft, elastic 

 pad — the frog and plantar cushion — at the heels to receive the sesamoid 

 expansion of the flexor tendon as it is forced downward by the pressure 

 of the coronet bone against the navicular. Extraordinary as these 

 means may appear for the destruction of shock, and ample as they are 

 when the animal is at a sli)w pace or unweighted by rider or load, they 

 fail to relieve completely the parts from concussion and excessive 

 pressure whenever the opposite conditions are present. The result, 

 then, is that the coronet bone forces the navicular hard against the 

 flexor tendon, which, in turn, presses firmly against the navicular as 

 the force of the contracting muscles lifts the tendon into place. It is 

 self-evident, then, Ihat the more rapid the pace and the greater the 

 load, the greater must these contending forces be, and the greater, the 

 liability to injury. For the same reason horses with excessive knee 

 action are more likely to suffer from this disease than others, concus- 

 sion of the foot and intense pressure on the tendon being common 

 among such horses. 



Besides the above exciting causes must be considered those which 

 predispose to the disease. Most prominent among these is heredity. 



