INFLAMMATION OF NAVICULAR BONE. 409 



NAVICULAR DISEASE. 



Navicular disease is an inflammation of the sesamoid sheath, in- 

 duced by repeated bruising or laceration, and complicated in many 

 cases by inflammation and caries of the navicular bone. In some 

 instances the disease undoubtedly begins in the bone, and the ses- 

 amoid sheath becomes involved subsequently by an extension of the 

 inflammatory process. (Plate XXXIV, 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 fully how navicular disease may be caused 

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

 sary 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 coffin 

 joint on the hind part of the coffin bone 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 slow pace or unweighted by rider or 

 load, they fail to relieve completely the parts from concussion and ex- 

 cessive 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, that the more rapid the pace and the greater the 

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



