396 



gins in the bone, and tlie sesamoid sheath becomes involved subse- 

 quently by an extension of the iniiammatory process. (Plate xxxii, 

 Fig. 5.) 



The thoronghbred horse is more commonly affected with the disease 

 than any other, yet no class or breed of horses is entirely exempt. The 

 mule, however, 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. As a general rule 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 comiirehendfull^' how navicular disease maybe 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 ani- 

 mal is at rest, and that the faster he moves the greater is the shock 

 which the fore feet must sustain as the body is thrown forward upon 

 them 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, and so nature has fur- 

 ther 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 in the cannon, knee, and arm bones. A still fur- 

 ther provision is made by placing a soft, elastic pad — the frog and 

 plantar cushion — at the heels to receive the sesamoid exp;rnsiou of the 

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

 bone against the navicular. Extraordinary as these means may ap- 

 pear for the destruction of shock, and ample as they are when the ani- 

 mal is at a slow pace or unweighted by rider or load, they fiiil to com- 

 pletely relieve the parts from concussion and excessive pressure when- 

 ever the opposite conditions are present. The result, then, is that 

 the coronet bone forces the navicular hard against the flexor tendon, 

 which, in turn, i:>resses firmly against the navicular as the force of the 

 contracting muscles lifts the tendon into place. It is self evident, then, 

 that tlie 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, concussion of the foot and intense 

 pressure on the tendon being common attendants upon their usage. Be- 

 sides these exciting causes must be considered those which predispose to 

 the disease. Most prominent among these is heredity. It may be claimed, 

 however, that an inherited predisposition to navicular disease consists 

 not so much in a special susceptibility of the tissues which are involved 



