436 DISEASES OF THE HOBSE. 



the greater is the shock ^Yhich the fore feet must receive as the body 

 is tlirown for-\vard 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. Even these 

 means, however, are not sufficient within themselves to protect the 

 foot from injury ; so nature has further supplemented them by plac- 

 ing 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 destruc- 

 tion 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 the parts com- 

 pletely 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 flexQr 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 liability to injury. For 

 the same reason horses with excessive knee action are more liable to 

 suffer from this disease than othfers, concussion of the foot and intense 

 pressure on the tendon being common among such horses. 



Besides the above-mentioned 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 predis- 

 position to navicular disease consists not so much in a special sus- 

 ceptibility of the tissues which are involved in the process as in a 

 vice of conformation which, as is well known, is liable to be trans- 

 mitted from parent to offspring. The faults of conformation most 

 likely to be followed by the development of navicular disease are 

 an insufficient plantar cushion, a small frog, high heels, excessive 

 knee action, and contracted heels. Finally, the environments of 

 domestication and use, such as dry stables, heavy pulling, bad 

 shoeing, punctured wounds, etc., all have their influence in develop- 

 ing this disease. 



Symptoms.— \n the early stages of navicular disease the symptoms 

 are generally very obscure. When the disease begins in inflamma- 

 tion of the navicular bone, the animal while at rest points the 



