409 



to recall the peciilifii" anatomy of the parts involved in the jirocess and 

 the functions which they perform in locomotion. It mnst he remem- 

 bered 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 mnst sustain as the bodj^ is thrown forward 

 upon them by the i^ropelling 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, tlie ease with which the arm closes on the shoul- 

 der 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 furtlier supplemented them b}' placing the coffin 

 joint on the hind i)art 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 further jirovision is made by j)lacing 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. Extraordi- 

 nary 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 completely relieve 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, 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 likely to suffer from this disease than 

 others, concussion of the foot and intense pressure on the tendon being 

 common attendants upon their usage. Besides these exciting causes 

 must be considered those which predispose to the disease. Most 

 prominent among these is heredity. It maybe claimed, however, that 

 an inherited in-edisposition to navicular disease consists not so much 

 in a special susceptibility of the tissues which are involved in the proc- . 

 ess as in a vice of conformation which, as is well known, is likely to 

 be transmitted 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. Finalh", the environments of 

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

 ing, punctured wounds, etc., all have their influence in developing 

 this disease. 



Symjifoms. — In the earlj- stages of navicular disease the sjmiptoms 

 are generally very obscure. When the disease begins in inflammation 



