436 DISEASES OF THE HORSE. 
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. 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 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 liable to 
suffer from this disease than others, 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.—tin 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 
