NAVICULAR DISEASE. 



337 



placed than those of the fore limbs ? I think the most devoted 

 advocate of the theory will scarcely venture to answer in the 

 affirmative; but should he feel inclined to think so, let him watch 

 a horse in motion with a load behind him, and he will very soon 

 see that the flexors of the hind limbs are called more powerfully 

 into action than those of the fore ones ; for in addition to acting 

 as flexors of the feet and pasterns, they are extensors of the 

 hocks. If strain were the cause, navicular disease, owing to 

 the double function the flexor perforans is called upon to perform, 

 and the more powerful strain thrown upon it in the removal 

 of heavy loads, would be found in the hind, and not in the fore 

 feet. 



Fig. 59 represents the phalangeal bones in their naturally oblique 

 position, and it will be seen that from the upper part of the 

 long pastern (a) bone, down to the toe of the os pedis {c) is a con- 

 tinuous oblique line. This obliquity of position enables the bone to 

 act as a spring, for the purpose of modifying concussion. Every 

 horseman knows that the more oblique the pastern, the greater the 

 elasticity of step and freedom from jar ; that the pace is easy for the 

 horse, and delightful to the rider. The coronary bone (b) rests en- 

 tirely upon the os pedis, and the navicular bone (d) placed posteriorly 

 bears no weight, but gives increased leverage power to the tendon (e). 

 It is a muscular appendage, like the sesamoids, and is not intended 

 to be a weight-supporting bone. 



Navicular disease is due to the rheumatoid diathesis and to 

 concussion, and the liability to suffer from the first-named 

 cause originates in hereditary predisposition and accidental 

 circumstances ; and to the second in conformation (as narrow 

 feet with short upright pasterns), and in the unnatural altera- 

 tion of the relative position of the navicular and weight-bearing 



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