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A MANUAL OF VETERINARY PHYSIOLOGY 



Its chief support is the broad expansion of the perforans tendon 

 which passes beneath it ; between the tendon and the bone the most 

 intimate fitting occurs, and a synovial apparatus exists to save 

 friction. It is probable that the perforans tendon and the inferior 

 face of the navicular are more closely adapted to each other than 

 any articulation in the body, excepting some of those found in the 

 knee and hock joints. Briefly, then, the small dense navicular bone 

 is enabled to form a yielding articulation in the foot, owing to the 

 manner in which it is supported in position by the powerful per- 

 forans tendon. It might be urged on purely theoretical grounds 

 that a small bone thus placed in the foot would be very liable to 

 damage, and such is clinically the case. There is no intention here 

 to touch on the subject of navicular disease, excepting in so far as 



Fig. 220. — Longitudinal Section of the Foot. 



The corona ; 2, the pedis ; 3, the navicular ; 4, the horn wall ; 5, the horn sole ; 

 6, 6, the foot-pad ; 7, 7, the plantar cushion ; 8, the perforans tendon passing 

 under the navicular bone, to be inserted in pedis ; 9, the wall-secreting 

 substance ; 10, the extensor pedis tendon ; n, junction of wall and sole, the 

 ' white line.' 



it helps to elucidate the physiology of the part, but it is permissible 

 to regard the lesions of navicular disease in the light of a physiological 

 experiment, and to learn from them how intimately the freedom 

 and elasticity of a horse's action depend upon the navicular bone, 

 and the stilty, pottering, shuffling gait conferred on the animal when 

 this bone is no longer capable of performing its functions properly. 

 The very close support afforded to the navicular by the perforans 

 tendon may possibly be a cause of disease, for the conclusion has 

 been forced on the writer that, under the influence of the weight of 

 the animal, and the counteracting influence of the perforans tendon, 

 the navicular bone must be exposed to considerable compression 

 (see Fig. 220). This compression exists, not only during locomotion, 

 but also during standing. The only complete rest from compression 

 which the navicular bone of the fore-limb obtains is while the 

 animal is lying down. Those of the hind-limbs are relieved from 

 pressure every time the horse rests the leg by flexing the hock, 



