348 DISEASES OF THE FEET. 



of pastern, is the great cause. In addition to this violent 

 interference with the designs of nature, the elevation of the 

 heels furnishes additional cause of concussion to those already 

 stated, by removing the frog from the ground ; the frog being 

 elastic, tough, and strong, intended to touch the ground and 

 diffuse concussion, as well as to prevent slipping at each step 

 the animal takes. When left in its natural condition, it is a 

 large tough, softish body, giving way to the pressure of the 

 fingers when manipulated, expanding each time the foot is put 

 to the ground, the commissures at its sides admitting of this 

 expansion without the other parts of the foot being interfered 

 with. So long as it touches the ground, the sensitive parts 

 within, and particularly the navicular bursa for which it 

 furnishes a soft bed, rests upon a resilient body, free from 

 concussive danger. Besides furnishing this soft bed, it per- 

 forms another important function ; being elastic within and 

 without, it assists the flexors in the first act of elevating the 

 foot from the ground, and it does this in virtue of its inherent 

 power of assuming its original shape when the superincumbent 

 weight is taken away from it. Thus, whilst on the ground, 

 and bearing its proportion of weight, it is flattened from side 

 to side, bulging into the commissures and cleft, ready at every 

 movement, like a piece of india-rubber, to assume its proper 

 form when the least portion of the weight is taken from it. 

 When the flexors are acting, it becomes narrowed from side 

 to side, and deeper from surface to surface, and during this 

 assumption of its natural form, it gives an upward impetus to 

 the foot, which is of essential importance to its easy elevation. 

 The removal of this frog pressure during long periods of rest in 

 the stable is also a fertile source of the disease, because the frog 

 becomes hardened, and does not furnish a soft cushion for the 

 bursa to rest upon. The question may be very properly asked. 

 Why, if concussion be the cause of the disease, the caries does 

 not appear upon both synovial surfaces of the navicular bone ? 

 The answer to this may be found in the following remarks, 

 namely — Ist. That the lower surface and tendon were intended 

 to rest upon the soft resilient frog, and not upon a hardened 

 body, like what we find in improperly shod horses. 2d. That 

 the changes of tissue consequent upon functional activity is 

 greater upon the surface under which the tendon plays. 3d. 



