GENERAL OBSERVATIONS 67 
This wedge-shaped structure we have already described as 
occupying the irregular space between the two lateral 
cartilages, the extremity of the perforans tendon, and the 
horny frog. 
Now, when weight or pressure is exerted from above on 
to this organ, and the frog is in contact with the ground 
below, it is clear from the position the cushion occupies 
that, whatever change of form pressure from above will 
cause it to take, it must certainly be limited in various 
directions. 
a 7 b 
Fie. 37. 
a, The dotted lines in this diagram represent the expansion of the solar edge 
of the hoof at the moment of over-extension of the fetlock-joint ; 5, the 
dotted line represents the change in form of the coronary edge under 
similar circumstances. 
Because of the shape of the cushion its change of form 
cannot be forwards (simultaneous pressure from above and 
below on to this wedge with its apex forwards must tend to 
give it a backward change of form). Because of the pastern 
being horizontal, and aiding in the downward pressure, its 
change of form cannot be upwards. And because of the 
ground it cannot be downwards. It follows, therefore, that 
the movement must be backwards and outwards, being 
especially directed outwards because of its shape and the 
median lacuna in its posterior half—this latter, the lacuna, 
5—2 
