PHYSIOLOGY OF THE FOOT. 263 



but both crusts are originally secreted in a fluid state, and 

 subsequently become of the requisite consistency. 



If we take an extended view of the matter, and call in 

 the aid of comparative anatomy, we shall be led to con- 

 sider the uses of the frog as of a mixed nature. Eminently 

 simple in form and structure, it combines even more than 

 the qualities possessed by the tendo-cartilaginous balls at 

 the posterior part of the feet of the digitated tribes, as 

 dogs, cats, &c. It is more dense in structure, to resist the 

 weight imposed on it ; yet more truly elastic, to enable it 

 to preserve its own integrity under the increased pressure 

 of so large an animal; and to transfer the shock of motion 

 more uniformly over the limb. It is peculiar also in being 

 united at its apex ; but it is wisely disunited by its cleft at 

 its posterior portion, where it can best act on the yielding 

 walls of the hoof. This union, or separation indeed, arises 

 from the frog being formed of a continuation of the coronet 

 (see Plate X. Fig Z . g ; Fig 4: . c). The frog claims, 

 likewise, importance as a natural wedge, to prevent slipping 

 and sliding on smooth surfaces. It is reasonable also to 

 suppose, that it forms a valuable antagonist to the down- 

 ward and backward direction of the internal parts of the 

 foot ; and in this way it may offer a point of solid support 

 to the flexor tendon, and an important protection to the 

 navicular bone ; it also most unquestionably acts very 

 favourably in promoting the expansion of the hoof. 



The bars (see Plate X. Fig 3 . b b). — We have already 

 fully explained that the crust of the hoof does not termi- 

 nate at the heels, as a superficial observation might lead 

 one to conclude ; but that having gained the heels, it turns 

 inwards and forwards {Plate X. Fig 3 . h b), decreasing 

 in extent, and laying itself on each side of the frog. 



THE INTERIOR OF THE FOOT. 



The part that presents itself when the hoof is removed, 

 is the sensitive or fleshy sole (see Plate IX. Fig 3 . c) ; 

 which is a vascular expansion covering a considerable 

 portion of the under surface of the coffin bone, and ex- 

 tending beyond its posterior part, but leaving a triangular 

 space for the vascular expansion of the sensitive frog 

 (see Plate X. Fig 3 . a). As the sensitive sole passes 



