262 THE ANATOMY AND 



the frog is little else than an adaptation of the coronet 

 turned downward, and forced to bear a certain amount of 

 weight {Plate X. Fig S . g; Fig 4 . c) . 



The horny sole is not so brittle as the crust ; it is thicker 

 at the circumference, where it unites with the outer wall of 

 the hoof, than it is towards its centre. Its concavity enables 

 the foot to retain its situation upon the ground ; while it 

 forms an arch which yields by its elasticity, thus affording 

 an admirable spring. The horny frog is the triangular 

 portion that fills up the natural excavation of the sole (see 

 Plate X. Fig 3 . a). This horny fi'og swells out, but not so 

 much in general cases as to extend beyond the level of the 

 outer surface of the crust ; it however is sufficiently promi- 

 nent in every healthy foot as to receive secondary pressure. 

 An inferior view of the foot will show that the frog (see 

 Plate X. Fig 3 . white line between a and h) presents a 

 distinct mark of separation between it and the other part, 

 which is called its commissure; wherefore connexion is 

 principally kept up by an internal prolongation of the 

 secretive substance of the coronet (see Plate X. Fig 

 S . g). The horny frog is intimately connected to the 

 sensitive frog, of which last it is indeed the secretion. 

 The frog is divided, or indented with a deep fissure, which 

 is called its cleft (see Plate X. Fig 3 . e), within which 

 are numerous sebaceous glands. It is these glands, when 

 diseased, which pour forth a stinking fluid which rots 

 the horn, and is known by the name of thrush. The 

 sensitive part of the secretive frog is covered by the elastic 



Horn is enthely unorganized, and when once divided 

 can never again unite like flesh. It does not bleed when 

 cut. It has no feeling when a knife is drawn across it. 

 It has none of the properties of living substance, and con- 

 sequently cannot be expected to possess the reparative 

 function that appertains to hving matter. Horn, although 

 a secretion, and as such pretty much the same, is not all 

 of the like hardness. The horny frog is the least hard, 

 and the most pliable ; the horny sole ranks the next in 

 these qualities. Then come the horny heels ; afterwards 

 the quarters and bars ; and lastly, as the most hard, the 

 toe. The inner crust is throughout softer than the outer, 



