80 THE PROTECTIVE STRUCTURES OF THE FOOT. 



the small holes accommodating the horn-secreting papillse of 

 the perioplic ring. Fresh sections of the entire foot (fig. 41, 5'') 

 show that its surface is convex and that it extends into the 

 depression exhibited by the coronary band. 



Although the perioplic horn is most distinct and easily seen 

 at the upper border of the hoof, at which point it forms a light- 

 coloured ring and extends towards the bulbs of the heels, it is 

 by no means confined to these points. With the exception of 

 the bars it covers all portions of the wall (fig. 44, a"), giving 

 the hoof a more or less shiny appearance. Hoofs which have 

 been rasped, and the hoofs of horses which work continuously 

 in loose ground, have usually lost this thin layer, though it may 

 be found at the heels close to the frog, and near the upper 

 margin of the wall, whence it is less frequently rasped away. 

 The hoofs of young equines always show it. 



This sheath has been the subject of most varying views. 

 Some altogether deny its existence ; others regard it as a pro- 

 longation of the cuticle, but such views chiefly depend on want 

 of close observation. Leisering regards it as a layer of soft horn, 

 produced by the perioplic ring. It is easily seen in animals 

 which have been shod and in which the sheath has been partially 

 torn away. Macerated dead feet exhibit it very distinctly. 

 The soft striated prolongation from the perioplic ring, which 

 is then easy to follow, shows beyond doubt the direct com- 

 munication between the sheath and perioplic ring, and the 

 correctness of the statement that the sheath grows from the 

 ring. On drying, the whitish look of the sheath disappears and 

 is succeeded by a brittle, shiny appearance. 



The essential diflerence between the periople (and its expan- 

 sion at the heels) and the horn of the wall or frog is well seen 

 by studying the development of the parts. In a 20-24-week 

 foetus, a sharply marked strip will be found between the future 

 cutis and the hoof, indicating the position of the perioplic ring, 

 and showing no visible horn formation, wliilst the wall, sole, and 

 frog are already quite covered by masses of young horn. 



B. The middle sheath is produced by the coronary band, and 

 is by far the strongest of the three. It consists of a very 

 tough, strong, and durable horn, which scarcely swells up in 

 water and is the most difhcuit to cut of all the varieties of 

 horn. It forms the principal mass of the wall. The middle 



