DISSECTION OF THE ANTERIOR LIMB. 37 



the sole presents a convex border, which unites it intimately to the 

 lower border of the wall, a line of whitish horn marking the junction of 

 the two structures. Posteriorly it has a deep V shaped indentation, 

 into the central point of which the frog penetrates, while behind that on 

 each side it is related to the bar. 



The sole of the hind hoof is distinguished from that of the fore by 

 being more vaulted, and by being more pointed (less circular) at the toe, 

 this latter difference affecting also the form of the wall in the same 

 region. The outer edge of the sole is more convex than the inner, 

 which enables one to readily distinguish between a right and a left hoof. 

 The Frog. This is a distinctly elastic mass of horn which, in a state 

 of nature, projects sufficiently to come into contact with the ground, 

 and thus give the animal a secure foothold. Its inferior surface shows 

 posteriorly a shallow cleft, or depression, termed the median lacunci. 

 The lateral lacnme lie at the sides of the frog, the outer boundary of 

 each lacuna being formed by the bar. The suj^erior surface shows, 

 vertically over the median lacuna, a projection termed the frog-stay. 

 On each side of the frog-stay this surface is depressed, and the whole is 

 moulded on the plantar cushion. This surface is punctated, and the 

 papillae of the plantar cushion are received into the minute apertures 

 The posterioi' extremity, or base, of the frog consists of two rounded emi- 

 nences — the hulhs, or glomes — separated from each other by the median 

 lacuna. The anterior extremity, or point, is wedged into the centre of the 

 sole. The lateral borders bring the frog into relation with the bars and 

 the sole, and there is an intimate union with each of these at the point 

 of contact. 



Minute Structure of the hoof. The entire hoof is an aggregation of 

 modified epithelial cells, which here repx'esent the horny layer of the 

 epidermis. When a thin section across the wall, sole, or frog is 

 examined, the horn substance is seen to be arranged in the form 

 of tubes, cemented together by an intertubular substance, and 

 containing within their lumen a quantity of intratubular material. 

 All of these — tubular, intertubular, and intratubular — are composed of 

 modified epithelial cells, differing in the three situations in the direction 

 of the cells, their state of aggregation, or the presence or absence of 

 contained pigment. The tubes of the wall are straight, and extend 

 parallel to the surface, from the coronary to the inferior edge of the 

 wall. The tubes of the sole have the same disposition, but those of the 

 frog are slightly flexuous. The upper end of each tube is occupied by 

 an elongated vascular papilla, Avhich belongs, in the case of the wall, to 

 the coronai-y cushion ; in the periople, to the perioplic ring ; and in 

 the sole and frog, to the sensitive structures of the same names. In the 

 growing hoof the bond of connection between these papillated surfaces 

 (which represent the corium of the skin) and the corresponding part of 



