.">> THE ANATOMY OF THE HORSE. 



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 represent 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, fiom 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 npper end of each tube is occupied by an 

 elongated vascular papilla, which belongs, in the case of the wall, to 

 the coronary cushion; iir 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 

 the hoof, is a stratum of soft protoplasmic epithelial cells by whose 

 growth and multiplication the hoof-horn is formed. This stratum of 

 cells represents the deepest cells of the rete mucosum in the skin, and 

 it is by its ready decomposition that the bond of connection between 

 the sensitive and insensitive structures is destroyed, permitting the 

 extremity of the digit to be extracted from its horny investment. 



Directions. — The student should next turn his attention to the ex- 

 tremity of the digit as exposed by the removal of the hoof, and he 

 will find it to present a configuration not unlike the exterior of the 

 hoof itself (Plate 10, Figs. 1 and 5). And in the first place, let 

 him examine that part which he will easily recognise as having been 

 separated from the inner surface of the wall. This is traversed by 

 a series of leaves which, in contradistinction to those already seen 

 on the inner surface of the wall, are termed the sensitive laminae, 

 and sometimes the podophyllous tissue. 



The Sensitive Lamina. Each lamina is fixed by one of its borders 

 to the periosteum of the os pedis, and extends in a vertical direction 

 from near the coronary cushion to the sharp edge of the bone, where it 

 terminates in five or six long papillae. When the laminae are slightly 

 magnified it can be seen that on each side they carry a number of 

 delicate secondary laminae. In the natural state the sensitive and the 

 horny laminae are interleaved, and the former here represent the corium, 



