The distribution of the laminæ over the foot we have discussed in the chapter devoted to the grosser anatomy. In a macerated foot the sensitive laminæ of the corium interdigitate with the horny laminæ of the hoof; that is to say, there is no union between the two, for the simple reason that it has been destroyed; they simply interlock like the _unglued_ junction of a finely dovetailed piece of joinery. But no further, however, than the irregularities of the underneath surface of the epidermis of the skin can be said to interlock with the papillæ of the corium does interlocking of the horny and sensitive laminæ occur. It is only apparent. The horny laminæ are simply beautifully regular epidermal ingrowths cutting up the corium into minute leaf-like projections. In a macerated specimen, then, the exposed sensitive structures of the foot exhibit the corium as (1) the _Coronary Cushion_, fitting into the cutigeral groove; (2) the _Sensitive Laminæ_, clothing the outer surface of the terminal phalanx, and extending to the bars; (3) the _Plantar Cushion_, or sensitive frog; and (4) the _Sensitive Sole_. The main portion of the wall is developed from the numerous papillæ covering the corium of the coronary cushion. We have in this way numberless down-growing tubes of horn. Professor Mettam describes their formation in a singularly happy fashion: "Let the human fingers represent the coronary papillæ, the tips of the fingers the summits of the papillæ, and the folds of skin passing from finger to finger in the metacarpo-phalangeal region the depressions between the papillæ. Imagine that all have a continuous covering of a proliferating epithelium. Then we shall have a more or less continuous column of cells growing from the tip of the finger or papilla (a hollow tube of cells gradually moving from off the surface of the finger or papilla like a cast), and similar casts are passing from off all the fingers or papillæ." From this description it will be noticed that each down-growing tube of horn bears a striking resemblance to the growth of a hair, described on p. 47. In fact, the horn tube may be regarded as what it really is, a modified hair. We next continue Professor Mettam's illustration, and note how the modified hairs or horn tubes become as it were matted together to form the hoof wall. The cells lining the depressions are also proliferating, and their progeny serve to cement together the hollow casts of the papillæ, thus giving the _inter_-tubular substance. We have thus produced hollow tubes, united together by cells, all arising from the rete Malpighii of the coronary corium. Section of the lower part of the horn tubes shows them to contain a cellular debris.