Professor Mettam commences by drawing attention to the error that has been made in this connection by studying the soft structures of the foot separated by ordinary putrefactive changes from the horny covering.  “In this way,” the writer points out, “a wholly erroneous idea has crept in as to the relation of the one to the other, and the two parts have been treated as two anatomical items, when, indeed, they are portions of one and the same thing.  As an illustration, and one very much to the point at issue, the soft structures of the foot are to the horny covering what the corium of the skin and the rete Malpighii are to the superficial portions of the epidermis.  Indeed, the point where solution of continuity occurs in macerating is along the line of the soft protoplasmic cells of the rete.”

In the foregoing description of the skin we have seen that the corium is not a plane surface, but that it is studded by numerous papillary projections, and that these projections, with the depressions between them, are covered by the cells of the epidermis.

The corium of the horse’s foot, however, although possessed of papillae in certain positions (as, for example, the papillae of the coronary cushion, and those of the sensitive frog and sole), has also most pronounced ridges (laminae) which run down the whole depth of the os pedis.  Each lamina again carries ridges (laminellae) on its lateral aspects, giving a section of a lamina the appearance of being studded with papillae.  We have already pointed out the ridge-like formation of the human nail-bed, and noted that, with the exception that the secondary ridges are not so pronounced, it is an exact prototype of the laminal formation of the corium of the horse’s foot.

The distribution of the laminae over the foot we have discussed in the chapter devoted to the grosser anatomy.  In a macerated foot the sensitive laminae of the corium interdigitate with the horny laminae 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 papillae of the corium does interlocking of the horny and sensitive laminae occur.  It is only apparent.  The horny laminae 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 Laminae, 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.