ing 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_.

The main portion of the wall is developed from the numerous papillae
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
papillae, the tips of the fingers the summits of the papillae, and the folds
of skin passing from finger to finger in the