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."