e rete Malpighii covering the corium of the
foot. It becomes even more clear when we remember the prompt appearance of
horn in cases where a portion, or the whole, of the wall has been removed
by operation or by accident (see reported cases in Chapter VII.).

The activity of the cells of the rete Malpighii of the corium covering the
remainder of the foot will be quite as necessary as the activity of the
cells of the coronary papillae which form the horn tubes themselves. 'For,'
in Professor Mettam's own words, 'I am inclined to believe that much of the
"white line" which is found uniting the wall of the hoof to the sole has
been derived from the horn formed from the rete of the foot corium. This
origin will explain the absence of pigment from this thin uniting "line,"
as it does from the horn lining the interior of the wall. The cells of the
rete are free of colouring matter.'

[Illustration: FIG. 30.--SECTION THROUGH HOOF AND SOFT TISSUES OF A FOAL AT
TERM. The horn of the wall is shown, and the horn-core ('horny laminae') of
the epithelial ingrowth. The latter has advanced far into the corium, and
is now provided with abundant secondary laminar ridges (Mettam).]

From the matter here given us it is easy to understand how, in a macerated
foot, the appearance is given of interlocking of the sensitive and horny
laminae. We see that the horny laminae are ingrowths of the rete Malpighii,
ploughing into and excavating the corium into the shape of leaves--the
sensitive laminae. Putrefactive changes simply break into two separate
portions what originally was one whole, by destroying the cells along its
weakest part. This part is the line of soft protoplasmic cells of the rete
Malpighii. Thus the more resistant parts (the horn on the one hand, and the
corium covering the foot on the other) are easily torn asunder.

As a result of the evidence we have quoted, we are able to answer our
original question in the affirmative. Seeing that the horny and the
sensitive laminae are both portions of the same thing--namely, a modified
skin, in which the epidermis is represented by the horny laminae, and the
corium by the sensitive--it is clear to see that the cells covering the
inspreading horny laminae are dependent for their growth and reproduction
upon the cells with which they are in immediate contact--namely, those
of the sensitive laminae--and that therefore the sensitive laminae are
responsible for the growth of the horny.

B.