(_c_) its anterior border, (_d_) its inferior border; 2, the os
pedis; 3, wing of os pedis.]

The upper border, sometimes convex, sometimes straight, is thin and
bevelled, and may easily be felt in the living animal. It is this border
that the digital vessels cross to gain the foot, and the border is often
broken by a deep notch to accommodate them. The inferior border is attached
in front to the basilar and retrossal processes, behind which it blends
with the plantar cushion. The posterior border is oblique from before to
behind, and above to below, and joins the preceding two. The anterior
border is oblique in the same direction, and is intimately attached to the
antero-lateral ligament of the pedal articulation. The cartilages of the
fore-feet are thicker and more extensive than those of the hind.

2. THE PLANTAR CUSHION on FIBRO-FATTY FROG.--Composed of a fibrous
meshwork, in the interstices of which are lodged fine elastic and
connective fibres and fat cells, this wedge-shaped body occupies the space
between the two lateral cartilages, the extremity of the perforans tendon,
and the horny frog. It offers for consideration an antero-superior and an
infero-posterior face, a base, an apex, and two borders.

The antero-superior face is in contact with the terminal expansion of the
perforans tendon. The infero-posterior face is covered by the keratogenous
membrane, and follows closely the shape of the horny frog, on whose inner
surface it is moulded. It presents, therefore, at its centre a single
conical prolongation, the _Pyramidal Body_, which is continued behind, as
is the horny frog, in the shape of two lateral ridges divided by a median
cleft. The _base_ of the cushion lies behind, and consists of two lateral
masses, _the Bulbs of the Plantar Cushion_. In front these are continuous
with the ridges of the pyramidal body, while behind they become confounded
with the lateral cartilages and the coronary cushion. The _apex_ is fixed
into the plantar surface of the os pedis, in front of its semilunar ridge.
The _borders_, right and left, are wider behind than before, and are in
relation with the inner faces of the lateral cartilages.

H. THE KERATOGENOUS MEMBRANE.

THE KERATOGENOUS, OR HORN-PRODUCING MEMBRANE, is in reality an extension
of the dermis of the digit. It covers the extremity of the digit as a sock
covers the foot, spreading over the insertion of the extensor pedis, the
lower half of the external face of