outside toe; 4, the quarter; 5, entigeral groove; 6, horny laminæ.]

For observation the wall offers two faces, two borders, and two
extremities.

_The External Face_ is convex from side to side, but straight from the
upper to the lower border. Examined closely, it is seen to be made up of
closely-arranged parallel fibres running in a straight line from the upper
to the lower border, and giving the surface of the foot a finely striated
appearance. In addition to these lines, which are really the horn tubules,
the external face is marked by a series of rings which run horizontally
from heel to heel. These are due to varying influences of food, climate,
and slight or severe disease. This will be noted again in a later page. In
a young and healthy horse the whole of the external face of the wall
is smooth and shining. This appearance is due to a thin layer of horn,
secreted independently of the wall proper, termed the periople.

[Illustration: FIG. 20.--INTERNAL FEATURES OF THE WALL, FROG, AND SOLE
(MESIAN SECTION OF HOOF). 1, Horny laminæ covering internal face of wall;
2, superior border of wall; 3, junction of wall with horny sole; 4, the
cutigeral groove; 5, the horny sole; 6, the horny frog (that portion of
it known as the 'frog-stay'); 7, inverted V-shaped ridge on wall and sole
(known as the 'toe-stay'); 8, anterior face of wall; 9, inferior border of
wall.]

_The Internal Face_ of the wall, that adapted to the sensitive laminæ,
is closely covered over its entire surface with white parallel leaves
_(Keraphyllæ_, or horn leaves, to distinguish them from the _Podophyllæ_,
or sensitive leaves). These keraphyllæ dovetail intimately with the
sensitive laminæ, covering the os pedis. Running along the superior portion