The section is cut just above the ridges of the frog and bars and
parallel with the ground surface. _1_, Wall; _2_, sole; _3_, median
ridge of frog; _4_, lateral ridge formed by junction of frog and
bar; _5_, median furrow over apex of frog; _6_, laminæ of wall; _7_,
laminæ of bar.
]
The =apex= occupies the central angle of the concave border of the sole,
and forms a blunt, round prominence a little in front of the middle of
the ground surface of the hoof.
=Structure of the Hoof.=—The hoof is composed of epithelial cells which
are more or less completely keratinized except in its deepest part, the
=stratum germinativum=; here the cells have not undergone cornification,
and by their proliferation maintain the growth of the hoof. The cells
are in part arranged to form tubes which are united by intertubular
epithelium, and inclose medullary cells and air-spaces.
[Illustration:
FIG. 579.—FRONTAL SECTION OF HOOF OF HORSE, POSTERIOR PART VIEWED FROM
IN FRONT.
_1_, Wall; _2_, sole; _3_, bar; _4_, frog; _5_, median ridge of frog;
_6_, lateral ridge formed by junction of frog and bar; _7_, laminæ
of bar; _8_, laminæ of wall; _9_, coronary groove; _10_, periople of
heel.
]
The =wall= may be regarded as consisting of three layers. The external
layer consists of the periople and the stratum vitreum. The =periople=
is composed of soft, non-pigmented tubular horn. It is continuous with
the epidermis above, and extends downward a variable distance. Usually
it forms a distinct band somewhat less than an inch wide, except at the
heels, where it is much wider, and caps the angle of inflection of the
wall proper. The =stratum vitreum= is a thin layer of horny scales which
gives the wall below the periople its smooth, glossy appearance. The
=middle= or =tubular layer= forms the bulk of the wall, and is the
densest part of the hoof. Its horn tubes run in a straight direction
from the coronary to the ground border. In dark hoofs it is pigmented
except in its deep part. The internal or laminar layer consists of the
=primary= and =secondary laminæ= and is non-pigmented. The laminæ are
narrow and thin at their origin at the lower margin of the coronary
groove, but become wider and thicker below. At the junction of the wall
and sole they are united by interlaminar horn to form the white line.
Only the central part of the laminæ becomes fully keratinized. They are
composed of non-tubular horn in the normal state.
The =sole= consists of tubular and intertubular horn. The tubes run
parallel with those of the wall and vary much in size.
The =frog= is composed of relatively soft horn, which is much more
elastic than that of the wall or sole, and is not fully keratinized. The
horn tubes in it are slightly flexuous.
The hoof is non-vascular and receives its nutrition from the matrix. It
is also destitute of nerves.