The =hoof=[226] (Unguis) is the horny covering of the distal end of the
digit. It is convenient to divide it for description into three parts,
termed the wall, sole, and frog.
1. The =wall= is defined as the part of the hoof which is visible when
the foot is placed on the ground.[227] It covers the front and sides of
the foot, and is reflected posteriorly at an acute angle to form the
=bars=. The latter (Pars inflexa medialis, lateralis) appear on the
ground surface of the hoof as convergent ridges, which subside in front
and are fused with the sole; they are united with each other by the
frog. For topographic purposes the wall may be divided into an anterior
part or “toe” (Pars ungulæ dorsalis), lateral parts or “quarters” (Pars
ungulæ medialis, lateralis), and the angles or “heels” (Pars ungularis
medialis, laterales). It presents two surfaces and two borders.
The outer surface is convex from side to side and slopes obliquely from
edge to edge. In front the angle of inclination on the ground plane is
about 50° for the fore limb, 55° for the hind limb; on the sides the
angle gradually increases and is 100° at the angles. The curve of the
wall is wider on the external than on the internal side, and the
external quarter is more oblique than the internal one. The surface is
smooth and is crossed by more or less distinct ridges, which are
parallel with the coronary border and indicate variations in the
activity of the growth of the hoof. It is also marked by fine parallel
striæ, which extend from border to border in an almost rectilinear
manner.
The =slope= of the wall varies considerably in apparently normal
hoofs. Lungwitz found by careful measurements of 56 fore and 36 hind
feet the following average angles:
FORE FOOT HIND FOOT
Toe 47.26° 54.1°
Inner angle 101.57° 96.50°
Outer angle 101.37° 96.1°
The wall of the fore foot may even be more upright than that of the
hind, and may have an angle of 60°. The =length= of the wall at the
toe, quarters, and heels is in the ratio of about 3 ∶ 2 ∶ 1 in the
fore foot and about 2 ∶ 1½ ∶ 1 in the hind hoof.
The inner surface is concave from side to side, and bears about six
hundred thin =primary laminæ= (Cristæ ungulæ), which extend from the
coronary groove to the junction of wall and sole. Each bears a hundred
or more =secondary laminæ= on its surfaces, so that the arrangement is
pennate on cross-section. These horny laminæ are continued on the inner
surface of the bars, and alternate with corresponding laminæ of the
matrix.
The =proximal= or =coronary border= (Margo occultus) is thin. Its outer
aspect is covered by a layer of soft horn known as the =periople=; this
appears as a ring-like prominence above and gradually fades out below;
at the angle it forms a wide cap or bulb and blends centrally with the
frog. The inner aspect of the border is excavated to form the =coronary
groove= (Sulcus coronalis ungulæ), which contains the thick coronary
matrix. The groove is deepest in front, narrows on the sides, and is
wide and shallow at the heels.[228] It is perforated by innumerable
small, funnel-like openings which are occupied by the papillæ of the
coronary matrix in the natural state.
[Illustration:
FIG. 573.—DIGIT OF HORSE, SHOWING SURFACE RELATIONS OF BONES AND
JOINTS. THE LATERAL CARTILAGE IS EXPOSED.
_a_, First phalanx; _b_, second phalanx; _c_, third phalanx; _d_,
lateral cartilage; _e_, third sesamoid or navicular bone; _f_,
pastern joint; _g_, coffin joint; _h′_, cut edge of wall of hoof
(_h_); _i_, laminar matrix. (After Ellenberger, in Leisering’s
Atlas.)
]
The =distal= or =ground border= (Margo liber) of the unshod hoof comes
in contact with the ground. Its thickness is greatest in front and
decreases considerably from before backward on the sides, but there is a
slight increase at the angles. Its inner face is united with the
periphery of the sole by horn of lighter color and softer texture, which
appears on the ground surface of the hoof as the =white line=.
In the case of horses at liberty the wall is usually worn off to the
level of the adjacent sole, but if the ground is too soft the wall is
likely to become unduly long and split or break or undergo
deformation. On very hard or rough ground, on the other hand, the wear
may be in excess of the growth. In the case of shod horses it is
necessary to remove the excess of growth of the wall at each shoeing.
The =thickness= of the wall at the toe, quarters, and heels is about
in the ratio of 4 ∶ 3 ∶ 2 for the fore foot and about 3 ∶ 2½ ∶ 2 for
the hind foot.
2. The =sole= (Solea ungulæ) forms the greater part of the ground
surface of the hoof. It is somewhat crescentic in outline, and presents
two surfaces and two borders.
The =superior= or =inner surface= is convex, and slopes with a varying
degree of obliquity downward to the convex border. It presents numerous
small funnel-like openings which contain the papillæ of the sole matrix
in the natural state.
[Illustration:
FIG. 574.—CROSS-SECTION OF FOOT OF HORSE, CUT PARALLEL WITH THE
CORONARY BORDER.
]
The =inferior= or =ground surface= is the converse of the preceding. It
is normally arched—and more strongly in the hind than in the fore
foot—but the curvature is subject to wide variation; in heavy draft
horses the sole is commonly less curved than in the lighter breeds and
may even be flat. The surface is usually rough, since the horn
exfoliates here in irregular flakes.
[Illustration:
FIG. 575.—HALF OF HOOF OF HORSE, INTERNAL SURFACE
]
The =convex border= is joined to the wall by relatively soft horn,
previously referred to as forming the white line on the ground surface
of the hoof. The angle of junction is rounded internally and presents a
number of low ridges and specially large openings for the papillæ of the
matrix. There is frequently a ridge of larger size at the toe.
The =concave border= has the form of a deep angle which is occupied by
the bars and frog. By its junction with these it forms two pronounced
ridges in the interior of the foot. The parts of the sole between the
wall and bars may be termed its angles.
3. The =frog= (Furca ungulæ) is a wedge-shaped mass which occupies the
angle between the bars and sole, and extends considerably below these on
the ground surface of the foot. It may be described as having four
surfaces, a base, and an apex.
[Illustration:
FIG. 576.—INTERNAL SURFACE OF PART OF HOOF OF HORSE. (After Schmaltz,
Atlas d. Anat. d. Pferdes.)
]
The =superior= or =internal surface= is deeply concave from side to
side. It bears a central ridge, the =spine= or “=frog-stay=” (Spina
furcæ ungulæ), which is high posteriorly and subsides abruptly in front.
On either side of this is a deep depression, bounded outwardly by the
rounded ridge formed by the junction of the frog with the bar and sole.
This surface presents fine striæ and openings for the papillæ of the
frog matrix.
The =inferior= or =ground surface= presents a =central furrow= (Sulcus
intercruralis), which is bounded by two ridges or crura (Crura furcæ
ungulæ); these converge and form the apex.
The =sides= (Facies medialis et lateralis) are united at the upper part
with the bars and sole, but are free below and form the central wall of
the deep =lateral furrows= (Sulci cruroparietales), which are bounded
outwardly by the bars.
The =base= is wide and high. It is depressed centrally and prominent at
the sides, where it unites with the angles of the walls; the junction
here is covered by the expanded periople and constitutes the bulb of the
hoof.
[Illustration: