The =matrix= of the hoof (Matrix ungulæ) is the specially modified and
highly vascular part of the corium of the common integument which
furnishes nutrition to the hoof. It is convenient to divide it into five
parts which nourish corresponding parts of the hoof.
1. The =perioplic matrix= or =ring= (Margo matricis ungulæ) is a band 5
to 6 mm. in width which lies in a groove between the periople and the
coronary border of the wall. It is continuous above with the corium of
the skin, and is marked off by a groove from the coronary matrix. At the
heels it widens and blends with the matrix of the frog. It bears very
fine, short papillæ which curve downward and are received in depressions
of the periople, to which it supplies nutrition.
2. The =coronary matrix= or =cushion= (Corona matricis ungulæ) is the
thick part of the corium which occupies the coronary groove, and
furnishes nutrition to the bulk of the wall. It diminishes in width and
thickness posteriorly, and along the upper border of the bar it is not
very clearly defined from the matrix of the frog. The convex superficial
surface is thickly covered with papillæ 4 to 6 mm. in length, which are
received into the funnel-like openings of the coronary groove. At the
heels and along the bars the papillæ are arranged in rows, separated by
fine furrows. The deep surface is attached to the extensor tendon and
the lateral cartilages by an abundant subcutis which contains many
elastic fibers and a rich venous plexus.
3. The =laminar matrix= (Latus matricis ungulæ)—also termed the
sensitive laminæ—bears primary and secondary laminæ which are
interleaved with the horny laminæ of the wall and bars in the natural
state. It is attached to the wall surface of the third phalanx by a
modified periosteum (Stratum periosteale) which contains a close-meshed
network of vessels, and to the lower part of the lateral cartilages by a
subcutis which contains a rich venous plexus. The laminæ are small at
their origin above, become wider below, and end in several papillæ 4 to
5 mm. in length. The sensitive laminæ supply nutrition to the horny
laminæ and to the interlaminar horn of the white line.
4. The =sole matrix= (Latus volare matricis ungulæ)—also termed the
sensitive sole—corresponds to the horny sole, to which it supplies
nutrition. It is often more or less pigmented and bears long papillæ,
which are specially large along the convex border and at the angles.
Centrally it is continuous with the matrix of the frog and bars. The
deep surface is attached to the sole surface of the third phalanx by a
modified and highly vascular periosteum.
[Illustration:
FIG. 580.—LATERAL VIEW OF FOOT OF HORSE AFTER REMOVAL OF HOOF AND PART
OF SKIN. (After Schmaltz, Atlas d. Anat. d. Pferdes.)
]
[Illustration:
FIG. 581.—GROUND SURFACE OF FOOT OF HORSE AFTER REMOVAL OF HALF OF
HOOF. (After Schmaltz, Atlas d. Anat. d. Pferdes.)
]
5. The =frog matrix= (Furca matricis ungulæ)—also called the sensitive
frog—is moulded on the deep surface of the frog and bears small papillæ.
Its deep face is blended with the plantar cushion. The germinal cells of
the frog derive their nutrition from this part of the matrix.
The =digital= or =plantar cushion= (Torus digitalis) is an important
part of the elastic apparatus of the foot. It is a wedge-shaped mass
which overlies the frog. It presents for description four surfaces, a
base, and an apex. The =superior surface= faces upward and forward and
is connected with the distal fibrous sheath of the deep flexor tendon.
The =inferior surface=, covered by the matrix of the frog, is moulded on
the upper face of the frog. The =lateral surfaces= are related chiefly
to the lateral cartilages; inferiorly the cushion is closely attached to
the cartilages, but higher up a rich venous plexus intervenes. The
=base=, situated posteriorly, is partly subcutaneous, and is divided by
a central depression into two rounded prominences termed the bulbs of
the plantar cushion. The =apex= lies under the terminal part of the deep
flexor tendon. The cushion is poorly supplied with vessels. It consists
of a feltwork of fibrous trabeculæ and elastic fibers, in the meshes of
which are masses of fat. The bulbs are soft and loose in texture and
contain a relatively large amount of fat, but toward the apex the
cushion becomes denser and more purely white fibrous in structure.
Branched =coil glands= occur chiefly in the part of the cushion which
overlies the central ridge of the frog. Their ducts pursue a slightly
flexuous course through the matrix and pass in a spiral manner through
the frog. Their secretion contains fat.
[Illustration:
FIG. 582.—CROSS-SECTION OF DIGIT OF HORSE, THROUGH DISTAL END OF
SECOND PHALANX.
_a_, Second phalanx; _b_, deep flexor tendon; _b′_, digital synovial
sheath; _c_, plantar cushion; _d_, lateral cartilage; _e_, skin;
_f_, hoof; _g_, anterior extensor tendon; _h_, cavity of coffin
joint; _i_, suspensory ligament of navicular bone; _k_, lateral
ligament of coffin joint; _l_, digital artery; _m_, digital nerve.
(After Ellenberger, in Leisering’s Atlas.)
]
=Vessels and Nerves.=—The matrix is richly supplied with blood by the
=digital arteries=. The =veins= are valveless, and form remarkable
plexuses which communicate freely with each other and are drained by the
digital veins. The =lymph vessels= form subpapillary plexuses in the
matrix of the sole and frog, and a wider-meshed plexus at the base of
the plantar cushion. A considerable lymph vessel lies in the attached
edge of each of the laminæ. The =nerves= are branches of the digital
nerves; some fibers end in lamellar corpuscles and end-bulbs.
The =ergot= is a small mass of horn which is situated in the tuft of
hair at the flexion surface of the fetlock. It is the vestige of the
second and fourth digits of extinct equidæ and hence is absent in cases
in which these digits are developed. A small tendinous band, 3 to 5 mm.
in width, extends downward and slightly forward from the fibrous basis
of the ergot on each side, crosses over the digital vessels and nerves
very obliquely, and blends below with the digital fascia and the plantar
cushion. It is known as the ligament or tendon of the ergot.
The term =chestnut= is applied to the masses of horn which occur on the
inner surface of the forearm about a handbreadth above the carpus, and
on the lower part of the inner face of the tarsus. They have an
elongated oval form and are flattened. They are regarded usually as
vestiges of the first digit. That of the hind limb is absent in the
donkey and very small in the mule.
These horny vestiges are quite variable in form and size and are
correlated with the fineness or coarseness of the integument in
general. The supracarpal chestnut is about 1½ to 2½ inches long, oval
in outline, the proximal end being pointed; it overlies the flexor
carpi internus at a quite variable distance above the carpus, and
hence should not be used as a surgical landmark. The tarsal chestnut
lies at a point behind the lower part of the internal lateral ligament
of the hock. When well developed it is about 2 to 2½ inches long,
broad below and produced above to form a long pointed end, with a
short blunt anterior process. They are composed of horn somewhat like
that of the frog.