The =third= or =ungual phalanx= (Phalanx tertia)[24] is entirely
inclosed by the hoof, to which it conforms in a general way. It presents
for examination three surfaces, three borders, and two angles or wings.
The =articular surface= (Facies articularis) faces upward and backward,
and is chiefly adapted to the distal surface of the second phalanx, but
a narrow flattened area along the posterior border articulates with the
third sesamoid. The =anterior= or =coronary border= forms a central
eminence, the =extensor= (or pyramidal) =process= (Processus
extensorius), to the front of which the anterior extensor tendon is
attached. On either side is a depression for the attachment of the
lateral ligament.
The =dorsal= or =wall surface= (Facies dorsalis) slopes downward and
forward. The angle of inclination on the ground plane is about 45 to 50
degrees in front. Laterally the height diminishes, and the slope becomes
steeper, especially on the inner side. From side to side the curvature
is almost semicircular. The surface is rough and porous, resembling
pumice stone somewhat. It is perforated by numerous foramina of various
sizes; a series of larger ones is situated on or near the lower border.
On either side the =dorsal= (or preplantar) =groove= (Sulcus dorsalis)
passes forward from the wing and ends at one of the larger foramina. In
the fresh state this surface is covered by the matrix of the wall of the
hoof. The =inferior= or =distal border= is thin, sharp, and irregularly
notched; there is commonly a wider notch in front.
The =volar= or =inferior surface= (Facies volaris) is arched, and
divided into two unequal parts by a curved rough line, the =semilunar
crest= (Crista semilunaris). The larger anterior area is
crescent-shaped, concave, and comparatively smooth; it corresponds to
the sole of the hoof, and may be termed the =sole surface=. The
posterior part is much smaller, and is semilunar; it is related to the
deep flexor tendon, and is hence called the =flexor= or =tendon surface=
(Facies flexoria). It presents a central prominent rough area, on either
side of which is the =volar= (or plantar) =foramen= (Foramen volare), to
which the =volar= (or plantar) =groove= (Sulcus volaris) conducts from
the wing. The foramina lead into the =semilunar canal= within the bone,
from which small canals lead to some of the foramina of the wall
surface. The deep flexor tendon is inserted into the semilunar crest and
the central rough area behind it.
The volar grooves and foramina transmit the terminations of the
digital arteries into the semilunar canal, where they meet and form a
terminal arch, from which branches pass through canals in the bone and
emerge through the foramina on the wall surface.
The =angles= or =wings= (Anguli) are prismatic masses which project
backward on either side; the inner one is usually the shorter. Each is
divided into upper and lower parts by a notch, or is perforated by a
foramen which leads to the vascular groove on the side of the wall
surface.[25] The upper border carries the lateral cartilage.
The =lateral cartilages= (Cartilagines ungulæ) are rhomboid curved
plates, which surmount the wings on either side. They are relatively
large and extend above the margin of the hoof sufficiently to be
distinctly palpable. The abaxial surface is convex, the axial concave.
The upper border is convex and thin; the lower is thicker and is in part
attached to the wing. The anterior end is attached by ligament to the
side of the second phalanx. The posterior end curves toward its fellow
at the heel, and is perforated by numerous foramina for the passage of
veins. The central part is mainly hyaline, the periphery mainly fibrous.
It will be noted that the size and form of the wings vary much in
different specimens. In the new-born foal the wing is a small, pointed
projection. Later the process of ossification invades the lower part
of the cartilage to a varying extent. In some cases the greater part
of the cartilage is ossified—a condition commonly termed “sidebone.”
=Development.=—The ossification of the terminal phalanx is peculiar.
While the proximal articular part is still cartilaginous, a
perichondrial cap of bone is formed in relation to the hoof. Later the
process extends into the upper part.
=Structure.=—The interior of this bone is channeled by numerous canals
for vessels, most of which radiate from the semilunar canal to the wall
surface; these are not canals for nutrient vessels of the bone, but
transmit arteries to the matrix of the hoof. Thick layers of compact
substance are found at the articular and flexor surfaces and the
extensor process, _i. e._, at the points of greatest pressure and
traction.