104 THE SKELETON OF THE HORSE 
face. 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 dorsal surface. 
The angles or wings (Anguli) are prismatic masses which project backward on 
either side; the medial 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 dorsal 
groove.! The proximal border carries the cartilage. 
The cartilages of the third phalanx (Cartilagines phalangis tertix)* are rhom- 
boid curved plates which surmount the angles on either side. They are relatively 
large and extend above the margin of the hoof sufficiently to be distinetly palpable. 
The abaxial surface is convex, the axial, concave. The proximal border is sinuous 
and thin; the distal 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 for- 
amina for the passage of veins. 
It will be noted that the size and form 
of the angles vary much in different specimens. 
In the new-born foal the angle is a small, 
pointed projection. Later the process of ossi- 
fication invades the lower part of the car- 
tilage to a varying extent. In some cases the 
greater part of the cartilage is ossified—a con- 
dition commonly termed ‘‘sidebone.”” In the 
young subject the cartilage is hyaline, but later 
it changes to the fibrous type. 
Development.—The ossification of 
the third phalanx is peculiar. While 
the proximal articular part is still ear- 
tilaginous, a perichondrial cap of bone 
is formed in relation to the hoof. Later 
the process extends into the upper part. 
Fic. 83.—PHALANGES OF Fic. 84.—PHALANGES AND Structure.—The interior of this 
NEWBORN OAL, DOR Distan Sesamor or — hone is channeled by numerous canals 
SAL VIEW. NeEw-BORN Foat, Vo- eee =O 
ore : View for vessels, most of which radiate from 
Cartilages of third phalanx LAR VIEW. . 
nenowed: Cartilages of third phalanx the semilunar canal to the dorsal sur- 
removed. face; these are not canals for nutrient 
vessels of the bone, but transmit 
arteries to the corium of the hoof. Thick layers of compact substance are found 
at the articular and flexor surfaces and the extensor process, 7. ¢., at the points of 
greatest pressure and traction. 
THE SESAMOID BONES 
The two proximal sesamoids (Ossa sesamoidea phalangis prime) are situated 
behind the distal end of the large metacarpal bone, and are closely attached to the 
first phalanx by strong ligaments. Each has the form of a three-sided pyramid. 
The articular surface (Facies articularis) conforms to the corresponding part of the 
1 The upper and lower divisions of the angle are sometimes termed the basilar and retrossal 
processes respectively. 
° These are usually called the lateral cartilages, but this designation could not be retained. 
They have the same relation to the third phalanx that the cartilage of the scapula has to the latter 
bone, and are here named in similar fashion, 
