The =second phalanx= (Phalanx secunda)[23] is situated between the first
and third phalanges, its direction corresponding to that of the first
phalanx. It is flattened from before backward, and its width is greater
than its height. It may be described as possessing four surfaces.
The =upper= or =proximal surface= presents two glenoid cavities
separated by a low ridge, and articulates with the first phalanx. The
middle of the anterior border is elevated and roughened in front for the
attachment of the anterior or common extensor tendon. The posterior
border is thick and overhanging; in the fresh state its middle part is
covered with cartilage, over which the deep flexor tendon passes. On
either side there is an eminence, to which the lateral ligament and the
superficial flexor tendon are attached.
The =inferior= or =distal surface= is trochlear, and articulates with
the third phalanx and third sesamoid bone. It resembles somewhat the
trochlea of the first phalanx, but is more extensive and encroaches more
on the anterior and posterior surfaces.
The =anterior= or =dorsal surface= is convex from side to side and
smooth in its middle; on its lower part are lateral rough depressions,
surmounted by tuberosities, to both of which ligaments are attached.
The =posterior= or =volar= surface is smooth, flattened, and slopes
obliquely downward and forward. The borders which separate the anterior
and posterior surfaces are concave from above downward, rounded from
before backward.
=Development.=—The second phalanx ossifies like the first, but the
proximal end unites with the shaft two or three months earlier.
[Illustration:
FIG. 58.—PHALANGES AND THIRD SESAMOID OF HORSE, DORSAL ASPECT.
]