_a_, Wing of third phalanx; _b_, coronary ridge; _c_, foramen for
digital artery; _d_, ungual surface of third phalanx; _e_, second
phalanx; _f_, first phalanx; _g_, elastic dorsal ligaments. (From
Leisering’s Atlas.)
]
The five =digits= have three phalanges each, except the first, which has
two. The third and fourth digits are the longest; the first is very
short and does not come in contact with the ground in walking. The
=first phalanges= of the chief digits have four-sided shafts, which are
slightly curved forward. The proximal end of each has a concave surface
for articulation with the metacarpal bone and is deeply notched behind.
The distal end has a trochlea for articulation with the second phalanx,
and lateral depressions for ligamentous attachment. The =second
phalanges= are about two-thirds of the length of the first phalanges.
The proximal articular surface consists of two cavities separated by a
sagittal ridge. The distal extremity is wider and flatter than that of
the first. The =third phalanges= correspond in general to the form of
the claws. The base has an articular surface adapted to the second
phalanx and is encircled by a collar of bone. The volar surface bears a
wing or tuberosity, and on either side of this is a foramen. The ungual
part is a curved rod with a blunt-pointed free end. It is rough and
porous. Its base forms with the collar previously mentioned a deep
groove, into which the proximal border of the claw is received. The two
phalanges of the first digit resemble in arrangement the first and third
phalanges of the other digits.
Nine =volar sesamoids= are usually present. Two are found at each
metacarpo-phalangeal joint of the chief digits. They are high and
narrow, articulate with the distal end of the metacarpal bone in front,
and have a small facet on the base for the first phalanx. On this joint
of the first digit there is usually a single flattened sesamoid, but
exceptionally two are present. The distal volar sesamoids remain
cartilaginous. A nodular =dorsal sesamoid= occurs in the anterior part
of the capsule of the metacarpo-phalangeal joints, and cartilaginous
nodules are found in a similar position in connection with the joints
between the first and second phalanges.