The forearm is covered on three sides by the muscles of this group,
leaving the inner surface of the radius for the most part subcutaneous.
The extensors of the carpus and digit lie on the anterior (dorsal) and
external part of the region, while the flexors occupy the posterior
(volar) surface.
The =fascia= of the =forearm= (Fascia antibrachii) forms a very strong
and complete investment for all the muscles of the region. The
=superficial fascia= is thin, and disappears at the carpus by fusing
with the deep fascia. The =deep fascia= is very strong and tendinous in
character. It furnishes insertion at its upper and inner part to the
tensor fasciæ antibrachii and posterior superficial pectoral muscles; at
its upper anterior and outer part, to aponeuroses from the
mastoido-humeralis and biceps. It is attached at the elbow by its deep
face to the outer tuberosities of the humerus and radius, to the ulna,
and to the lateral ligaments. On the inner surface of the forearm it
blends with the periosteum on the subcutaneous surface of the radius. It
is closely adherent to the surface of the extensor muscles, but is
rather loosely attached to the flexors. From its deep face are detached
=intermuscular septa=, which form sheaths for the muscles and are
attached to the underlying bones. The principal septa are: (_a_) One
which passes between the common extensor (in front) and the lateral
extensor and flexor carpi externus (behind); (_b_) one between the
common extensor and the extensor carpi radialis; (_c_) one between the
inner and middle flexors of the carpus.
The =carpal fascia= (Fascia carpi) is a direct continuation of that of
the forearm. It is attached chiefly to the tuberosities at the distal
end of the radius, to the accessory carpal (pisiform) bone, and to the
lateral ligaments. In front it forms the so-called =anterior annular
ligament= (Ligamentum carpi dorsale), bridging over the grooves and
binding down the extensor tendons and their synovial sheaths. Behind it
is greatly thickened and forms the =posterior annular= or =transverse
ligament= of the carpus (Ligamentum carpi transversum). This stretches
across from the accessory carpal bone to the internal lateral ligament
and the proximal extremity of the inner metacarpal bone. It thus
completes the carpal canal, in which lie the flexor tendons, their
synovial sheath, and the principal vessels and nerves of the region.
The =fascia= of the =metacarpus= and =digit= (Fascia metacarpea et
digitalis) is, in general, thinner than the preceding. It is attached to
the tendons, ligaments, and the exposed bony prominences—especially to
the small metacarpal bones. On the flexion surface of the fetlock joint
it is much thickened by fibers passing transversely from one sesamoid
bone to the other, forming an =annular ligament= which binds down the
flexor tendons in the sesamoid groove or canal. Distal to this is a
second thick quadrilateral sheet (Ligamentum vaginale) which covers and
is adherent to the tendon of the flexor perforatus. It is attached on
either side by two bands to the borders of the first phalanx, thus
firmly binding down the flexor tendons. A little further down a
crescentic =fibro-elastic sheet= covers the terminal expansion of the
deep flexor tendon. It is attached on either side by a strong band to
the side of the first phalanx about its middle.