_a_, Abductor pollicis brevis et opponens pollicis; _b_, flexor
pollicis brevis; _c_, adductor pollicis; _d_, adductor digiti
secundi; _e_, adductor digiti quinti; _f_, flexor digiti quinti
brevis; _g_, abductor digiti quinti; _h_, interossei; _1_,
accessory carpal bone; _2_, first digit; _3–6_, sesamoids of
metacarpo-phalangeal joints. (Ellenberger-Baum, Anat. d. Hundes.)
]
The =ulnaris medialis= or =flexor carpi medius= (s. ulnaris) consists of
two quite distinct heads. The larger, =humeral head= arises on the
flexor epicondyle, while the smaller, superficial =ulnar head= arises on
the posterior border of the ulna. The tendons of the two end together on
the accessory carpal bone.
The =radialis volaris= or =flexor carpi internus= (s. radialis) arises
on the flexor epicondyle of the humerus and is inserted by a bifid
tendon into the second and third metacarpal bones.
The tendons of the foregoing muscles are provided with synovial sheaths
at the carpus.
The =pronator teres= is a round muscle, which is situated superficially
on the inner border of the proximal part of the radius. It arises on the
flexor epicondyle of the humerus, and is inserted into the internal
border of the radius. Its action is to flex the elbow and rotate the
forearm inward.
The =superficial digital flexor= is situated superficially on the
posterior and inner surfaces of the forearm. It arises on the flexor
epicondyle of the humerus and terminates near the carpus on a tendon
which passes downward outside of the carpal canal and receives below the
carpus two reinforcing bands, one from the accessory carpal, the other
from the sesamoid bone at the inner side of the carpus. Below this it
divides into four branches, which are inserted into the second phalanges
of the second, third, fourth, and fifth digits.
The =deep digital flexor= has =humeral=, =ulnar=, and =radial heads=,
the latter arising from the inner border of the radius. They unite on a
common tendon which passes down through the carpal canal, gives off a
branch to the first digit, and divides into four branches. These
perforate the tendons of the superficial flexor and are inserted into
the third phalanges of the second to the fifth digits. The tendons are
provided with synovial sheaths from the middle of the metacarpus
downward, and are held in place by three digital annular ligaments.
The =palmaris longus accessorius= (?) is a small muscle which arises
from the deep flexor below the middle of the forearm and ends by two
tendons which unite with those of the superficial flexor for the third
and fourth digits.
The =supinator= is a short, wide muscle which arises on the humerus
beneath the external lateral ligament, crosses the anterior surface of
the radius, and is inserted into the inner border of the radius. (A
pouch of the capsule of the elbow joint lies under the tendon of
origin.) Its action is to rotate the forearm outward (supination).
The =pronator quadratus= consists of fibers which cross the inner
surface of the interosseous ligament of the forearm, except at the two
extremities of the latter. It is attached to the inner border of the
ulna and the anterior surface of the radius. It rotates the forearm
inward (pronation).
The =palmaris brevis= (?) is a very small muscle, which arises on the
tendon of the superficial digital flexor for the fifth digit, and is
inserted at the fifth metacarpo-phalangeal joint into the sheath and
annular ligament of the deep flexor.
The =lumbricales= are three very delicate muscles, which arise on the
tendons of the deep flexor, and are inserted into the first phalanges of
the third, fourth, and fifth digits.
The =abductor pollicis brevis et opponens pollicis=, a very small pale
muscle, arises on the fibrous band which connects the superficial flexor
tendon with the inner carpal sesamoid, and ends on the distal end of the
first metacarpal bone and the first phalanx of the first digit. It
abducts the first digit.[76]
The =flexor pollicis brevis= arises on the posterior carpal ligament
over the second metacarpal bone, and ends on the volar sesamoid of the
first digit.
The =adductor pollicis=, situated externally to the preceding, is the
largest of the thumb muscles. It arises between the preceding and the
second interosseous muscle, and is inserted into the first phalanx of
the first digit.
The =adductor digiti secundi= is situated between the second
interosseous muscle and the adductor digiti quinti. It arises on the
carpal ligament, and ends on the first phalanx of the second digit.
The =adductor digiti quinti= arises close to the preceding muscle, and
passes outward to end on the first phalanx of the fifth digit.
The =flexor digiti quinti= arises on the ligament connecting the
accessory carpal to the third and fourth metacarpal bones, crosses the
corresponding interosseous muscle, and ends on the fifth digit with the
next muscle.
The =abductor digiti quinti= is larger than the two preceding muscles;
it arises on the accessory carpal bone, and ends on the outer sesamoid
of the fifth digit and on the lateral ligament.
There are four =interossei= which lie on the volar (posterior) surface
of the metacarpus. They are well developed and fleshy. They arise on the
distal row of the carpus and on the proximal ends of the metacarpals.
Each divides distally into two branches, which are inserted by small
tendons on the corresponding sesamoid bones, and detach slips to the
extensor tendons.