_a_, Triceps brachii; _b_, brachialis; _c_, extensor carpi radialis;
_d_, common or anterior digital extensor; _d′_, _d″_, _d‴_, _d⁗_,
tendons of preceding; _e_, lateral digital extensor; _e′_, _f_,
tendons of preceding; _g_, extensor carpi ulnaris; _h_, _h′_, flexor
carpi ulnaris; _i_, extensor carpi obliquus (s. abductor pollicis
longus); _k_, interossei; _l_, branches from preceding to extensor
tendons; _1_, olecranon; _2_, radius; _3_, extensor epicondyle of
humerus. (After Ellenberger, in Leisering’s Atlas.)
]
The =biceps brachii= lies almost entirely on the inner surface of the
humerus. It is long and fusiform. The tendon of origin is round, and
passes through the capsule of the shoulder joint. The tendon of
insertion is bifid, one branch being attached to the ulnar, the other to
the radial tuberosity.
The =brachialis= is very little curved, and is inserted chiefly into the
tuberosity and inner border of the ulna.
The =tensor fasciæ antibrachii= is thin and narrow. It arises on the
outer surface of the latissimus dorsi, and ends on the olecranon and the
fascia of the forearm.
The =triceps= has an additional deep head (Caput accessorium), which
arises just below the head of the humerus.
The =brachio-radialis= is a long, narrow, delicate muscle, situated
superficially on the anterior surface of the forearm. It arises with the
extensor carpi on the crest above the extensor epicondyle of the
humerus, and is inserted into the distal part of the inner border of the
radius. It is often much reduced, and is sometimes absent. It rotates
the forearm and paw outward.
The =extensor carpi= divides into two parts. The larger outer part, the
=extensor carpi radialis brevis=, ends on the proximal end of the third
metacarpal bone. The inner and more superficial part, the =extensor
carpi radialis longus=, ends on the second metacarpal bone. (A tendon to
the fourth metacarpal may occur.)
The =extensor carpi obliquus= or =abductor pollicis longus= arises from
the outer border and anterior surface of the ulna, the interosseous
ligament, and the outer border of the radius. It is inserted into the
first metacarpal bone by a tendon which contains a small (sesamoid?)
bone. It abducts the first digit.
There are three (or four) =extensors of the digits=.
1. The =common digital extensor= (M. extensor digitalis communis) arises
on the extensor epicondyle of the humerus and the lateral ligament of
the elbow joint. It has four bellies, each terminating in a tendon.
These are inserted into the third phalanges of the second, third,
fourth, and fifth digits.
2. The =extensor of the first and second digits= (M. extensor pollicis
longus et extensor indicis proprius) is small, and is covered by the
common and lateral extensors. It arises on the proximal part of the
ulna. Its tendon passes down with that of the common extensor and
divides into two branches. The delicate inner branch ends on the first
digit, while the other blends with the tendon of the common extensor for
the second digit.
3. The =lateral digital extensor= (M. extensor digitalis lateralis)
consists of two muscles which are not rarely fused. They arise on the
extensor epicondyle of the humerus and the lateral ligament of the elbow
joint. The larger, =superficial belly= (M. extensor digiti tertii et
quarti) terminates on a tendon which divides at or near the carpus into
two branches, which are inserted into the third phalanges of the third
and fourth digits, blending with the corresponding tendons of the common
extensor. The =posterior belly= (M. extensor digiti quinti) terminates
by a tendon which fuses with that of the common extensor for the fifth
digit.
The =ulnaris lateralis= or =extensor carpi ulnaris= corresponds to the
flexor carpi externus of the horse, but is an extensor of the carpus. It
is a large flat muscle which lies on the outer surface of the ulna. It
arises on the extensor epicondyle of the humerus, and is inserted into
the proximal end of the fifth metacarpal and the accessory carpal bone.
[Illustration: