This group consists of five muscles which are grouped around the
humerus. They arise from the scapula and the humerus, and are inserted
into the forearm. They act on the elbow joint and the fascia of the
forearm.
[Illustration:
FIG. 188.—BICEPS AND BRACHIALIS MUSCLES OF HORSE. (After
Ellenberger-Baum, Anat. für Künstler.)
]
1. =Biceps brachii= (Coraco-radialis; flexor brachii).—This is a strong,
somewhat rounded muscle, which lies on the anterior surface of the
humerus.
_Origin._—The tuberosity of the scapula.
_Insertion._—(1) The bicipital tuberosity of the radius; (2) the
internal lateral ligament of the elbow joint; (3) the fascia of the
forearm and the tendon of the extensor carpi radialis.
_Action._—To flex the elbow joint, to fix the shoulder, elbow, and
carpus in standing, and to tense the fascia of the forearm.
_Structure._—The muscle is inclosed in a double sheath of fascia, which
is attached to the lips of the bicipital groove and the deltoid ridge of
the humerus. The tendon of origin is moulded on the bicipital groove; it
is very strong and dense and is partly cartilaginous. Its play over the
groove is facilitated by the large =bicipital bursa= (Bursa
intertubercularis). The synovial membrane covers not only the deep face
of the tendon, but extends somewhat over the edges to the superficial
face. A well-marked tendinous intersection runs through the belly of the
muscle and divides distally into two portions. Of these, the short,
thick one is inserted into the bicipital tuberosity and detaches fibers
to the internal lateral ligament. The long tendon (Lacertus fibrosus) is
thinner, blends with the fascia of the forearm, and ends by fusing with
the tendon of the extensor carpi radialis.
_Relations._—Externally, the mastoido-humeralis and brachialis muscles;
internally, the posterior deep pectoral and the superficial pectoral
muscles; in front, the anterior deep pectoral muscle; behind, the
humerus, the coraco-brachialis muscle, the anterior circumflex and
anterior radial vessels, and the musculo-cutaneous nerve.
_Blood-supply._—Branches of the brachial and anterior radial arteries.
_Nerve-supply._—Musculo-cutaneous nerve.
2. =Brachialis= (Humeralis obliquus s. externus; brachialis
anticus).—This muscle occupies the musculo-spiral groove of the humerus.
_Origin._—The proximal third of the posterior surface of the humerus.
_Insertion._—The inner surface of the neck of the radius (under cover of
the lateral ligament) and the arciform ligament.
_Action._—To flex the elbow joint.
_Structure._—The peculiar spiral course of this muscle gave rise to the
name often applied to it—humeralis obliquus. Beginning on the posterior
surface of the shaft, close to the head of the humerus, it winds over
the lateral surface, crosses the biceps very obliquely, and finally
reaches the inner side of the forearm by passing between the biceps and
the extensor carpi. It is entirely fleshy, with the exception of its
relatively slender tendon of insertion.
_Relations._—Externally the skin and fascia, the teres minor, deltoid,
triceps (lateral head), biceps, and mastoido-humeralis muscles. The
anterior radial artery crosses the deep face of the muscle in its distal
third, and the radial nerve accompanies the muscle in the distal half of
the musculo-spiral groove.
_Blood-supply._—Brachial artery.
_Nerve-supply._—Radial nerve.
3. =Tensor fasciæ antibrachii= (Fig. 187)[58] (Scapulo-ulnaris; long
extensor of the forearm; accessory anconeus of the latissimus
dorsi).—This is a thin muscle which lies on the inner surface of the
triceps.
_Origin._—The tendon of insertion of the latissimus dorsi and the
posterior border of the scapula.
_Insertion._—(1) The fascia of the forearm; (2) a small eminence on the
posterior border of the olecranon.
_Action._—(1) To tense the fascia of the forearm and to extend the elbow
joint.
_Structure._—The origin consists of a very thin aponeurosis which blends
with those of the caput longum and the latissimus dorsi. The muscular
portion is quite thin in its anterior part, somewhat thicker behind, and
is narrower than the aponeurotic origin. It is succeeded by an
aponeurotic insertion, which ends chiefly by blending with the fascia of
the forearm a little below the elbow. There is, however, a small but
constant tendinous attachment to the olecranon.
_Relations._—Externally, the panniculus, triceps (long and internal
heads), the inner and middle flexors of the carpus, and the ulnar
vessels and nerve; internally, the latissimus dorsi, serratus magnus,
and posterior pectoral muscles.
_Blood-supply._—Subscapular, ulnar, and deep brachial arteries.
_Nerve-supply._—Radial nerve.
4. =Triceps brachii= (Figs. 178, 179, 187) (Brachial triceps; triceps
extensor cubiti).—This, together with the preceding muscle, constitutes
the large muscular mass which fills the angle between the posterior
border of the scapula and the humerus. It is clearly divisible into
three heads, as described below.
(=a=) =Long head= (Caput longum tricipitis; anconeus longus; caput
magnum).—This, the largest and longest of the three heads, is a
powerful, thick, triangular muscle, which extends from the posterior
border of the scapula to the olecranon.
_Origin._—The posterior border of the scapula.
_Insertion._—The outer and posterior part of the summit of the
olecranon.
_Action._—(1) To extend the elbow joint; (2) to flex the shoulder joint.
_Structure._—The muscle arises by a wide, strong aponeurosis from the
posterior border of the scapula. From this the bundles of the fleshy
portion converge to the short, strong tendon of insertion. A careful
examination will show that the muscle is penetrated by a tendinous
intersection from which many fibers take origin obliquely. The
superficial face is covered by an aponeurosis which is specially
developed at its distal part. A small =bursa= occurs under the tendon of
insertion.
_Relations._—Externally, the panniculus, deltoid, infraspinatus, teres
minor, and the external head; internally, the tensor fasciæ antibrachii,
teres major, latissimus dorsi, and posterior deep pectoral muscles, and
the subscapular vessels; in front, the brachialis, and the inner head,
the deep brachial and posterior circumflex vessels, and the axillary and
radial nerves; behind, the skin and fascia.
_Blood-supply._—Subscapular and deep brachial arteries.
_Nerve-supply._—Radial nerve.
(=b=) =External head= (Caput laterale tricipitis s. anconeus lateralis
s. externus; caput medium).—This is a strong, quadrilateral muscle,
which lies on the outer surface of the arm. Its proximal third is
covered by the deltoid and teres minor muscles; the remainder only by
the thin panniculus and the skin.
_Origin._—The deltoid tuberosity and the curved rough line which extends
from it to the neck of the humerus.
_Insertion._—(1) A small prominent area on the outer surface of the
olecranon; (2) the tendon of the long head.
_Action._—To extend the elbow joint.
_Structure._—The origin consists of short tendinous fibers. The belly is
thick, and is composed of parallel bundles which are directed obliquely
downward and backward. They are inserted partly into the tendon of the
long head and partly into the olecranon below and in front of that
tendon.
_Relations._—Externally, the deltoid, teres minor, and panniculus
muscles; internally, the long and inner heads and the brachialis muscle.
Branches of the circumflex vessels and axillary nerve emerge between the
posterior edge of the muscle and the long head. The deep face of the
muscle is related to the branches of the deep brachial artery and of the
radial nerve.
_Blood-supply._—Posterior circumflex and deep brachial arteries.
_Nerve-supply._—Radial nerve.
(=c=) =Internal head= (Fig. 193) (Caput mediale tricipitis; anconeus
medialis s. internus; caput parvum).—This is much the smallest of the
three heads. It is situated on the inner surface of the arm, and extends
from the middle third of the humerus to the olecranon.
_Origin._—The middle third of the inner surface of the shaft of the
humerus, behind and below the internal tubercle.
_Insertion._—The inner and fore part of the summit of the olecranon,
between the insertion of the long head and the origin of the ulnar head
of the flexor perforans.
_Action._—To extend the elbow joint.
_Structure._—The muscle is fleshy except at its insertion, where it has
a flat tendon, under which a small bursa may be found.
_Relations._—Externally, the humerus, brachialis, anconeus, and the
external head; internally, the posterior deep pectoral,
coraco-brachialis, teres major, latissimus dorsi, and tensor fasciæ
antibrachii muscles, the brachial and deep brachial vessels, and the
median and ulnar nerves; behind, the long head, branches of the deep
brachial vessels, and the radial nerve.
_Blood-supply._—Deep brachial and ulnar arteries.
_Nerve-supply._—Radial nerve.
5. =Anconeus= (M. anconeus parvus s. subanconeus).—This is a small
fleshy muscle which covers the olecranon fossa and is covered by the
triceps. It is somewhat difficult to separate from the outer head.
_Origin._—The distal third of the posterior surface of the humerus.
_Insertion._—The outer surface of the olecranon.
_Action._—To extend the elbow joint, and to raise the capsule of the
joint and prevent its being pinched during extension.
_Structure._—It is almost entirely fleshy, and is adherent by its deep
face to the joint capsule.
_Relations._—Superficially, the triceps muscle; deeply, the humerus and
the elbow joint.
_Blood-supply._—Deep brachial artery.
_Nerve-supply._—Radial nerve.
[Illustration: