_3_, _4_, Rhomboideus; _5_, latissimus dorsi; _8_, posterior deep
pectoral; _9_, anterior deep pectoral; _11_, supraspinatus; _15_,
subscapularis; _16_, teres major; _20_, long head of triceps; _22a_,
_22b_, tensor fasciæ antibrachii; _23_, internal head of triceps.
(After Ellenberger, in Leisering’s Atlas.)
]
_Structure._—The muscle is flat and triangular, with the base upward.
The latter is thin and interdigitates with the scapular attachments of
the serratus. Below this the belly thickens and becomes narrower. It is
covered by an aponeurosis, and contains a considerable amount of
tendinous tissue. The tendon of insertion is crossed by the tendon of
origin of the coraco-brachialis; it is intimately adherent to the
capsule of the shoulder joint, and may be regarded as replacing the
internal ligament of the latter. A small bursa may occur here.
_Relations._—Superficially, the scapula and shoulder joint, the
supraspinatus, triceps, and teres major muscles; deeply, the serratus
magnus muscle, the brachial vessels, and the chief branches of the
brachial plexus. The subscapular vessels run along or near the posterior
edge of the muscle.
_Blood-supply._—Subscapular artery.
_Nerve-supply._—Subscapular nerves (from the brachial plexus).
2. =Teres major= (Teres internus; adductor of the arm).—This muscle is
flat, widest about its middle, and lies chiefly on the deep face of the
triceps.
_Origin._—The dorsal angle and the adjacent part of the posterior border
of the scapula.
_Insertion._—The tubercle on the inner surface of the shaft of the
humerus, in common with the latissimus dorsi.
_Action._—To flex the shoulder joint and adduct the arm.
_Structure._—It is for the most part fleshy, but the origin consists of
an aponeurosis which blends with that of the tensor fasciæ antibrachii.
The insertion is by a flat tendon which fuses with that of the
latissimus dorsi.
_Relations._—Superficially, the triceps, infraspinatus, and deltoid
muscles; deeply, the serratus magnus muscle. The subscapular vessels lie
in a groove between the anterior edge of this muscle and the posterior
border of the subscapularis; near the shoulder joint the posterior
circumflex artery and the axillary nerve emerge between the two muscles.
The deep face of the muscle is crossed by the thoracic branches of the
brachial plexus, and by the branch of the subscapular artery which
supplies the latissimus dorsi.
_Blood-supply._—Subscapular artery.
_Nerve-supply._—Axillary nerve.
3. =Coraco-brachialis= (Coraco-humeralis).—This muscle lies on the inner
surface of the shoulder joint and the arm.
_Origin._—The coracoid process of the scapula.
_Insertion._—(1) A small area above the internal tubercle of the
humerus; (2) the middle third of the anterior surface of the humerus.
_Action._—To adduct the arm and to flex the shoulder joint.
_Structure._—The long tendon of origin emerges between the subscapularis
and the inner branch of the supraspinatus. It passes over the terminal
part of the subscapularis and is provided with a =synovial sheath=. The
muscular part spreads out and divides into two portions. The smaller and
shorter portion is inserted into the proximal third of the shaft of the
humerus, close to the origin of the lateral head of the triceps; the
larger and longer portion is inserted into the middle third of the
humerus, in front of the internal tubercle and the inner head of the
triceps.
_Relations._—Externally, the subscapularis muscle and the humerus;
internally, the deep pectoral and brachialis muscles. The anterior
circumflex artery and the nerve to the biceps usually emerge between the
two insertions, and the brachial vessels lie along the posterior border
of the muscle.
_Blood-supply._—Anterior circumflex artery.
_Nerve-supply._—Musculo-cutaneous nerve.
4. =Capsularis= (Scapulo-humeralis posticus s. gracilis).—This is a very
small muscle, which lies on the back of the capsule of the shoulder
joint.
_Origin._—The scapula, close to the rim of the glenoid cavity.
_Insertion._—The posterior surface of the shaft of the humerus, a short
distance below the head.
_Action._—It may perhaps tense the capsule of the shoulder joint and
prevent its being pinched during flexion.
_Structure._—It is fleshy and usually about the breadth of a finger. It
may, however, consist of only a few bundles of fibers; sometimes it is
double. Its attachment to the joint capsule is slight. It passes through
the brachialis muscle to reach its insertion.
_Relations._—Superficially, the teres minor and triceps muscles; deeply,
the teres major and subscapularis muscles, and the capsule of the joint.
_Blood-supply._—Posterior circumflex artery.
_Nerve-supply._—Axillary nerve.