1. =Deltoid= (M. deltoideus; long abductor of the arm; scapular portion
of the deltoid of man).—This lies partly on the triceps in the angle
between the scapula and humerus, partly on the infraspinatus and teres
minor.
_Origin._—(1) The upper part of the posterior border of the scapula; (2)
the spine of the scapula, by means of the strong aponeurosis which
covers the infraspinatus.
_Insertion._—The deltoid tuberosity of the humerus.
_Action._—To flex the shoulder joint and abduct the arm.
_Structure._—The origin of the muscle is partly aponeurotic, partly
fleshy. The aponeurosis fuses with that of the infraspinatus; the
posterior part is attached to the scapula immediately in front of the
origin of the long head of the triceps. The belly of the muscle lies for
the most part in a cavity formed in the triceps. It is widest about its
middle.
_Relations._—Superficially, the skin, fascia, panniculus, and
mastoido-humeralis; deeply, the infraspinatus, teres minor, triceps, and
brachialis muscles, and branches of the posterior circumflex artery and
axillary nerve.
_Blood-supply._—Subscapular artery (chiefly through the posterior
circumflex).
_Nerve-supply._—Suprascapular and axillary nerves.
2. =Supraspinatus.=—This muscle occupies the supraspinous fossa, which
it fills, and beyond which it extends, thus coming in contact with the
subscapularis.
_Origin._—The supraspinous fossa, the spine, and the lower part of the
cartilage of the scapula.
_Insertion._—The inner and outer lips of the bicipital groove.
_Action._—To extend the shoulder joint. It also assists in preventing
dislocation.
_Structure._—The surface of the muscle is covered by a strong
aponeurosis, from the deep face of which many fibers arise. The muscle
is thin at its origin from the cartilage, but becomes considerably
thicker below. At the neck of the scapula it divides into two branches,
between which the tendon of origin of the biceps emerges. These
branches, fleshy superficially, tendinous deeply, are inserted into the
lips of the bicipital groove. They are united by a fibrous membrane
already mentioned in connection with the deep pectoral muscle; some
fibers are attached to this membrane and the capsule of the shoulder
joint.
_Relations._—Superficially, the skin, fascia, panniculus, trapezius, and
mastoido-humeralis; deeply, the scapula and its cartilage, the
subscapularis muscle, and the suprascapular vessels and nerve; in front,
the anterior deep pectoral muscle; behind, the spine of the scapula and
infraspinatus muscle.
_Blood-supply._—Suprascapular and posterior circumflex arteries.
_Nerve-supply._—Suprascapular nerve.
3. =Infraspinatus.=—This muscle occupies the greater part of the
infraspinous fossa.
_Origin._—The infraspinous fossa and the scapular cartilage.
_Insertion._—(1) The outer tuberosity of the humerus, distal to the
outer insertion of the supraspinatus; (2) the posterior eminence of the
outer tuberosity.
_Action._—To abduct the arm and rotate it outward.[57] It also plays the
part of a lateral ligament.
_Structure._—This muscle is also covered by a strong aponeurosis, from
which many fibers arise, and by means of which the deltoid is attached
to the spine of the scapula. A thick tendinous layer partially divides
the muscle into two strata, and, coming to the surface at the shoulder
joint, constitutes the chief means of insertion. This tendon, an inch or
more (3 cm.) in width, passes over the posterior eminence of the
external tuberosity of the humerus; it is bound down by a fibrous sheet,
and a =synovial bursa= is interposed between the tendon and the bone.
When the long insertion is cut and reflected, the short insertion,
partly tendinous, partly fleshy, is exposed.
_Relations._—Superficially, the skin, fascia, panniculus, trapezius, and
deltoid; deeply, the scapula and its cartilage, the shoulder joint and
capsule, the long head of the triceps, the teres minor, and the nutrient
artery of the scapula.
_Blood-supply._—Subscapular artery.
_Nerve-supply._—Suprascapular and axillary nerves.
4. =Teres minor.=—This is a much smaller muscle than the foregoing. It
lies chiefly on the triceps, under cover of the deltoid and
infraspinatus.
_Origin._—(1) The rough lines on the distal and posterior part of the
infraspinous fossa; (2) a small part of the posterior border of the
scapula, about its middle; (3) a tubercle near the rim of the glenoid
artery.
_Insertion._—The deltoid tuberosity and a small area just above it.
_Action._—To flex the shoulder joint and to abduct the arm; also to
assist in outward rotation.
_Structure._—The muscle is not rounded in the horse and ox, but flat and
triangular. Its origin from the posterior border of the scapula is by
means of an aponeurosis which also gives origin to fibers of the
infraspinatus and triceps. A =bursa= is commonly found between the
terminal part of the muscle and the capsule of the shoulder joint, and
is often continuous with that of the infraspinatus.
_Relations._—Superficially, the deltoid and infraspinatus muscles;
deeply, the scapula, the shoulder joint, and the triceps muscle.
_Blood-supply._—Subscapular artery (circumflex branches).
_Nerve-supply._—Axillary nerve.