_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.