Now we'll complete our picture by looking at three big external muscles: pectoralis major, latissimus dorsi, and deltoid.
Of these, the first two have much in common - pectoralis major on the front, and latissimus dorsi on the back. These two are alike, in that they both pass directly from the trunk to the humerus, bypassing the scapula. Between them they define the posterior and anterior walls of the axilla.
Pectoralis major arises from the medial third of the clavicle, from the front of the sternum, and from the front of the first six costal cartilages. It's inserted here, on the anterior edge of the bicipital groove.
Pectoralis major is a powerful adductor of the humerus. When its adducting effect is held in check by other muscles, it also produces internal rotation.
Latissimus dorsi has a very wide origin. It starts here, under the tail end of trapezius, at T7, and goes all the way down to the sacrum, and out onto the posterior iliac crest. It also has some fibers arising from the lower four ribs, and occasionally from the tip of the scapula.
It inserts here, on the floor of the bicipital groove. To get to its insertion, the latissimus tendon has to spiral around teres major. Here's teres major. Latissimus spirals from the back, to the front, with the lowest fibers of origin ending up highest.
Latissimus dorsi, like pectoralis major, is a powerful adductor of the humerus. Acting through the humerus, it's also a powerful depressor of the scapula, powerful enough to overcome the whole weight of the body, as in doing a pull-up.
Last of all, here's the deltoid muscle. It completely surrounds the shoulder joint from the front, to the back. It arises from the spine of the scapula, from the acromion, and from the lateral third of the clavicle. It's inserted here on the deltoid tuberosity of the humerus.
The deltoid muscle has multiple functions: it's almost like three different muscles. Its anterior part is a powerful flexor, its posterior part is a powerful extensor, and its lateral part is a powerful abductor.