Before we look at any shoulder muscles, we need to take note of the tendons of two long elbow muscles, which arise very close to the shoulder joint, and lie deep to everything else.
They're the tendons of the long head of the biceps, and the long head of the triceps muscles. The long head of triceps arises here, from the infraglenoid tubercle. The long head of biceps arises, surprisingly, here from the supraglenoid tubercle. To get there, it passes inside the joint capsule, and right over the top of the head of the humerus.
Now let's look at the four short muscles which hold the shoulder joint together. There are three on the back, one on the front. The one on the front is subscapularis. It arises from almost all of the anterior, or costal aspect of the scapula. Its tendon inserts here, on the lesser tubercle.
Subscapularis, acting alone, produces internal rotation of the humerus. Acting with the other three short muscles, it holds the humeral head and the glenoid fossa together, while other, more powerful muscles are at work.
On the back, there are two muscles below the scapular spine, and one above it. The one above is supraspinatus. It arises from almost all of the supraspinous fossa. It passes under the acromion and inserts here, on the greater tubercle.
The tendon of supraspinatus runs through a tight spot, between the acromion and the head of the humerus. There's a synovial lined pocket, a bursa, here between it and the acromion. Supraspinatus initiates abduction of the humerus.
The two muscles below the spine are infraspinatus and teres minor. Between them, they arise from almost all of the infraspinous fossa, infraspinatus here, teres minor here. Infraspinatus inserts here on the back of the greater tubercle, teres minor just below it. Both these muscles produce external rotation of the humerus.
These four short muscles, subscapularis, supraspinatus, infraspinatus, and teres minor, converge on the humerus to form an almost continuous cuff of flat, supporting tendons, often referred to as the rotator cuff. It's these tendons together with the long head of the triceps down here, which keep the head of the humerus from sliding out of its very shallow socket.
There are two other muscles to note, that also run from the scapula to the humerus, one on the front, and one on the back. The one on the back is teres major. It arises here, from the lower lateral border of the scapula, and inserts here, on the posterior lip of the biciptal groove. Teres major is quite a powerful adductor of the humerus..
On the front here's coraco-brachialis. It arises from the coracoid process. It inserts down here, on the humerus. Coraco-brachialis helps to flex the shoulder joint.
Altogether there are seven muscles that go from the scapula to the humerus, and so far we've seen six of them. The last one, the deltoid, is so big that it covers up almost everything else, so we'll leave it out of the picture till the very end.