The Shoulder Joint. — The shoulder joint, formed as it is by the shoulder-blade (scapula), collarbone (clavicle), and the arm bone, is most beautifully designed and executed. It comprises a shallow ball and socket joint, the supporting the freest rotary movements. The shallowness of the socket, however, accounts for the frequent dislocations [...]

The large triangular muscle of the shoulder — the deltoid — is one of great strength, as in fact are all the muscles of the arm.  If you grasp the arm tightly just above the elbow joint, and then bend the forearm, you will feel the biceps muscle of the arm become firm, hard and [...]