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 prominent; now straighten it again and it becomes relaxed, whilst the muscles on the back of the arm become hard and prominent. The muscles of the forearm are the flexors and pronators; that is, they flex the arm and turn the palm downward.  In each upper extremity or arm there are fifty-three muscles, and we observe here the nicest and most economical method of packing away the muscles that could be improvised, securing strength, giving elegance to its form and shape and facilitating its mobility.