The shoulder is a specialised joint which has an extreme range
of motion at the connection between the upper limb and the
trunk. The joint classification of the shoulder is as one of the
ball and socket joints but this structure is much clearer in the
hip than the shoulder. The humeral head, the upper end of the
arm bone, is a large rounded ball-like structure with some
obvious relationship to the ball of the hip. The shoulder socket
however is quite different from the hip in that the joint
surface is very flat and small compared to the head. The scapula
or shoulder blade is a large flat piece of bone that lies over
the ribs each side of the upper back, and the outer ends of the
scapula are formed into the glenoid or shoulder socket. The
capsule of the shoulder joint, the fibrous bag which surrounds
the joint, and supports it strongly in many joints, is
particularly lax and baggy in the shoulder to allow a large
range of motion. The rotator cuff muscles start (have their
origin) from the flat plate of the sca