The spine of the scapula divides its back surface into unequal portions. This spine leads to a head, that bears two processes - the acromion process that forms the tip of the shoulder and a coracoid process that curves forward and down below the clavicle (collarbone). The acromion, a bony prominence at the top of the shoulder blade, joins the clavicle and provides attachments for muscles of the arm and chest muscles. On the head of the scapula, between the processes mentioned above, is a depression called the glenoid cavity. It joins with the head of the upper arm bone (humerus).