Among mammals, the monotremes have two coracoid processes, which articulate medially with the presternum and laterally with the scapula.
The head of the humerus, the long bone of the upper arm, fits into the glenoid cavity, a depression in the scapula.
The range of motion of the pectoral girdle and in particular of the scapula is enormously greater than that of the pelvic girdle.
In mammals the supracoracoideus retains its attachment to the humerus, but its previous point of origin (the coracoid plate) disappears, and the muscle now appears as two separate blocks of muscle arising on either side of the spine of the scapula as an abductor muscle (supraspinatus) and a rotator and flexor (infraspinatus).
In more typical tetrapods, there are two major derivatives of the external oblique attaching the scapula (shoulder blade) to the body: the serratus, made up of numerous fingerlike slips running from the scapula to the neighbouring ribs, and the levator scapulae, which are fused with serratus along its caudal (tail-end) border.
The connection is by means of muscles only, including the trapezius, rhomboids, and levator scapulae, while the serratus anterior connects the scapula to the rib cage.
The components of the girdle of the upper extremity, the pectoral girdle, are the shoulder blade, or scapula, and the collarbone, or clavicle.
The scapula, or shoulder blade, floats over the thoracic surface by reason of the arrangement of the fibres of the serratus anterior muscle and the support against gravity that is provided by the trapezius, rhomboid, and levator scapulae muscles.
Grossly, bone tissue is organized into a variety of shapes and configurations adapted to the function of each bone: broad, flat plates, such as the scapula, serve as anchors for large muscle masses, while hollow, thick-walled tubes, such as the femur, the radius, and the ulna, support weight or serve as a lever arm.
The human pectoralis minor muscle has forsaken its attachment to the humerus, the long bone of the upper arm, and presumably derives some stability from attaching to the coracoid process, a projection from the scapula, instead of gliding over it.
It achieves a pulleylike action by means of a tendon that passes through the canal at the junction of the coracoid, furcula, and scapula and attaches to the dorsal side of the head of the humerus.
