Scapula

The scapula (shoulder bone, shoulder blade, wing bone) is a flat bone, triangular in shape. It is placed on a posterolateral aspect of the thoracic cage, ranging from the level of the second rib to the level of the seventh rib.

Scapula connects with the humerus (upper arm bone) in the glenohumeral (shoulder) joint and the clavicula (collar bone) in the acromioclavicular joint. There is no direct connection between the thoracic cage and scapula. It is held in place thanks to the surrounding muscles.

The name scapula comes from early Roman times, meaning a small shovel.

Functions
The scapula is the origin and instertion of various muscles

The scapula protects the thoracic cavity form the dorsal side

Moving the upper limb
As is, the humerus can only move 90° upwards, into the horizontal plane. Moving any more, the humerus' head would collide with the coracoid process of the scapula. To move the upper limb above the horizontal plane, the coracoacromial ligament (fornix humeri) needs to be engaged.

The scapula and humerus move in a 1:2 ratio. When the upper limb is abducted 180°, 60° of those 180 occur by rotation of the scapula and 120° by movement of the humerus itself.

Borders
The three borders of the scapula are:


 * Margo medialis:Medial border, parallel to the longitudinal axis of the spine
 * Margo lateralis:Lateral, or axillary border
 * Margo superior:Superior border

Angles
Borders of the scapula connect in three angles:


 * Angulus superior:Superior and medial border
 * Angulus inferior:Medial and lateral border
 * Angulus lateralis:Lateral and superior border

Dorsal surface
The back of the scapula is divided into two unequal parts by the spine of the scapula. The spine ends in a process called the acromion. The acromion forms the cavity of the glenohumeral joint.

The portion of the dorsal surface above the spine is called the supraspinous fossa. M. supraspinatus originates there.

The portion of the dorsal surface below the spine is called the infraspinous fossa. It is much larger than the supraspionous fossa. M. infraspinatus originates there.

Both muscles are parts of the rotator cuff, rotating the humerus and providing stability for the glenohumeral joint.

The coracoid process originates from the superior border.

Ventral surface
The front of the scapula has a concavity called the subscapular fossa. M. subscapularis originates in the fossa.

Lateral surface
The glenoid cavity can be found at the lateral angle. The acromion and the coracoid process make its top border.

Above and below the cavity, two tubercles serve as origins of muscles:


 * Supraglenoid tubercle
 * Origin of the long head of m. biceps brachii


 * Infraglenoid tubercle
 * Origin of the long head of m. triceps brachii