Bone: shoulder blades
Posterior view of the thorax and shoulder girdle. (Scapula visible at either side.)
Gray's	subject #50 202
MeSH	Scapula

In anatomy, the scapula, omo (Medical Latin), or shoulder blade, is the bone that connects the humerus (arm bone) with the clavicle (collar bone).

The scapula forms the posterior (back) located part of the shoulder girdle. In humans, it is a flat bone, roughly triangular in shape, placed on a posterolateral aspect of the thoracic cage.

[edit] Structure

[edit] Surfaces

[edit] Costal (Front, Ventral, Anterior)

The costal or ventral surface [Fig. 1] presents a broad concavity, the subscapular fossa.

The medial two-thirds of this fossa are marked by several oblique ridges, which run lateralward and upward. The ridges give attachment to the tendinous insertions, and the surfaces between them to the shelby, of the Subscapularis. The lateral third of the fossa is smooth and covered by the fibers of this muscle.

|

Figure 1 : Left scapula. Costal surface.

|

Figure 2 : Left scapula. Dorsal surface.

|

Figure 3 : Left scapula. Lateral surface.

At the upper part of the fossa is a transverse depression, where the bone appears to be bent on itself along a line at right angles to and passing through the center of the glenoid cavity, forming a considerable angle, called the subscapular angle; this gives greater strength to the body of the bone by its arched form, while the summit of the arch serves to support the spine and acromion.

[edit] Dorsal (Back, Posterior)

The dorsal surface [Fig. 2] is arched from above downward, and is subdivided into two unequal parts by the spine; the portion above the spine is called the supraspinous fossa, and that below it the infraspinous fossa.

• The supraspinous fossa, the smaller of the two, is concave, smooth, and broader at its vertebral than at its humeral end; its medial two-thirds give origin to the Supraspinatus.

• The infraspinous fossa is much larger than the preceding; toward its vertebral margin a shallow concavity is seen at its upper part; its center presents a prominent convexity, while near the axillary border is a deep groove which runs from the upper toward the lower part. The medial two-thirds of the fossa give origin to the Infraspinatus; the lateral third is covered by this muscle.

The dorsal surface is marked near the axillary border by an elevated ridge, which runs from the lower part of the glenoid cavity, downward and backward to the vertebral border, about 2.5 cm above the inferior angle.

The ridge serves for the attachment of a fibrous septum, which separates the Infraspinatus from the Teres major and Teres minor.

The surface between the ridge and the axillary border is narrow in the upper two-thirds of its extent, and is crossed near its center by a groove for the passage of the scapular circumflex vessels; it affords attachment to the Teres minor.

Its lower third presents a broader, somewhat triangular surface, which gives origin to the Teres major, and over which the Latissimus dorsi glides; frequently the latter muscle takes origin by a few fibers from this part.

The broad and narrow portions above alluded to are separated by an oblique line, which runs from the axillary border, downward and backward, to meet the elevated ridge: to it is attached a fibrous septum which separates the Teres muscles from each other.

[edit] Borders

There are three borders of the scapula:

• The superior border is the shortest and thinnest; it is concave, and extends from the medial angle to the base of the coracoid process. It is referred to as the cranial border in animals.

• The axillary border (or "lateral border") is the thickest of the three. It begins above at the lower margin of the glenoid cavity, and inclines obliquely downward and backward to the inferior angle. It is referred to as the caudal border in animals.

• The vertebral border (or "medial border") is the longest of the three, and extends from the medial to the inferior angle. It is referred to as the dorsal border in animals.

[edit] The acromion

The acromion forms the summit of the shoulder, and is a large, somewhat triangular or oblong process, flattened from behind forward, projecting at first lateralward, and then curving forward and upward, so as to overhang the glenoid cavity.

[edit] Development

The larger part of the scapula undergoes membranous ossification.[1]. Some of the outer parts of the scapula are cartilagenous at birth, and would therefore undergo endochondral ossification [2].

The head, processes, and the thickened parts of the bone, contain cancellous tissue; the rest consists of a thin layer of compact tissue.

The central part of the supraspinatous fossa and the upper part of the infraspinatous fossa, but especially the former, are usually so thin as to be semitransparent; occasionally the bone is found wanting in this situation, and the adjacent muscles are separated only by fibrous tissue.

[edit] Muscular attachments

The following muscles attach to the scapula:

[edit] Movements

Movements of the scapula are brought about by scapular muscles:

Elevation, Depression, Protraction, Retraction, Lateral rotation, Medial rotation, Upward Rotation, Downward Rotation, Anterior Tipping, and Posterior Tipping

[edit] Injury

Because of its sturdy structure and protected location, scapular fractures are uncommon; when they do occur, they are an indication that severe chest trauma has occurred.^[1]

A winged scapula is a condition in which the medial border (the side nearest the spine) of a person's scapula is abnormally positioned outward and backward. The resulting appearance of the upper back is said to be wing-like because the inferior angle of the shoulder blade protrudes backward rather than lying mostly flat like in people without the condition.

[edit] In other animals

In fish, the scapular blade is a structure attached to the upper surface of the articulation of the pectoral fin, and is accompanied by a similar coracoid plate on the lower surface. Although sturdy in cartilagenous fish, both plates are generally small in most other fish, and may be partially cartilagenous, or consist of multiple bony elements.^[2]

In the early tetrapods, these two structures respectively became the scapula and a bone referred to as the procoracoid (commonly called simply the "coracoid", but not homologous with the mammalian structure of that name). In amphibians, birds, and reptiles, these two bones are distinct, but together form a single structure bearing many of the muscle attachments for the forelimb. In such animals, the scapula is usually a relatively simple plate, lacking the projections and spine that it possesses in mammals. However, the detailed structure of these bones varies considerably in living groups. For example, in frogs, the procoracoid bones may be braced together at the animal's underside to absorb the shock of landing, while in turtles, the combined structure forms a Y-shape in order to allow the scapula to retain a connection to the clavicle (which is part of the shell). In birds, the procoracoids help to brace the wing against the top of the sternum.^[2]

In the fossil therapsids, a third bone, the true coracoid, formed just behind the procoracoid. The resulting three-boned structure is still seen in modern monotremes, but in all other living mammals, the procoracoid has disappeared, and the coracoid bone has fused with the scapula, to become the coracoid process. These changes are associated with the upright gait of mammals, compared with the more sprawling limb arrangement of reptiles and amphibians; the muscles formerly attached to the procoracoid are no longer required. The altered musculature is also responsible for the alteration in the shape of the rest of the scapula; the forward margin of the original bone became the spine and acromion, from which the main shelf of the shoulder blade arises as a new structure.^[2]

[edit] Additional images

Pectoral girdle - front	Human arm bones diagram	Diagram of the human shoulder joint	Left scapula. Lateral view.
The scapular and circumflex arteries.

[edit] See also

Wikimedia Commons has media related to: Scapula

• Bone terminology
• Scapulimancy/Oracle bone
• Terms for anatomical location

[edit] External links

• SUNY Labs 10:st-0301 - "Joints of the Upper Extremity: Scapula"

[edit] References

1. ^ Livingston DH, Hauser CJ (2003). "Trauma to the chest wall and lung". in Moore EE, Feliciano DV, Mattox KL. Trauma. Fifth Edition. McGraw-Hill Professional. pp. 516. ISBN 0071370692. 
2. ^ ^{a b c} Romer, Alfred Sherwood; Parsons, Thomas S. (1977). The Vertebrate Body. Philadelphia, PA: Holt-Saunders International. pp. 186-187. ISBN 0-03-910284-X. 

• Nickel, Schummer, & Seiferle; Lehrbuch der Anatomie der Haussäugetiere.

This article was originally based on an entry from a public domain edition of Gray's Anatomy. As such, some of the information contained within it may be outdated.