Scapula
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In anatomy, the scapula, or shoulder blade, is the bone that connects the humerus (arm bone) with the clavicle (collar bone).
The scapula forms the posterior part of the shoulder girdle. In humans, it is a flat bone, roughly triangular in shape. It has two surfaces, three borders, and three angles.
The anterior (front) side of the scapula shows the fossa subscapularis (subscapular fossa) to which the subscapularis muscle attaches.
The posterior surface of the scapula is divided by a bony projection, the spina scapulae (opposite to the fossa subscapularis) into the supraspinous fossa and the infraspinous fossa. This projection is called the spine of the scapula. It begins flat at the base of the shoulder bone, ascends in distal direction for all animals but carnivores and humans to its peak at about the middle of the scapula, this peak is called tuber scapulae. After this peak the spina scapulae steeply decays in height. For humans and carnivores and bovinae the spina runs into a forward pointing hook called acromion, which continues past the main part of the bone.
Another hook-like projection comes off the lateral angle of the scapula, and is called the coracoid process. The end of this hook is the site of attachment of many muscles, such as the coracobrachialis muscle.
Near the base of the coracoid process, so also on the lateral angle, there is a depression called the glenoid cavity. This forms the socket that the head of the humerus articulates with.
The scapula also articulates with the clavicle, via the acromion process (the acromioclavicular joint).
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Surfaces
The costal or ventral surface [Fig. 1] presents a broad concavity, the subscapular fossa. The medial two-thirds of the 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 fleshy fibers, of the Subscapularis. The lateral third of the fossa is smooth and covered by the fibers of this muscle. The fossa is separated from the vertebral border by smooth triangular areas at the medial and inferior angles, and in the interval between these by a narrow ridge which is often deficient. These triangular areas and the intervening ridge afford attachment to the Serratus anterior. 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. 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 supraspinatous 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.
The Spine ('spina scapulæ')
The spine is a prominent plate of bone, which crosses obliquely the medial four-fifths of the dorsal surface of the scapula at its upper part, and separates the supra- from the infraspinatous fossa. It begins at the vertical border by a smooth, triangular area over which the tendon of insertion of the lower part of the Trapezius glides, and, gradually becoming more elevated, ends in the acromion, which overhangs the shoulder joint. The spine is triangular, and flattened from above downward, its apex being directed toward the vertebral border. It presents two surfaces and three borders. Its superior surface is concave; it assists in forming the supraspinous fossa, and gives origin to part of the Supraspinatus. Its inferior surface forms part of the infraspinous fossa, gives origin to a portion of the Infraspinatus, and presents near its center the orifice of a nutrient canal. Of the three borders, the anterior is attached to the dorsal surface of the bone; the posterior, or crest of the spine, is broad, and presents two lips and an intervening rough interval. The Trapezius is attached to the superior lip, and a rough tubercle is generally seen on that portion of the spine which receives the tendon of insertion of the lower part of this muscle. The Deltoid is attached to the whole length of the inferior lip. The interval between the lips is subcutaneous and partly covered by the tendinous fibers of these muscles. The lateral border, or base, the shortest of the three, is slightly concave; its edge, thick and round, is continuous above with the under surface of the acromion, below with the neck of the scapula. It forms the medial boundary of the great scapular notch, which serves to connect the supra- and infraspinous fossæ.
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. Its superior surface, directed upward, backward, and lateralward, is convex, rough, and gives attachment to some fibers of the Deltoideus, and in the rest of its extent is subcutaneous. Its inferior surface is smooth and concave. Its lateral border is thick and irregular, and presents three or four tubercles for the tendinous origins of the Deltoideus. Its medial border, shorter than the lateral, is concave, gives attachment to a portion of the Trapezius, and presents about its center a small, oval surface for articulation with the acromial end of the clavicle.
Figure 1 : Left scapula. Costal surface.
Its apex, which corresponds to the point of meeting of these two borders in front, is thin, and has attached to it the coracoacromial ligament.
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Figure 2 : Left scapula. Dorsal surface.
Borders
Of the three borders of the scapula, the superior is the shortest and thinnest; it is concave, and extends from the medial angle to the base of the coracoid process. At its lateral part is a deep, semicircular notch, the scapular notch, formed partly by the base of the coracoid process. This notch is converted into a foramen by the superior transverse ligament, and serves for the passage of the suprascapular nerve; sometimes the ligament is ossified. The adjacent part of the superior border affords attachment to the Omohyoideus. The axillary 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. Immediately below the glenoid cavity is a rough impression, the infraglenoid tuberosity, about 2.5 cm in length, which gives origin to the long head of the Triceps brachii; in front of this is a longitudinal groove, which extends as far as the lower third of this border, and affords origin to part of the Subscapularis. The inferior third is thin and sharp, and serves for the attachment of a few fibers of the Teres major behind, and of the Subscapularis in front. The vertebral border is the longest of the three, and extends from the medial to the inferior angle. It is arched, intermediate in thickness between the superior and the axillary borders, and the portion of it above the spine forms an obtuse angle with the part below. This border presents an anterior and a posterior lip, and an intermediate narrow area. The anterior lip affords attachment to the Serratus anterior; the posterior lip, to the Supraspinatus above the spine, the Infraspinatus below; the area between the two lips, to the Levator scapulæ above the triangular surface at the commencement of the spine, to the Rhomboideus minor on the edge of that surface, and to the Rhomboideus major below it; this last is attached by means of a fibrous arch, connected above to the lower part of the triangular surface at the base of the spine, and below to the lower part of the border.
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Figure 3 : Posterior view of the thorax and shoulder girdle. (Morris.)
Angles
Of the three angles, the medial, formed by the junction of the superior and vertebral borders, is thin, smooth, rounded, inclined somewhat lateralward, and gives attachment to a few fibers of the Levator scapulæ. The inferior angle, thick and rough, is formed by the union of the vertebral and axillary borders; its dorsal surface affords attachment to the Teres major and frequently to a few fibers of the Latissimus dorsi. The lateral angle is the thickest part of the bone, and is sometimes called the head of the scapula. On it is a shallow pyriform, articular surface, the glenoid cavity, which is directed lateralward and forward and articulates with the head of the humerus; it is broader below than above and its vertical diameter is the longest. The surface is covered with cartilage in the fresh state; and its margins, slightly raised, give attachment to a fibrocartilaginous structure, the glenoidal labrum, which deepens the cavity. At its apex is a slight elevation, the supraglenoid tuberosity, to which the long head of the Biceps brachii is attached. The neck of the scapula is the slightly constricted portion which surrounds the head; it is more distinct below and behind than above and in front.
The Coracoid Process ('processus coracoideus')
The coracoid process is a thick curved process attached by a broad base to the upper part of the neck of the scapula; it runs at first upward and medialward; then, becoming smaller, it changes its direction, and projects forward and lateralward. The ascending portion, flattened from before backward, presents in front a smooth concave surface, across which the Subscapularis passes. The horizontal portion is flattened from above downward; its upper surface is convex and irregular, and gives attachment to the Pectoralis minor; its under surface is smooth; its medial and lateral borders are rough; the former gives attachment to the Pectoralis minor and the latter to the coracoacromial ligament; the apex is embraced by the conjoined tendon of origin of the Coracobrachialis and short head of the Biceps brachii and gives attachment to the coracoclavicular fascia. On the medial part of the root of the coracoid process is a rough impression for the attachment of the conoid ligament; and running from it obliquely forward and lateralward, on to the upper surface of the horizontal portion, is an elevated ridge for the attachment of the trapezoid ligament.
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Figure 4 : Left scapula. Lateral view.
Structure
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.
Ossification
(Fig. 206) The scapula is ossified from seven or more centers: one for the body, two for the coracoid process, two for the acromion, one for the vertebral border, and one for the inferior angle. Ossification of the body begins about the second month of fetal life, by the formation of an irregular quadrilateral plate of bone, immediately behind the glenoid cavity. This plate extends so as to form the chief part of the bone, the spine growing up from its dorsal surface about the third month. At birth, a large part of the scapula is osseous, but the glenoid cavity, the coracoid process, the acromion, the vertebral border, and the inferior angle are cartilaginous. From the fifteenth to the eighteenth month after birth, ossification takes place in the middle of the coracoid process, which as a rule becomes joined with the rest of the bone about the fifteenth year. Between the fourteenth and twentieth years, ossification of the remaining parts takes place in quick succession, and usually in the following order; first, in the root of the coracoid process, in the form of a broad scale; secondly, near the base of the acromion; thirdly, in the inferior angle and contiguous part of the vertebral border; fourthly, near the extremity of the acromion; fifthly, in the vertebral border. The base of the acromion is formed by an extension from the spine; the two separate nuclei of the acromion unite, and then join with the extension from the spine. The upper third of the glenoid cavity is ossified from a separate center (subcoracoid), which makes its appearance between the tenth and eleventh years and joins between the sixteenth and the eighteenth. Further, an epiphysical plate appears for the lower part of the glenoid cavity, while the tip of the coracoid process frequently presents a separate nucleus. These various epiphyses are joined to the bone by the twenty-fifth year. Failure of bony union between the acromion and spine sometimes occurs, the junction being effected by fibrous tissue, or by an imperfect articulation; in some cases of supposed fracture of the acromion with ligamentous union, it is probable that the detached segment was never united to the rest of the bone.
Figure 5 : Plan of ossification of the scapula. From seven centers.
This article is based on an entry from the 1918 edition of Gray's Anatomy, which is in the public domain. As such, some of the information contained herein may be outdated. Please edit the article if this is the case, and feel free to remove this notice when it is no longer relevant. Additions have been made from "Nickel; Schummer; Seiferle; Lehrbuch der Anatomie der Haussäugetiere."