Scapula
The shoulder blade connects upper arm to collarbone.
The scapula, or shoulder blade, is a paired bone that links the upper arm bone (humerus) to the collarbone (clavicle). Each scapula is roughly the mirror image of the other. Its name comes from the Classical Latin word for a small shovel or trowel, which the bone was thought to resemble. In medical terms, the prefix *omo-* refers to the shoulder blade, stemming from the Ancient Greek word for shoulder. This bone forms the rear part of the shoulder girdle. In humans, it is a flat, roughly triangular bone positioned on the back and side of the ribcage. The scapula is a thick, flat bone lying against the thoracic wall. It serves as an attachment point for three muscle groups: intrinsic, extrinsic, and those that stabilize and rotate the bone. The intrinsic muscles—the rotator cuff (subscapularis, supraspinatus, infraspinatus, and teres minor)—attach directly to the scapula’s surface and control internal and external rotation of the shoulder joint, as well as arm abduction. The extrinsic muscles—biceps, triceps, and deltoid—attach to specific parts of the scapula (the coracoid process, supraglenoid tubercle, infraglenoid tubercle, and spine) and drive various glenohumeral joint actions. The third group, which primarily stabilizes and rotates the scapula, includes the trapezius, serratus anterior, levator scapulae, and rhomboid muscles; these attach along the bone’s medial, superior, and inferior borders. The head, processes, and thicker parts of the scapula contain cancellous (spongy) tissue, while the rest is a thin layer of compact bone. In humans, the central part of the supraspinous fossa and the upper infraspinous fossa—especially the former—are often so thin they become semitransparent; sometimes bone is missing there entirely, leaving only fibrous tissue between the muscles. The scapula has two surfaces, three borders, three angles, and three processes. The front (costal or ventral surface) features a broad concavity called the subscapular fossa, where the subscapularis muscle attaches. The medial two-thirds of this fossa have three longitudinal oblique ridges, plus another thick ridge near the lateral border, all running outward and upward. These ridges anchor the tendinous insertions, while the surfaces between them hold the fleshy fibers of the subscapularis. The lateral third is smooth and covered by this muscle. At the upper part of the fossa, a transverse depression makes the bone appear bent along a line at a right angle through the center of the glenoid cavity, forming the subscapular angle. This arch gives the bone greater strength, with the arch’s summit supporting the spine and acromion. The upper costal surface also provides the origin for the first digitation of the serratus anterior. The back (dorsal or posterior surface) is arched from top to bottom and split into two unequal parts by the spine of the scapula. The area above the spine is the supraspinous fossa; below it is the infraspinous fossa. These two fossae connect via the spinoglenoid notch, located lateral to the spine’s root. The supraspinous fossa is concave, smooth, and wider at its vertebral end than its humeral end; its medial two-thirds give rise to the supraspinatus muscle. On its lateral side lies the spinoglenoid fossa, near the glenoid’s medial margin, which houses the suprascapular canal—a passage between the suprascapular notch and spinoglenoid notch that carries the suprascapular nerve and vessels. The infraspinous fossa is much larger. Near its vertebral margin, a shallow concavity appears at the top; its center is prominently convex, and near the axillary border, a deep groove runs from top to bottom. The medial two-thirds of this fossa give origin to the infraspinatus, while the lateral third is covered by it. A ridge on the outer back of the scapula runs from the lower part of the glenoid cavity downward and backward to the vertebral border, about 2.5 cm above the inferior angle. A fibrous septum attached to this ridge separates the infraspinatus from the teres major and teres minor. The upper two-thirds of the surface between this ridge and the axillary border is narrow, crossed near its center by a groove for the scapular circumflex vessels; the teres minor attaches here. The broad and narrow portions are separated by an oblique line from the axillary border downward and backward to the ridge, where a fibrous septum splits the teres muscles.
- Type
- Bone
- Location
- Posterolateral aspect of the thoracic cage
- Connections
- Humerus and clavicle
- Shape
- Flat, roughly triangular
- Key Feature
- Forms back of shoulder girdle
Lore & Background
The scapula is a thick, flat bone lying on the thoracic wall that provides attachment for three groups of muscles: intrinsic, extrinsic, and stabilizing and rotating muscles. The intrinsic muscles include the rotator cuff muscles—subscapularis, supraspinatus, infraspinatus, and teres minor—which attach to the surface of the scapula and are responsible for internal and external rotation of the shoulder joint, along with humeral abduction. The extrinsic muscles include the biceps, triceps, and deltoid muscles, attaching to the acromion and spine of the scapula, responsible for several actions of the glenohumeral joint. The third group, mainly responsible for stabilization and rotation of the scapula, consists of the trapezius, serratus anterior, levator scapulae, and rhomboid muscles, attaching to the medial, superior, and inferior borders.
Reader's Guide
The scapula is a critical bone in human anatomy, serving as the structural link between the upper arm and the torso via its connections to the humerus and clavicle. Its flat, triangular shape and placement on the thoracic cage allow it to form the back of the shoulder girdle, providing a broad surface for muscle attachment. The three groups of muscles that attach to the scapula—intrinsic, extrinsic, and stabilizing—enable a wide range of shoulder movements, including rotation, abduction, and stabilization of the glenohumeral joint. The scapula's structure includes two surfaces, three borders, three angles, and three processes, with the glenoid fossa articulating with the head of the humerus. Its name, derived from the Latin for trowel or small shovel, reflects its perceived resemblance to that tool. The scapula's role in facilitating upper limb mobility and strength makes it essential for everyday activities and athletic performance, and its study is fundamental in fields such as orthopedics, physical therapy, and evolutionary biology.
Did You Know?
- The name 'scapula' derives from the Classical Latin word for trowel or small shovel.
- The prefix 'omo-' used for the shoulder blade in medical terminology comes from the Ancient Greek word for shoulder.
- The scapula has two surfaces, three borders, three angles, and three processes.
- The central part of the supraspinatus fossa and the upper part of the infraspinatous fossa are often so thin in humans as to be semitransparent.
The Ventral Concavity
The costal, or ventral, surface of the scapula is defined by a single prominent feature: a broad concavity known as the subscapular fossa. This wide, shallow depression occupies the inner face of the shoulder blade and serves as the primary attachment surface for the subscapularis muscle. Unlike the more angular posterior surface, this medial-facing region presents a smooth, curved basin that spans a considerable portion of the bone's width. Within this basin, the medial two-thirds are not entirely featureless; they carry a series of oblique ridges that break up the otherwise uniform concavity. These ridges are not bony projections in the traditional sense but rather represent the paths of intramuscular tendons belonging to the subscapularis muscle. Together, the broad fossa and its internal ridge pattern create a highly specialized surface, one that is shaped by and for the muscle that anchors itself there. The fossa thus functions as both a structural landmark and a functional platform, its geometry dictated by the demands of the muscle it houses.
The Oblique Ridge System
Crossing the subscapular fossa in an oblique orientation are several distinct ridges that run parallel to the scapular spine. These ridges traverse the medial two-thirds of the concavity, extending from the superomedial region toward the inferiolateral direction. Their formation is not a product of bone growth or developmental patterning in the usual sense; rather, they are created by the intramuscular tendons of the subscapularis muscle itself. In other words, the ridges are essentially the visible, surface-level expression of tendinous tissue that weaves through the muscle belly. The ridges serve as attachment points for the tendinous insertions of the subscapularis, while the smooth surfaces between them provide the anchoring ground for the fleshy, contractile fibers. This alternating pattern of ridge and groove creates a highly organized topography on the fossa floor, one that reflects the internal architecture of the muscle rather than any independent bony structure. The parallelism to the scapular spine suggests a coordinated mechanical alignment between the fossa's internal features and the bone's external landmarks.
Tendon and Flesh: The Muscle-Bone Interface
The relationship between the subscapular fossa and the subscapularis muscle is one of precise, interlocking attachment. The ridges that cross the fossa provide specific sites where the tendinous insertions of the muscle grip the bone, anchoring the muscle's structural framework. Between these ridged attachment lines, the smooth concave surfaces offer the fleshy, contractile fibers of the subscapularis a broad area on which to rest and generate force. This division of labor—tendon on ridge, flesh in the valleys between—means that the fossa is not a passive socket but an active participant in the muscle's mechanical function. The intramuscular tendons that form the ridges are not merely attached to the bone; they are integral to the muscle's internal structure, threading through the belly and emerging at the surface to create the visible ridge pattern. The result is a surface where bone, tendon, and muscle fiber are so interwoven that the boundary between them becomes difficult to delineate. The fossa, in this sense, is as much a muscular feature as a bony one.
A Legacy in Anatomical Literature
The description of the subscapular fossa and its ridge system carries a lineage that stretches back to one of the most influential anatomical texts in the English-speaking world. That text, now in the public domain, established the precise terminology and spatial relationships that continue to define how these features are described in medical education and reference literature. The language of the costal or ventral surface, the broad concavity, and the intramuscular tendons all trace back to this era of systematic anatomical documentation. The fact that this description has persisted, essentially unchanged, for over a century speaks to the clarity and accuracy of the original observations. The subscapular fossa and its ridge pattern remain a defining feature of the scapula's inner face, a feature whose description has become part of the permanent anatomical record.
Frequently Asked Questions
What is the scapula?
The scapula is the flat, roughly triangular bone most people call the shoulder blade. It sits on the posterolateral side of the thoracic cage and serves as the rear component of the shoulder girdle.
What bones does the scapula connect?
It links the humerus (upper arm bone) to the clavicle (collar bone), acting as the structural bridge between the arm and the torso.
Where exactly is the scapula positioned in the body?
Each scapula rests on the posterolateral aspect of the thoracic cage, with one on each side of the body forming a near mirror image of the other.
Where does the name "scapula" come from?
The term traces back to a Classical Latin word meaning trowel or small shovel, reflecting an ancient observation about the bone's shape. In medical compound terms, the prefix omo- (from Ancient Greek for shoulder) is used instead.
Why is the scapula important to the shoulder girdle?
It forms the back of the shoulder girdle, providing the flat surface against which the arm can pivot. Without it, the humerus would lack its posterior anchor point to the torso.
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