The scapula (: scapulae or scapulas), also known as the shoulder blade, is the

bone A bone is a rigid organ that constitutes part of the skeleton in most vertebrate animals. Bones protect the various other organs of the body, produce red and white blood cells, store minerals, provide structure and support for the body, ...

that connects the

humerus The humerus (; : humeri) is a long bone in the arm that runs from the shoulder to the elbow. It connects the scapula and the two bones of the lower arm, the radius (bone), radius and ulna, and consists of three sections. The humeral upper extrem ...

(upper arm bone) with the

clavicle The clavicle, collarbone, or keybone is a slender, S-shaped long bone approximately long that serves as a strut between the scapula, shoulder blade and the sternum (breastbone). There are two clavicles, one on each side of the body. The clavic ...

(collar bone). Like their connected bones, the scapulae are paired, with each scapula on either side of the body being roughly a mirror image of the other. The name derives from the

Classical Latin Classical Latin is the form of Literary Latin recognized as a Literary language, literary standard language, standard by writers of the late Roman Republic and early Roman Empire. It formed parallel to Vulgar Latin around 75 BC out of Old Latin ...

word for

trowel A trowel is a small hand tool used for digging, applying, smoothing, or moving small amounts of viscous or particulate material. Common varieties include the masonry trowel, garden trowel, and float trowel. A power trowel is a much larger ga ...

or small

shovel A shovel is a tool used for digging, lifting, and moving bulk materials, such as soil, coal, gravel, snow, sand, or ore. Most shovels are hand tools consisting of a broad blade fixed to a medium-length handle. Shovel blades are usually made ...

, which it was thought to resemble. In compound terms, the prefix omo- is used for the shoulder blade in medical terminology. This prefix is derived from ὦμος (ōmos), the Ancient Greek word for shoulder, and is cognate with the Latin , which in Latin signifies either the shoulder or the upper arm bone. The scapula forms the back of the

shoulder girdle The shoulder girdle or pectoral girdle is the set of bones in the appendicular skeleton which connects to the arm on each side. In humans, it consists of the clavicle and scapula; in those species with three bones in the shoulder, it consists o ...

. In humans, it is a

flat bone Flat bones are bones whose principal function is either extensive protection or the provision of broad surfaces for muscular attachment. These bones are expanded into broad, flat plates,'' Gray's Anatomy'' (1918). (See infobox) as in the cranium ...

, roughly triangular in shape, placed on a posterolateral aspect of the thoracic cage.

Structure

The scapula is a thick, flat bone lying on the thoracic wall that provides an attachment for three groups of muscles: intrinsic, extrinsic, and stabilizing and rotating muscles. The intrinsic muscles of the scapula include the muscles of the

rotator cuff The rotator cuff (SITS muscles) is a group of muscles and their tendons that act to stabilize the human shoulder and allow for its extensive range of motion. Of the seven scapulohumeral muscles, four make up the rotator cuff. The four muscles a ...

(SITS muscle)—the subscapularis,

supraspinatus The supraspinatus (: supraspinati) is a relatively small muscle of the upper back that runs from the supraspinous fossa superior portion of the scapula (shoulder blade) to the greater tubercle of the humerus. It is one of the four rotator cuff m ...

, infraspinatus and teres minor. These muscles attach to the surface of the scapula and are responsible for the internal and external rotation of the

shoulder joint The shoulder joint (or glenohumeral joint from Greek ''glene'', eyeball, + -''oid'', 'form of', + Latin ''humerus'', shoulder) is structurally classified as a synovial joint, synovial ball-and-socket joint and functionally as a diarthrosis and m ...

, along with humeral abduction. The extrinsic muscles include the

biceps The biceps or biceps brachii (, "two-headed muscle of the arm") is a large muscle that lies on the front of the upper arm between the shoulder and the elbow. Both heads of the muscle arise on the scapula and join to form a single muscle bel ...

triceps The triceps, or triceps brachii (Latin for "three-headed muscle of the arm"), is a large muscle on the ventral, back of the upper limb of many vertebrates. It consists of three parts: the medial, lateral, and long head. All three heads cross the ...

, and

deltoid muscle The deltoid muscle is the muscle forming the rounded contour of the shoulder, human shoulder. It is also known as the 'common shoulder muscle', particularly in other animals such as the domestic cat. Anatomically, the deltoid muscle is made up o ...

s and attach to the

coracoid process The coracoid process (from Greek κόραξ, raven) is a small hook-like structure on the lateral edge of the superior anterior portion of the scapula (hence: coracoid, or "like a raven's beak"). Pointing laterally forward, it, together with the ...

and supraglenoid tubercle of the scapula, infraglenoid tubercle of the scapula, and

spine of the scapula The spine of the scapula or scapular spine is a prominent plate of bone, which crosses obliquely the medial four-fifths of the scapula at its upper part, and separates the supra- from the infraspinatous fossa. Structure It begins at the vertica ...

. These muscles are responsible for several actions of the glenohumeral joint. The third group, which is mainly responsible for stabilization and rotation of the scapula, consists of the

trapezius The trapezius is a large paired trapezoid-shaped surface muscle that extends longitudinally from the occipital bone to the lower thoracic vertebrae of the human spine, spine and laterally to the spine of the scapula. It moves the scapula and ...

, serratus anterior, levator scapulae, and

rhomboid muscles The rhomboid muscles (), often simply called the rhomboids, are rhombus-shaped muscles associated with the scapula. There are two rhomboid muscles on each side of the upper back: * Rhomboid major muscle * Rhomboid minor muscle The large rhomb ...

. These attach to the medial, superior, and inferior borders of the scapula. 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 supraspinatus fossa and the upper part of the

infraspinatous fossa The infraspinous fossa (infraspinatus fossa or infraspinatous fossa) of the scapula The scapula (: scapulae or scapulas), also known as the shoulder blade, is the bone that connects the humerus (upper arm bone) with the clavicle (collar bone ...

, but especially the former, are usually so thin in humans as to be semitransparent; occasionally the bone is found wanting in this situation, and the adjacent muscles are separated only by fibrous tissue. The scapula has two surfaces, three borders, three angles, and three processes.

Surfaces

Front or subscapular fossa

The front of the scapula (also known as the costal or ventral surface) has a broad concavity called the subscapular fossa, to which the

subscapularis muscle The subscapularis is a large triangular muscle which fills the subscapular fossa and inserts into the lesser tubercle of the humerus and the front of the capsule of the shoulder-joint. Structure The subscapularis is covered by a dense fasci ...

attaches. The medial two-thirds of the fossa have 3 longitudinal oblique ridges, and another thick ridge adjoins the lateral border; they run outward and upward. The ridges give attachment to the tendinous insertions, and the surfaces between them to the fleshy fibers, of the subscapularis muscle. The lateral third of the fossa is smooth and covered by the fibers of this muscle. 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 The glenoid fossa of the scapula or the glenoid cavity is a bone part of the shoulder. The word ''glenoid'' is pronounced or (both are common) and is from , "socket", reflecting the shoulder joint's ball-and-socket form. It is a shallow, pyri ...

, 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 Spine or spinal may refer to: Science Biology * Spinal column, also known as the backbone * Dendritic spine, a small membranous protrusion from a neuron's dendrite * Thorns, spines, and prickles, needle-like structures in plants * Spine (zoology), ...

and

acromion In human anatomy, the acromion (from Greek: ''akros'', "highest", ''ōmos'', "shoulder", : acromia) or summit of the shoulder is a bony process on the scapula (shoulder blade). Together with the coracoid process, it extends laterally over the sh ...

. The costal surface superior of the scapula is the origin of 1st digitation for the serratus anterior origin.

Back

The back of the scapula (also called the dorsal or posterior surface) is arched from above downward, and is subdivided into two unequal parts by the spine of the scapula. The portion above the spine is called the supraspinous fossa, and that below it the infraspinous fossa. The two fossae are connected by the spinoglenoid notch, situated lateral to the root of the spine. * The ''supraspinous fossa'', above the spine of scapula, is concave, smooth, and broader at its vertebral than at its humeral end; its medial two-thirds give origin to the

Supraspinatus The supraspinatus (: supraspinati) is a relatively small muscle of the upper back that runs from the supraspinous fossa superior portion of the scapula (shoulder blade) to the greater tubercle of the humerus. It is one of the four rotator cuff m ...

. At its lateral surface resides the spinoglenoid fossa which is situated by the medial margin of the glenoid. The spinoglenoid fossa houses the suprascapular canal which forms a connecting passage between the suprascapular notch and the spinoglenoid notch conveying the suprascapular nerve and vessels. * 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. There is a ridge on the outer part of the back of the scapula. This runs from the lower part of the glenoid cavity, downward and backward to the vertebral border, about 2.5 cm above the inferior angle. Attached to the ridge is a fibrous septum, which separates the infraspinatus muscle from the Teres major and Teres minor muscles. The upper two-thirds of the surface between the ridge and the axillary border is narrow, and is crossed near its center by a groove for the scapular circumflex vessels; the Teres minor attaches here. 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. Its lower third presents a broader, somewhat triangular surface, the inferior angle of the scapula, which gives origin to the Teres major, and over which the

Latissimus dorsi The latissimus dorsi () is a large, flat muscle on the back that stretches to the sides, behind the arm, and is partly covered by the trapezius on the back near the midline. The word latissimus dorsi (plural: ''latissimi dorsi'') comes from L ...

glides; frequently the latter muscle takes origin by a few fibers from this part.

Side

The

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

Angles

There are 3 angles: The superior angle of the scapula or medial angle, is covered by the

trapezius muscle The trapezius is a large paired trapezoid-shaped surface muscle that extends longitudinally from the occipital bone to the lower thoracic vertebrae of the human spine, spine and laterally to the spine of the scapula. It moves the scapula and ...

. This angle is formed by the junction of the superior and medial borders of the scapula. The superior angle is located at the approximate level of the

second The second (symbol: s) is a unit of time derived from the division of the day first into 24 hours, then to 60 minutes, and finally to 60 seconds each (24 × 60 × 60 = 86400). The current and formal definition in the International System of U ...

thoracic vertebra In vertebrates, thoracic vertebrae compose the middle segment of the vertebral column, between the cervical vertebrae and the lumbar vertebrae. In humans, there are twelve thoracic vertebra (anatomy), vertebrae of intermediate size between the ce ...

. The superior angle of the scapula is thin, smooth, rounded, and inclined somewhat lateralward, and gives attachment to a few fibers of the

levator scapulae muscle The levator scapulae is a slender skeletal muscle Skeletal muscle (commonly referred to as muscle) is one of the three types of vertebrate muscle tissue, the others being cardiac muscle and smooth muscle. They are part of the somatic nervous s ...

. The inferior angle of the scapula is the lowest part of the scapula and is covered by the

latissimus dorsi muscle The latissimus dorsi () is a large, flat muscle on the back that stretches to the sides, behind the arm, and is partly covered by the trapezius on the back near the midline. The word latissimus dorsi (plural: ''latissimi dorsi'') comes from ...

. It moves forwards round the chest when the arm is abducted. The inferior angle is formed by the union of the medial and lateral borders of the scapula. It is thick and rough and its posterior or back surface affords attachment to the teres major and often to a few fibers of the latissimus dorsi. The

anatomical plane An anatomical plane is a hypothetical plane used to transect the body, in order to describe the location of structures or the direction of movements. In human anatomy and non-human anatomy, four principal planes are used: the median plane, ...

that passes vertically through the inferior angle is named the

scapular line The scapular line, also known as the linea scapularis, is a vertical line passing through the inferior angle of the scapula. It has been used in the evaluation of brachial plexus birth palsy. References External links

* http://www.meddean.l ...

. The lateral angle of the scapula or glenoid angle, also known as the head of the scapula, is the thickest part of the scapula. It is broad and bears the glenoid fossa on its articular surface which is directed forward, laterally and slightly upwards, and articulates with the head of the humerus. The inferior angle 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 Fibrocartilage consists of a mixture of white fibrous tissue and Cartilage, cartilaginous tissue in various proportions. It owes its inflexibility and toughness to the former of these constituents, and its Elasticity (physics), elasticity to the ...

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 The biceps or biceps brachii (, "two-headed muscle of the arm") is a large muscle that lies on the front of the upper arm between the shoulder and the elbow. Both heads of the muscle arise on the scapula and join to form a single muscle bell ...

is attached. The anatomic neck of the scapula is the slightly constricted portion which surrounds the head and is more distinct below and behind than above and in front. The surgical neck of the scapula passes directly medial to the base of the coracoid process. File:Superior angle of left scapula01.png, Superior angle shown in red File:Head of scapula02.png, Lateral angle shown in red File:Scapula ant - anatomical neck and surgical neck.png, Anatomic neck: red, Surgical neck: purple File:Inferior angle of the left scapula01.png, Inferior angle shown in red

Borders

There are three borders of the scapula: * The superior border is the shortest and thinnest; it is concave, and extends from the superior angle to the base of the

. It is referred to as the cranial border in animals. ::At its lateral part is a deep, semicircular notch, the scapular notch, formed partly by the base of the

. This notch is converted into a

foramen In anatomy and osteology, a foramen (; : foramina, or foramens ; ) is an opening or enclosed gap within the dense connective tissue (bones and deep fasciae) of extant and extinct amniote animals, typically to allow passage of nerves, artery, ...

by the superior transverse scapular ligament, and serves for the passage of the suprascapular nerve; sometimes the ligament is

ossified Ossification (also called osteogenesis or bone mineralization) in bone remodeling is the process of laying down new bone material by cells named osteoblasts. It is synonymous with bone tissue formation. There are two processes resulting in t ...

. ::The adjacent part of the superior border affords attachment to the omohyoideus. File:Superior border of left scapula01.png, Costal surface of left scapula. Superior border shown in red. File:Superior border of left scapula - animation01.gif, Left scapula. Superior border shown in red. File:Superior border of scapula - animation02.gif, Animation. Superior border shown in red. * The axillary border (or "lateral border") is the thickest of the three. It begins above at the lower margin of the

, and inclines obliquely downward and backward to the inferior angle. It is referred to as the caudal border in animals. ::It begins above at the lower margin of the

, and inclines obliquely downward and backward to the inferior angle. ::Immediately below the glenoid cavity is a rough impression, the infraglenoid tuberosity, about . in length, which gives origin to the long head of the

triceps brachii The triceps, or triceps brachii (Latin for "three-headed muscle of the arm"), is a large muscle on the back of the upper limb of many vertebrates. It consists of three parts: the medial, lateral, and long head. All three heads cross the elbow jo ...

; 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 subscapularis is a large triangular muscle which fills the subscapular fossa and inserts into the lesser tubercle of the humerus and the front of the capsule of the Glenohumeral joint, shoulder-joint. Structure The subscapularis is covere ...

. ::The inferior third is thin and sharp, and serves for the attachment of a few fibers of the teres major behind, and of the

in front. File:Lateral border of left scapula01.png, Dorsal surface of left scapula. Lateral border shown in red. File:Lateral border of left scapula - animation.gif, Left scapula. Lateral border shown in red. File:Lateral border of scapula - animation.gif, Animation. Lateral border shown in red. * The medial border (also called the vertebral border or medial margin) is the longest of the three borders, and extends from the superior angle to the inferior angle. In animals it is referred to as the dorsal border. ::Four muscles attach to the medial border. Serratus anterior has a long attachment on the anterior lip. Three muscles insert along the posterior lip, the levator scapulae (uppermost), rhomboid minor (middle), and to the rhomboid major (lower middle). File:Medial boder of left scapula - animation.gif, Left scapula. Medial border shown in red. File:Medial boder of scapula - animation.gif, Animation. Medial border shown in red. File:Medial border of scapula01.png, Still image. Medial border shown in red. File:Levator scapulae.png,

Levator scapulae muscle The levator scapulae is a slender skeletal muscle Skeletal muscle (commonly referred to as muscle) is one of the three types of vertebrate muscle tissue, the others being cardiac muscle and smooth muscle. They are part of the somatic nervous s ...

(red) File:Rhomboideus minor.png,

Rhomboid minor muscle In human anatomy, the rhomboid minor is a small skeletal muscle of the back that connects the scapula to the vertebrae of the spinal column. It arises from the nuchal ligament, the 7th cervical and 1st thoracic vertebrae and intervening sup ...

(red) File:Rhomboideus major.png,

Rhomboid major muscle The rhomboid major is a skeletal muscle of the back that connects the scapula with the vertebrae of the spinal column. It originates from the spinous processes of the thoracic vertebrae T2–T5 and supraspinous ligament; it inserts onto the lower ...

(red)

Development

The scapula is

from 7 or more centers: one for the body, two for the

, two for the

, one for the vertebral border, and one for the inferior angle. Ossification of the body begins about the second month of fetal life, by an irregular quadrilateral plate of bone forming, immediately behind the

. This plate extends to form the chief part of the bone, the

scapular spine The spine of the scapula or scapular spine is a prominent plate of bone, which crosses obliquely the medial four-fifths of the scapula at its upper part, and separates the Supraspinatous fossa, supra- from the infraspinatous fossa. Structure It b ...

growing up from its dorsal surface about the third month. Ossification starts as membranous ossification before birth. After birth, the cartilaginous components would undergo endochondral

ossification Ossification (also called osteogenesis or bone mineralization) in bone remodeling is the process of laying down new bone material by cells named osteoblasts. It is synonymous with bone tissue formation. There are two processes resulting in t ...

. The larger part of the scapula undergoes membranous ossification. Some of the outer parts of the scapula are cartilaginous at birth, and would therefore undergo endochondral ossification. 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 Cartilage is a resilient and smooth type of connective tissue. Semi-transparent and non-porous, it is usually covered by a tough and fibrous membrane called perichondrium. In tetrapods, it covers and protects the ends of long bones at the joints ...

. From the 15th to the 18th month after birth, ossification takes place in the middle of the coracoid process, which as a rule becomes joined with the rest of the

about the 15th year. Between the 14th and 20th years, the remaining parts ossify 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 outer end of the acromion; fifthly, in the vertebral border. The base of the acromion is formed by an extension from the spine; the two 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 (sub coracoid), which appears between the 10th and 11th years and joins between the 16th and the 18th years. Further, an epiphysial plate appears for the lower part of the glenoid cavity, and the tip of the coracoid process frequently has a separate nucleus. These various

epiphyses An epiphysis (; : epiphyses) is one of the rounded ends or tips of a long bone that ossify from one or more secondary centers of ossification. Between the epiphysis and diaphysis (the long midsection of the long bone) lies the metaphysis, inc ...

are joined to the bone by the 25th year. Failure of bony union between the acromion and spine sometimes occurs (see os acromiale), the junction being effected by

fibrous tissue Connective tissue is one of the four primary types of animal tissue, a group of cells that are similar in structure, along with epithelial tissue, muscle tissue, and nervous tissue. It develops mostly from the mesenchyme, derived from the mesode ...

, or by an imperfect articulation; in some cases of supposed

fracture Fracture is the appearance of a crack or complete separation of an object or material into two or more pieces under the action of stress (mechanics), stress. The fracture of a solid usually occurs due to the development of certain displacemen ...

of the acromion with ligamentous union, it is probable that the detached segment was never united to the rest of the

Function

The following muscles attach to the scapula:

Movements

Movements of the scapula are brought about by the scapular muscles. The scapula can perform six actions: * Elevation: upper

and levator scapulae * Depression: lower trapezius * Retraction (

adduction Motion, the process of movement, is described using specific anatomical terms. Motion includes movement of organs, joints, limbs, and specific sections of the body. The terminology used describes this motion according to its direction relativ ...

): rhomboids and middle trapezius * Protraction (abduction): serratus anterior * Upward rotation: upper and lower trapezius, serratus anterior * Downward rotation: rhomboids, Levator Scapulae, and Pec Minor

Clinical significance

Scapular fractures

Scapula ant - anatomical neck and surgical neck

Because of its sturdy structure and protected location, fractures of the scapula are uncommon. When they do occur, they are an indication that severe

chest trauma A chest injury, also known as chest trauma, is any form of physical injury to the chest including the ribs, heart and lungs. Chest injuries account for 25% of all deaths from traumatic injury. Typically chest injuries are caused by blunt mechanism ...

has occurred. Scapular fractures involving the neck of the scapula have two patterns. One (rare) type of fracture is through the anatomical neck of the scapula. The other more common type of fracture is through the surgical neck of the scapula. The surgical neck exits medial to the

. An abnormally protruding inferior angle of the scapula is known as a winged scapula and can be caused by

paralysis Paralysis (: paralyses; also known as plegia) is a loss of Motor skill, motor function in one or more Skeletal muscle, muscles. Paralysis can also be accompanied by a loss of feeling (sensory loss) in the affected area if there is sensory d ...

of the

serratus anterior muscle The serratus anterior is a muscle of the chest. It originates at the side of the chest from the upper 8 or 9 ribs; it inserts along the entire length of the anterior aspect of the medial border of the scapula. It is innervated by the long tho ...

. In this condition the sides of the scapula nearest the spine are positioned outward and backward. The appearance of the upper back is said to be wing-like. In addition, any condition causing weakness of the serratus anterior muscle may cause scapular "winging".

Scapular dyskenesis

The scapula plays an important role in shoulder impingement syndrome.Kibler, BW. (1998). The role of the scapula in athletic shoulder function. The American Journal of Sports Medicine, 26(2), 325-337. Abnormal scapular function is called scapular dyskinesis. The scapula performs elevation of the acromion process during a throwing or serving motion, in order to avoid impingement of the rotator cuff tendons. If the scapula fails to properly elevate the acromion, impingement may occur during the cocking and acceleration phase of an overhead activity. The two muscles most commonly inhibited during this first part of an overhead motion are the serratus anterior and the lower trapezius. These two muscles act as a force couple within the glenohumeral joint to properly elevate the acromion process, and if a muscle imbalance exists, shoulder impingement may develop. Other conditions associated with scapular dyskenesis include thoracic outlet syndrome and the related pectoralis minor syndrome.

Etymology

The name ''scapula'' as synonym of ''shoulder blade'' is of Latin origin.Anderson, D.M. (2000). ''Dorland’s illustrated medical dictionary'' (29th edition). Philadelphia/London/Toronto/Montreal/Sydney/Tokyo: W.B. Saunders Company. It is commonly used in medical EnglishDorland, W.A.N. & Miller, E.C.L. (1948). ‘’The American illustrated medical dictionary.’’ (21st edition). Philadelphia/London: W.B. Saunders Company.Dirckx, J.H. (Ed.) (1997).''Stedman’s concise medical dictionary for the health professions.'' (3rd edition). Baltimore: Williams & Wilkins. and is part of the current official Latin nomenclature, ''

Terminologia Anatomica ''Terminologia Anatomica'' (commonly abbreviated TA) is the international standard for human anatomy, human anatomical terminology. It is developed by the Federative International Programme on Anatomical Terminology (FIPAT) a program of the Inter ...

''.Federative Committee on Anatomical Terminology (FCAT) (1998). ''Terminologia Anatomica''. Stuttgart: Thieme Shoulder blade is the colloquial name for this bone.

In other animals

In fish, the scapular blade is a structure attached to the upper surface of the articulation of the

pectoral fin Fins are moving appendages protruding from the body of fish that interact with water to generate thrust and help the fish aquatic locomotion, swim. Apart from the tail or caudal fin, fish fins have no direct connection with the vertebral column ...

, 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. In the early

tetrapod A tetrapod (; from Ancient Greek :wiktionary:τετρα-#Ancient Greek, τετρα- ''(tetra-)'' 'four' and :wiktionary:πούς#Ancient Greek, πούς ''(poús)'' 'foot') is any four-Limb (anatomy), limbed vertebrate animal of the clade Tetr ...

s, these two structures respectively became the scapula and a bone referred to as the procoracoid (commonly called simply the "

coracoid A coracoid is a paired bone which is part of the shoulder assembly in all vertebrates except therian mammals (marsupials and placentals). In therian mammals (including humans), a coracoid process is present as part of the scapula, but this is n ...

", but not homologous with the mammalian structure of that name). In amphibians and reptiles (birds included), 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 The sternum (: sternums or sterna) or breastbone is a long flat bone located in the central part of the chest. It connects to the ribs via cartilage and forms the front of the rib cage, thus helping to protect the heart, lungs, and major bl ...

. 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

monotreme Monotremes () are mammals of the order Monotremata. They are the only group of living mammals that lay eggs, rather than bearing live young. The extant monotreme species are the platypus and the four species of echidnas. Monotremes are typified ...

s, 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.

In dinosaurs

dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic Geological period, period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the #Evolutio ...

s the main bones of the

pectoral girdle The shoulder girdle or pectoral girdle is the set of bones in the appendicular skeleton which connects to the arm on each side. In humans, it consists of the clavicle and scapula; in those species with three bones in the shoulder, it consists o ...

were the scapula (shoulder blade) and the

, both of which directly articulated with the

. The clavicle was present in saurischian dinosaurs but largely absent in ornithischian dinosaurs. The place on the scapula where it articulated with the

(upper bone of the forelimb) is called the glenoid. The scapula serves as the attachment site for a dinosaur's back and forelimb muscles.

Gallery

References

* * Nickel, Schummer, & Seiferle; ''Lehrbuch der Anatomie der Haussäugetiere''.

External links

* - "Joints of the Upper Extremity: Scapula" *
shoulder/bones/bones2
at the Dartmouth Medical School's Department of Anatomy *
shoulder/surface/surface2
at the Dartmouth Medical School's Department of Anatomy * () {{Authority control Bones of the upper limb Flat bones Shoulder