The shoulder girdle or pectoral girdle is the set of
bones
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, an ...
in the
appendicular skeleton
The appendicular skeleton is the portion of the skeleton of vertebrates consisting of the bones that support the appendages. There are 126 bones. The appendicular skeleton includes the skeletal elements within the limbs, as well as supporting shou ...
which connects to the
arm
In human anatomy, the arm refers to the upper limb in common usage, although academically the term specifically means the upper arm between the glenohumeral joint (shoulder joint) and the elbow joint. The distal part of the upper limb between th ...
on each side. In humans it consists of the
clavicle
The clavicle, or collarbone, is a slender, S-shaped long bone approximately 6 inches (15 cm) long that serves as a strut between the shoulder blade and the sternum (breastbone). There are two clavicles, one on the left and one on the rig ...
and
scapula
The scapula (plural scapulae or scapulas), also known as the shoulder blade, is the bone that connects the humerus (upper arm bone) with the clavicle (collar bone). Like their connected bones, the scapulae are paired, with each scapula on eithe ...
; in those
species
In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...
with three bones in the shoulder, it consists of the clavicle, scapula, and
coracoid
A coracoid (from Greek κόραξ, ''koraks'', raven) 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 prese ...
. Some
mammal
Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or ...
ian species (such as the
dog
The dog (''Canis familiaris'' or ''Canis lupus familiaris'') is a domesticated descendant of the wolf. Also called the domestic dog, it is derived from the extinct Pleistocene wolf, and the modern wolf is the dog's nearest living relative. Do ...
and the
horse
The horse (''Equus ferus caballus'') is a domesticated, one-toed, hoofed mammal. It belongs to the taxonomic family Equidae and is one of two extant subspecies of ''Equus ferus''. The horse has evolved over the past 45 to 55 million y ...
) have only the scapula.
The pectoral girdles are to the upper limbs as the
pelvic girdle
The pelvis (plural pelves or pelvises) is the lower part of the trunk, between the abdomen and the thighs (sometimes also called pelvic region), together with its embedded skeleton (sometimes also called bony pelvis, or pelvic skeleton).
The p ...
is to the lower limbs; the girdles are the parts of the
appendicular skeleton
The appendicular skeleton is the portion of the skeleton of vertebrates consisting of the bones that support the appendages. There are 126 bones. The appendicular skeleton includes the skeletal elements within the limbs, as well as supporting shou ...
that anchor the
appendage
An appendage (or outgrowth) is an external body part, or natural prolongation, that protrudes from an organism's body.
In arthropods, an appendage refers to any of the homologous body parts that may extend from a body segment, including anten ...
s to the axial skeleton.
In humans, the only true anatomical
joint
A joint or articulation (or articular surface) is the connection made between bones, ossicles, or other hard structures in the body which link an animal's skeletal system into a functional whole.Saladin, Ken. Anatomy & Physiology. 7th ed. McGraw ...
s between the shoulder girdle and the axial skeleton are the
sternoclavicular joints on each side. No anatomical joint exists between each scapula and the
rib cage
The rib cage, as an enclosure that comprises the ribs, vertebral column and sternum in the thorax of most vertebrates, protects vital organs such as the heart, lungs and great vessels.
The sternum, together known as the thoracic cage, is a semi- ...
; instead the
muscular
Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of muscle ...
connection or physiological joint between the two permits great mobility of the shoulder girdle compared to the compact
pelvic girdle
The pelvis (plural pelves or pelvises) is the lower part of the trunk, between the abdomen and the thighs (sometimes also called pelvic region), together with its embedded skeleton (sometimes also called bony pelvis, or pelvic skeleton).
The p ...
; because the
upper limb
The upper limbs or upper extremities are the forelimbs of an upright-postured tetrapod vertebrate, extending from the scapulae and clavicles down to and including the digits, including all the musculatures and ligaments involved with the shoulde ...
is not usually involved in weight bearing, its stability has been sacrificed in exchange for greater mobility. In those species having only the scapula, no joint exists between the
forelimb
A forelimb or front limb is one of the paired articulated appendages (limbs) attached on the cranial ( anterior) end of a terrestrial tetrapod vertebrate's torso. With reference to quadrupeds, the term foreleg or front leg is often used instead. ...
and the
thorax
The thorax or chest is a part of the anatomy of humans, mammals, and other tetrapod animals located between the neck and the abdomen. In insects, crustaceans, and the extinct trilobites, the thorax is one of the three main divisions of the cre ...
, the only attachment being muscular.
In humans
The shoulder girdle is the anatomical mechanism that allows for all upper arm and shoulder movement in humans. The shoulder girdle consists of five muscles that attach to the
clavicle
The clavicle, or collarbone, is a slender, S-shaped long bone approximately 6 inches (15 cm) long that serves as a strut between the shoulder blade and the sternum (breastbone). There are two clavicles, one on the left and one on the rig ...
and
scapula
The scapula (plural scapulae or scapulas), also known as the shoulder blade, is the bone that connects the humerus (upper arm bone) with the clavicle (collar bone). Like their connected bones, the scapulae are paired, with each scapula on eithe ...
and allow for the motion of the
sternoclavicular joint
The sternoclavicular joint or sternoclavicular articulation is a synovial saddle joint between the manubrium of the sternum, and the clavicle, as well as the first rib. The joint possesses a joint capsule, and an articular disk, and is reinfor ...
(connection between sternum and clavicle) and
acromioclavicular joint
The acromioclavicular joint, or AC joint, is a joint at the top of the shoulder. It is the junction between the acromion (part of the scapula that forms the highest point of the shoulder) and the clavicle. It is a plane synovial joint.
Structure ...
(connection between clavicle and scapula). The five muscles that comprise the function of the shoulder girdle are 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 spine and laterally to the spine of the scapula. It moves the scapula and supports the ...
(upper, middle, and lower),
levator scapulae muscle
The levator scapulae is a slender skeletal muscle situated at the back and side of the neck. As the Latin name suggests, its main function is to lift the scapula.
Anatomy
Attachments
The muscle descends diagonally from its origin to its inserti ...
,
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 rhom ...
(major and minor),
serratus anterior muscle
The serratus anterior is a muscle that originates on the surface of the 1st to 8th ribs at the side of the chest and inserts along the entire anterior length of the medial border of the scapula. The serratus anterior acts to pull the scapula fo ...
, and
pectoralis minor muscle
Pectoralis minor muscle () is a thin, triangular muscle, situated at the upper part of the chest, beneath the pectoralis major in the human body.
Structure
Attachments
Pectoralis minor muscle arises from the upper margins and outer surface ...
.
Joints
The shoulder girdle is a complex of 5 joints that can be divided into two groups. 3 of these joints are true
anatomical
Anatomy () is the branch of biology concerned with the study of the structure of organisms and their parts. Anatomy is a branch of natural science that deals with the structural organization of living things. It is an old science, having it ...
joints, while 2 are
physiological
Physiology (; ) is the scientific study of functions and mechanisms in a living system. As a sub-discipline of biology, physiology focuses on how organisms, organ systems, individual organs, cells, and biomolecules carry out the chemical ...
("false") joints.
[Physiological joints are called "false" joints because they lack the characteristics of "true" anatomical joints, such as tendons and a capsule, but they are still joints because the gliding movements they provide play an important biomechanical role. ()] Within each group, the joints are mechanically linked so that both groups simultaneously contribute to the different movements of the shoulder to variable degrees.
In the first group, the scapulohumeral or
glenohumeral joint
The shoulder joint (or glenohumeral joint from Greek ''glene'', eyeball, + -''oid'', 'form of', + Latin ''humerus'', shoulder) is structurally classified as a synovial ball-and-socket joint and functionally as a diarthrosis and multiaxial joint. ...
is the anatomical joint mechanically linked to the physiological subdeltoid or
suprahumeral joint (the "second shoulder joint") so that movements in the suprahumeral joint results in movements in the
glenohumeral
The shoulder joint (or glenohumeral joint from Greek ''glene'', eyeball, + -''oid'', 'form of', + Latin ''humerus'', shoulder) is structurally classified as a synovial ball-and-socket joint and functionally as a diarthrosis and multiaxial joint. ...
joint.
In the second group, the scapulocostal or
scapulothoracic joint
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 of t ...
is the important physiological joint that can not function without the two anatomical joints in the group, the
acromioclavicular
The acromioclavicular joint, or AC joint, is a joint at the top of the shoulder. It is the junction between the acromion (part of the scapula that forms the highest point of the shoulder) and the clavicle. It is a plane synovial joint.
Structure ...
and
sternoclavicular joint
The sternoclavicular joint or sternoclavicular articulation is a synovial saddle joint between the manubrium of the sternum, and the clavicle, as well as the first rib. The joint possesses a joint capsule, and an articular disk, and is reinfor ...
s, i.e. they join both ends of the
clavicle
The clavicle, or collarbone, is a slender, S-shaped long bone approximately 6 inches (15 cm) long that serves as a strut between the shoulder blade and the sternum (breastbone). There are two clavicles, one on the left and one on the rig ...
.
Glenohumeral joint
The
glenohumeral joint
The shoulder joint (or glenohumeral joint from Greek ''glene'', eyeball, + -''oid'', 'form of', + Latin ''humerus'', shoulder) is structurally classified as a synovial ball-and-socket joint and functionally as a diarthrosis and multiaxial joint. ...
is the articulation between the
head of the humerus
The humerus (; ) 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 and ulna, and consists of three sections. The humeral upper extremity consists of a round ...
and 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 el, gléne, "socket", reflecting the shoulder joint's ball-and-socket form. It is a sha ...
of the
scapula
The scapula (plural scapulae or scapulas), also known as the shoulder blade, is the bone that connects the humerus (upper arm bone) with the clavicle (collar bone). Like their connected bones, the scapulae are paired, with each scapula on eithe ...
. It is a
ball and socket
The ball-and-socket joint (or spheroid joint) is a type of synovial joint in which the ball-shaped surface of one rounded bone fits into the cup-like depression of another bone. The distal bone is capable of motion around an indefinite number of ...
type of
synovial joint with three rotatory and three translatory degrees of freedom. The glenohumeral joint allows for
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 relative ...
,
abduction,
medial and
lateral rotation
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 ...
,
flexion
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 relative ...
and
extension
Extension, extend or extended may refer to:
Mathematics
Logic or set theory
* Axiom of extensionality
* Extensible cardinal
* Extension (model theory)
* Extension (predicate logic), the set of tuples of values that satisfy the predicate
* E ...
of the arm.
Acromioclavicular joint
The
acromioclavicular joint
The acromioclavicular joint, or AC joint, is a joint at the top of the shoulder. It is the junction between the acromion (part of the scapula that forms the highest point of the shoulder) and the clavicle. It is a plane synovial joint.
Structure ...
is the articulation between the
acromion process
In human anatomy, the acromion (from Greek: ''akros'', "highest", ''ōmos'', "shoulder", plural: acromia) is a bony process on the scapula (shoulder blade). Together with the coracoid process it extends laterally over the shoulder joint. The acr ...
of the scapula and the
lateral
Lateral is a geometric term of location which may refer to:
Healthcare
*Lateral (anatomy), an anatomical direction
*Lateral cricoarytenoid muscle
*Lateral release (surgery), a surgical procedure on the side of a kneecap
Phonetics
*Lateral cons ...
end of the
clavicle
The clavicle, or collarbone, is a slender, S-shaped long bone approximately 6 inches (15 cm) long that serves as a strut between the shoulder blade and the sternum (breastbone). There are two clavicles, one on the left and one on the rig ...
. It is a
plane
Plane(s) most often refers to:
* Aero- or airplane, a powered, fixed-wing aircraft
* Plane (geometry), a flat, 2-dimensional surface
Plane or planes may also refer to:
Biology
* Plane (tree) or ''Platanus'', wetland native plant
* ''Planes' ...
type of synovial joint. The acromion of the scapula rotates on the acromial end of the clavicle.
Sternoclavicular joint
The
sternoclavicular joint
The sternoclavicular joint or sternoclavicular articulation is a synovial saddle joint between the manubrium of the sternum, and the clavicle, as well as the first rib. The joint possesses a joint capsule, and an articular disk, and is reinfor ...
is the articulation of the
manubrium of the
sternum
The sternum 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 blood vessels from injury. Sh ...
and the first
costal cartilage
The costal cartilages are bars of hyaline cartilage that serve to prolong the ribs forward and contribute to the elasticity of the walls of the thorax. Costal cartilage is only found at the anterior ends of the ribs, providing medial extension.
...
with the
medial end of the
clavicle
The clavicle, or collarbone, is a slender, S-shaped long bone approximately 6 inches (15 cm) long that serves as a strut between the shoulder blade and the sternum (breastbone). There are two clavicles, one on the left and one on the rig ...
. It is a
saddle
The saddle is a supportive structure for a rider of an animal, fastened to an animal's back by a girth. The most common type is equestrian. However, specialized saddles have been created for oxen, camels and other animals. It is not kno ...
type of synovial joint but functions as a plane joint. The sternoclavicular joint accommodates a wide range of scapula movements and can be raised to a 60° angle.
Scapulocostal joint
The scapulocostal joint (also known as the scapulothoracic joint) is a
physiological
Physiology (; ) is the scientific study of functions and mechanisms in a living system. As a sub-discipline of biology, physiology focuses on how organisms, organ systems, individual organs, cells, and biomolecules carry out the chemical ...
joint formed by an articulation of the
anterior
Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...
scapula
The scapula (plural scapulae or scapulas), also known as the shoulder blade, is the bone that connects the humerus (upper arm bone) with the clavicle (collar bone). Like their connected bones, the scapulae are paired, with each scapula on eithe ...
and the
posterior thoracic rib cage. It is musculotendinous in nature and is formed predominantly by 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 spine and laterally to the spine of the scapula. It moves the scapula and supports the ...
,
rhomboids and
serratus anterior
The serratus anterior is a muscle that originates on the surface of the 1st to 8th ribs at the side of the chest and inserts along the entire anterior length of the medial border of the scapula. The serratus anterior acts to pull the scapula fo ...
muscles. The
pectoralis minor
Pectoralis minor muscle () is a thin, triangular muscle, situated at the upper part of the chest, beneath the pectoralis major in the human body.
Structure
Attachments
Pectoralis minor muscle arises from the upper margins and outer surfaces ...
also plays a role in its movements. The gliding movements at the scapulocostal joint are
elevation
The elevation of a geographic location is its height above or below a fixed reference point, most commonly a reference geoid, a mathematical model of the Earth's sea level as an equipotential gravitational surface (see Geodetic datum § Vert ...
,
depression,
retraction,
protraction
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 relative ...
and superior and inferior rotation of the scapula. Disorders of the scapulocostal joint are not very common and usually restricted to
snapping scapula.
Suprahumeral joint
The suprahumeral joint (also known as the subacromial joint) is a physiological joint formed by an articulation of the
coracoacromial ligament
The coracoacromial ligament is a strong triangular ligament between the coracoid process and the acromion. It protects the head of the humerus. Its acromial attachment may be repositioned to the clavicle during reconstructive surgery of the acrom ...
and the
head of the humerus
The humerus (; ) 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 and ulna, and consists of three sections. The humeral upper extremity consists of a round ...
. It is formed by the gap between the humerus and the acromion process of the scapula. This space is filled mostly by the
subacromial bursa
The subacromial bursa is the synovial cavity located just below the acromion, which communicates with the subdeltoid bursa in most individuals, forming the so-called subacromial-subdeltoid bursa (SSB).
The SSB bursa is located deep to the deltoid ...
and the
tendon
A tendon or sinew is a tough, high-tensile-strength band of dense fibrous connective tissue that connects muscle to bone. It is able to transmit the mechanical forces of muscle contraction to the skeletal system without sacrificing its ability ...
of
supraspinatus
The supraspinatus (plural ''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 rotat ...
. This joint plays a role during complex movements while the arm is fully flexed at the glenohumeral joint, such as changing a lightbulb, or painting a ceiling.
Movements
From its neutral position, the shoulder girdle can be rotated about an imaginary vertical axis at the medial end of the clavicle (the sternoclavicular joint). Throughout this movement the scapula is rotated around the chest wall so that it moves laterally and the glenoid cavity is rotated 40–45° in the horizontal plane. When the scapula is moved medially it lies in a frontal plane with the glenoid cavity facing directly laterally. At this position, the lateral end of the clavicle is rotated posteriorly so that the angle at the acromioclavicular joint opens up slightly. When the scapula is moved laterally it lies in a sagittal plane with the glenoid cavity facing anteriorly. At this position, the lateral end of the clavicle is rotated anteriorly so that the clavicle lies in a frontal plane. While this slightly closes the angle between the clavicle and the scapula, it also widens the shoulder.
The scapula can be elevated and depressed from the neutral position to a total range of ; at its most elevated position the scapula is always tilted so that the glenoid cavity is facing superiorly. During this tilting, the scapula rotates to a maximum angle of 60° about an axis passing perpendicularly through the bone slightly below the spine; this causes the inferior angle to move and the lateral angle .
Injury
Shoulders are a common place for tissue injuries, especially if the person plays overhead sports such as tennis,
volleyball
Volleyball is a team sport in which two teams of six players are separated by a net. Each team tries to score points by grounding a ball on the other team's court under organized rules. It has been a part of the official program of the Summ ...
, baseball, swimming, etc. According to Bahr's major injury related statistics, shoulder dislocations or subluxations account for 4% of injuries in adults ages 20–30 and 20% of shoulder injuries are fractures. Damage to the shoulder and adjacent features can fluctuate in severity depending on the person's age, sport, position, recurring shoulder dysfunction, and many other factors. Some other common shoulder injuries are fractures to any shoulder girdle bones i.e.
clavicle
The clavicle, or collarbone, is a slender, S-shaped long bone approximately 6 inches (15 cm) long that serves as a strut between the shoulder blade and the sternum (breastbone). There are two clavicles, one on the left and one on the rig ...
, ligamentous sprains such as
AC joint or GH ligaments, rotator cuff injuries, different labral tears, and other acute or chronic conditions related to shoulder anatomy.
Shoulder girdle pain can be
acute
Acute may refer to:
Science and technology
* Acute angle
** Acute triangle
** Acute, a leaf shape in the glossary of leaf morphology
* Acute (medicine), a disease that it is of short duration and of recent onset.
** Acute toxicity, the adverse eff ...
or
chronic and be due to a number of causes.
Inflammation
Inflammation (from la, wikt:en:inflammatio#Latin, inflammatio) is part of the complex biological response of body tissues to harmful stimuli, such as pathogens, damaged cells, or Irritation, irritants, and is a protective response involving im ...
or injury of associated
tendons
A tendon or sinew is a tough, high-tensile-strength band of dense fibrous connective tissue that connects muscle to bone. It is able to transmit the mechanical forces of muscle contraction to the skeletal system without sacrificing its ability ...
,
bone
A bone is a Stiffness, rigid Organ (biology), organ that constitutes part of the skeleton in most vertebrate animals. Bones protect the various other organs of the body, produce red blood cell, red and white blood cells, store minerals, provid ...
,
muscles
Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of muscle ...
,
nerves
A nerve is an enclosed, cable-like bundle of nerve fibers (called axons) in the peripheral nervous system.
A nerve transmits electrical impulses. It is the basic unit of the peripheral nervous system. A nerve provides a common pathway for the e ...
,
ligaments
A ligament is the fibrous connective tissue that connects bones to other bones. It is also known as ''articular ligament'', ''articular larua'', ''fibrous ligament'', or ''true ligament''. Other ligaments in the body include the:
* Peritoneal li ...
, and
cartilage
Cartilage is a resilient and smooth type of connective tissue. In tetrapods, it covers and protects the ends of long bones at the joints as articular cartilage, and is a structural component of many body parts including the rib cage, the neck an ...
can all cause pain. Also, past injury compensation, and stress can result in complicated shoulder pain.
Disorders
In humans,
winged scapula
A winged scapula (scapula alata) is a skeletal medical condition in which the shoulder blade protrudes from a person's back in an abnormal position.
In rare conditions it has the potential to lead to limited functional activity in the upper ex ...
is a condition in which the shoulder blade protrudes from a person's back in an abnormal position.
Other animals
The pectoral girdle demonstrates an enormous variation in
amniote
Amniotes are a clade of tetrapod vertebrates that comprises sauropsids (including all reptiles and birds, and extinct parareptiles and non-avian dinosaurs) and synapsids (including pelycosaurs and therapsids such as mammals). They are disti ...
s (vertebrates adapted to a full terrestrial life), both among extant species and along evolutionary lines, and determining
homologies for individual pectoral elements is difficult. Except for the sternum, these elements (along with the
pelvic girdle
The pelvis (plural pelves or pelvises) is the lower part of the trunk, between the abdomen and the thighs (sometimes also called pelvic region), together with its embedded skeleton (sometimes also called bony pelvis, or pelvic skeleton).
The p ...
) were, however, present in early
bony fishes before there were even limbs, arising from their ancestral external armor plates. In digitless
choanates the
cleithrum
The cleithrum (plural cleithra) is a membrane bone which first appears as part of the skeleton in primitive bony fish, where it runs vertically along the scapula. Its name is derived from Greek κλειθρον = " key (lock)", by analogy with "cl ...
, clavicle, and
interclavicle
An interclavicle is a bone which, in most tetrapods, is located between the clavicles. Therian mammals (marsupials and placentals) are the only tetrapods which never have an interclavicle, although some members of other groups also lack one. In t ...
are
dermal
The dermis or corium is a layer of skin between the epidermis (with which it makes up the cutis) and subcutaneous tissues, that primarily consists of dense irregular connective tissue and cushions the body from stress and strain. It is divided ...
and linked to the caudal part of the head while the humerus articulates with a small
scapulocoracoid The scapulocoracoid is the unit of the pectoral girdle that contains the coracoid and scapula.
The coracoid itself is a beak-shaped bone that is commonly found in most vertebrates with a few exceptions.
The scapula is commonly known as the ''shoulde ...
bone. As the first digits appeared, the pectoral structure lost its direct connection to the head skeleton while the scapulocoracoid grew more prominent and started to face laterally. In true tetrapods the dermal part of the girdle was gradually reduced and the scapulocoracoid split into a dorsal scapula and a ventral coracoid.
[Fig. 1]
/ref> As for the sternum, it also came from the fusion of the inner ends of the pectoral girdles in tetrapods
Tetrapods (; ) are four-limbed vertebrate animals constituting the superclass Tetrapoda (). It includes extant and extinct amphibians, sauropsids (reptiles, including dinosaurs and therefore birds) and synapsids (pelycosaurs, extinct therapsids ...
, subsequently growing between the ribs
The rib cage, as an enclosure that comprises the ribs, vertebral column and sternum in the thorax of most vertebrates, protects vital organs such as the heart, lungs and great vessels.
The sternum, together known as the thoracic cage, is a semi- ...
.
In dinosaurs
In dinosaurs
Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...
the main bones of the pectoral girdle were the scapula
The scapula (plural scapulae or scapulas), also known as the shoulder blade, is the bone that connects the humerus (upper arm bone) with the clavicle (collar bone). Like their connected bones, the scapulae are paired, with each scapula on eithe ...
(shoulder blade) and the coracoid
A coracoid (from Greek κόραξ, ''koraks'', raven) 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 prese ...
, both of which directly articulated with the clavicle
The clavicle, or collarbone, is a slender, S-shaped long bone approximately 6 inches (15 cm) long that serves as a strut between the shoulder blade and the sternum (breastbone). There are two clavicles, one on the left and one on the rig ...
. The clavicle was present in saurischian
Saurischia ( , meaning "reptile-hipped" from the Greek ' () meaning 'lizard' and ' () meaning 'hip joint') is one of the two basic divisions of dinosaurs (the other being Ornithischia), classified by their hip structure. Saurischia and Ornithis ...
dinosaurs but largely absent in ornithischian
Ornithischia () is an extinct order of mainly herbivorous dinosaurs characterized by a pelvic structure superficially similar to that of birds. The name ''Ornithischia'', or "bird-hipped", reflects this similarity and is derived from the Greek st ...
dinosaurs. The place on the scapula where it articulated with the humerus
The humerus (; ) 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 and ulna, and consists of three sections. The humeral upper extremity consists of a roun ...
(upper bone of the forelimb) is the called the glenoid
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 el, gléne, "socket", reflecting the shoulder joint's ball-and-socket form. It is a sha ...
. The scapula served as the attachment site for a dinosaur's back and forelimb muscles.
In primates
Chimpanzee
The chimpanzee (''Pan troglodytes''), also known as simply the chimp, is a species of great ape native to the forest and savannah of tropical Africa. It has four confirmed subspecies and a fifth proposed subspecies. When its close relative th ...
s are far better at brachiation than humans. Their clavicles possess a cranially oriented twist on the acromial end, conducive to better force transfer through it - a very important function in arboreal locomotion. Chimpanzee scapulas also possess a considerably larger supraspinous fossa, allowing for a larger supraspinatus muscle. Through the process of evolution, humans have lost the Atlantoclavicularis muscle, originating on the atlas of the vertebral column, and inserting onto the acromial clavicle. This muscle acts to elevate the clavicle.
See also
* Shoulder
The human shoulder is made up of three bones: the clavicle (collarbone), the scapula (shoulder blade), and the humerus (upper arm bone) as well as associated muscles, ligaments and tendons. The articulations between the bones of the shoulder mak ...
* Human skeletal changes due to bipedalism
The evolution of human bipedalism, which began in primates approximately four million years ago, or as early as seven million years ago with '' Sahelanthropus'', or approximately twelve million years ago with ''Danuvius guggenmosi'', has led to m ...
Notes
References
Further reading
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Skeletal system