Pectoral girdle
Encyclopedia
The pectoral girdle or shoulder girdle is the set of bone
s which connects the upper limb
to the axial skeleton
on each side. It consists of the clavicle
and scapula
in humans and, in those species
with three bones in the pectoral girdle, the coracoid. Some mammal
ian species (e.g. the dog
and the horse
) have only the scapula.
In humans, the only true anatomical joint
s between the shoulder girdle and axial skeleton
are the sternoclavicular joints
on each side. No anatomical joint exists between each scapula and the rib cage
; instead the muscular
connection or physiological joint between the two permits great mobility of the shoulder girdle compared to the compact pelvic girdle
— i.e. because the upper limb
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
and the thorax
, the only attachment being muscular.
joints while two are physiological
("false") joints.Physiological joints are called "false" joints because they lack the characteristics of "true" anatomcial joints, such as ligaments 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
is the anatomical joint mechanically linked to the physiological subdeltoid or suprahumeral joint (the "second shoulder joint") so that movements in the latter results in movements in the former. In the second group, the scapulocostal or scapulothoracic joint is the important physiological joint that can not function without the two anatomical joints in the group, the acromioclavicular
and sternoclavicular joints, i.e. the joints at both ends of the clavicle
.
is the articulation between the head of the humerus and the glenoid cavity
of the scapula
. It is a ball and socket type of synovial
joint. The glenohumeral joint allows for adduction
, abduction
, medial and lateral rotation, flexion
and extension
of the arm.
is the articulation between the acromion process
of the scapula and the lateral
end of the clavicle
. It is a plane type of synovial joint. The acromion of the scapula rotates on the acromial end of the clavicle.
of the sternum and the first costal cartilage with the medial
end of the clavicle
. It is a saddle
type of synovial joint but functions as a ball and socket joint. The sternoclavicular joint accommodates a wide range of scapula movements and can be raised to a 60° angle during elevation
of the scapula.
joint formed by an articulation of the anterior scapula
and the posterior thoracic rib cage
. It is musculotendinous in nature and is formed predominantly by the trapezius
, rhomboids
and serratus anterior muscles. The pectoralis minor also plays a role in its movements. The gliding movements at the scapulocostal joint are elevation
, depression
, retraction
, protraction
and superior and inferior rotation of the scapula.
and the head of the humerus. 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
and the tendon
of supraspinatus. 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.
The scapula can be elevated and depressed from the neutral position to a total range of 10 to 12 cm (3.9 to 4.7 in); at its most elevated position the scapula is always tilted so that the glenoid cavity is facing superiorly. During this tilting, the scapula rotate 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 10 to 12 cm (3.9 to 4.7 in) and the lateral angle 5 to 6 cm (2 to 2.4 in).
Bone
Bones are rigid organs that constitute part of the endoskeleton of vertebrates. They support, and protect the various organs of the body, produce red and white blood cells and store minerals. Bone tissue is a type of dense connective tissue...
s which connects the upper limb
Upper limb
The upper limb or upper extremity is the region in an animal extending from the deltoid region to the hand, including the arm, axilla and shoulder.-Definition:...
to the axial skeleton
Skeleton
The skeleton is the body part that forms the supporting structure of an organism. There are two different skeletal types: the exoskeleton, which is the stable outer shell of an organism, and the endoskeleton, which forms the support structure inside the body.In a figurative sense, skeleton can...
on each side. It consists of the clavicle
Clavicle
In human anatomy, the clavicle or collar bone is a long bone of short length that serves as a strut between the scapula and the sternum. It is the only long bone in body that lies horizontally...
and scapula
Scapula
In anatomy, the scapula , omo, or shoulder blade, is the bone that connects the humerus with the clavicle ....
in humans and, in those species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
with three bones in the pectoral girdle, the coracoid. Some mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
ian species (e.g. the dog
Dog
The domestic dog is a domesticated form of the gray wolf, a member of the Canidae family of the order Carnivora. The term is used for both feral and pet varieties. The dog may have been the first animal to be domesticated, and has been the most widely kept working, hunting, and companion animal in...
and the horse
Horse
The horse is one of two extant subspecies of Equus ferus, or the wild horse. It is a single-hooved mammal belonging to the taxonomic family Equidae. The horse has evolved over the past 45 to 55 million years from a small multi-toed creature into the large, single-toed animal of today...
) have only the scapula.
In humans, the only true anatomical joint
Joint
A joint is the location at which two or more bones make contact. They are constructed to allow movement and provide mechanical support, and are classified structurally and functionally.-Classification:...
s between the shoulder girdle and axial skeleton
Axial skeleton
The axial skeleton consists of the 80 bones along the central axis of the human body. It is composed of six parts; the human skull, the ossicles of the middle ear, the hyoid bone of the throat, the rib cage, sternum and the vertebral column...
are the sternoclavicular joints
Sternoclavicular articulation
The sternoclavicular articulation is a synovial double-plane joint composed of two portions separated by an articular disc. The parts entering into its formation are the sternal end of the clavicle, the upper and lateral part of the manubrium sterni , and the cartilage of the first rib, visible...
on each side. No anatomical joint exists between each scapula and the rib cage
Human rib cage
-See also:*Terms for bones*Terms for anatomical location*Articulation of head of rib-References:* Clinically Oriented Anatomy, 4th ed. Keith L. Moore and Robert F. Dalley. pp. 62–64...
; instead the muscular
Muscle
Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
connection or physiological joint between the two permits great mobility of the shoulder girdle compared to the compact pelvic girdle
Pelvis
In human anatomy, the pelvis is the lower part of the trunk, between the abdomen and the lower limbs .The pelvis includes several structures:...
— i.e. because the upper limb
Upper limb
The upper limb or upper extremity is the region in an animal extending from the deltoid region to the hand, including the arm, axilla and shoulder.-Definition:...
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
Forelimb
A forelimb is an anterior limb on an animal's body. When referring to quadrupeds , the term foreleg is often instead used....
and the thorax
Thorax
The thorax is a division of an animal's body that lies between the head and the abdomen.-In tetrapods:...
, the only attachment being muscular.
Joints
The pectoral girdle is a complex of five joints that can be divided into two groups. Three of these joints are true anatomicalAnatomy
Anatomy is a branch of biology and medicine that is the consideration of the structure of living things. It is a general term that includes human anatomy, animal anatomy , and plant anatomy...
joints while two are physiological
Physiology
Physiology is the science of the function of living systems. This includes how organisms, organ systems, organs, cells, and bio-molecules carry out the chemical or physical functions that exist in a living system. The highest honor awarded in physiology is the Nobel Prize in Physiology or...
("false") joints.Physiological joints are called "false" joints because they lack the characteristics of "true" anatomcial joints, such as ligaments 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
Glenohumeral joint
The glenohumeral joint, or shoulder joint, is a multiaxial synovial ball and socket joint and involves articulation between the glenoid fossa of the scapula and the head of the humerus...
is the anatomical joint mechanically linked to the physiological subdeltoid or suprahumeral joint (the "second shoulder joint") so that movements in the latter results in movements in the former. In the second group, the scapulocostal or scapulothoracic joint is the important physiological joint that can not function without the two anatomical joints in the group, the acromioclavicular
Acromioclavicular joint
The acromioclavicular joint, or AC joint, is a joint at the top of the shoulder. It is the junction between the acromion and the clavicle. -Function:...
and sternoclavicular joints, i.e. the joints at both ends of the clavicle
Clavicle
In human anatomy, the clavicle or collar bone is a long bone of short length that serves as a strut between the scapula and the sternum. It is the only long bone in body that lies horizontally...
.
Glenohumeral joint
The glenohumeral jointGlenohumeral joint
The glenohumeral joint, or shoulder joint, is a multiaxial synovial ball and socket joint and involves articulation between the glenoid fossa of the scapula and the head of the humerus...
is the articulation between the head of the humerus and the glenoid cavity
Glenoid cavity
The glenoid cavity is a shallow pyriform, articular surface, which is located on the lateral angle of the scapula. It is directed laterally and forward and articulates with the head of the humerus; it is broader below than above and its vertical diameter is the longest.This cavity forms the...
of the scapula
Scapula
In anatomy, the scapula , omo, or shoulder blade, is the bone that connects the humerus with the clavicle ....
. It is a ball and socket type of synovial
Synovial
Synovial may refer to:* Synovial fluid* Synovial joint* Synovial membrane...
joint. The glenohumeral joint allows for adduction
Adduction
Adduction is a movement which brings a part of the anatomy closer to the middle sagittal plane of the body. It is opposed to abduction.-Upper limb:* of arm at shoulder ** Subscapularis** Teres major** Pectoralis major** Infraspinatus...
, abduction
Abduction (kinesiology)
Abduction, in functional anatomy, is a movement which draws a limb away from the median plane of the body. It is thus opposed to adduction.-Upper limb:* of arm at shoulder ** Supraspinatus** Deltoid* of hand at wrist...
, medial and lateral rotation, flexion
Flexion
In anatomy, flexion is a position that is made possible by the joint angle decreasing. The skeletal and muscular systems work together to move the joint into a "flexed" position. For example the elbow is flexed when the hand is brought closer to the shoulder...
and extension
Extension
Extension may refer to:* A cheerleading stunt* The building of community capacity by outsiders, for instance agricultural extension* Extension , relating to the pulling apart of the Earth's crust and lithosphere...
of the arm.
Acromioclavicular joint
The acromioclavicular jointAcromioclavicular joint
The acromioclavicular joint, or AC joint, is a joint at the top of the shoulder. It is the junction between the acromion and the clavicle. -Function:...
is the articulation between the acromion process
Acromion process
The acromion process, or simply the acromion , is an anatomical feature on the shoulder blade , together with the coracoid process extending laterally over the shoulder joint.-In humans:...
of the scapula and the lateral
Lateral
Lateral may refer to:*Lateral , an anatomical direction*Lateral canal, a canal built beside another stream*Lateral consonant, an ℓ-like consonant in which air flows along the sides of the tongue...
end of the clavicle
Clavicle
In human anatomy, the clavicle or collar bone is a long bone of short length that serves as a strut between the scapula and the sternum. It is the only long bone in body that lies horizontally...
. It is a plane type of synovial joint. The acromion of the scapula rotates on the acromial end of the clavicle.
Sternoclavicular joint
The sternoclavicular joint is the articulation of the manubriumManubrium
The manubrium or manubrium sterni is the broad, upper part of the sternum. Located ventrally with a quadrangular shape, wider superiorly and narrower inferiorly, it articulates with the clavicles and the first two ribs.-Borders:The superior border is the thickest and presents at its center the...
of the sternum and the first costal cartilage with the medial
Medial
- Medial magmas :In abstract algebra, a medial magma is a set with a binary operation which satisfies the identity \cdot = \cdot , or more simply, xy\cdot uv = xu\cdot yv...
end of the clavicle
Clavicle
In human anatomy, the clavicle or collar bone is a long bone of short length that serves as a strut between the scapula and the sternum. It is the only long bone in body that lies horizontally...
. It is a saddle
Saddle joint
In a saddle joint the opposing surfaces are reciprocally concave-convex.-Movements:...
type of synovial joint but functions as a ball and socket joint. The sternoclavicular joint accommodates a wide range of scapula movements and can be raised to a 60° angle during elevation
Elevation
The elevation of a geographic location is its height above a fixed reference point, most commonly a reference geoid, a mathematical model of the Earth's sea level as an equipotential gravitational surface ....
of the scapula.
Scapulocostal joint
The scapulocostal joint (also known as the scapulothoracic joint) is a physiologicalPhysiology
Physiology is the science of the function of living systems. This includes how organisms, organ systems, organs, cells, and bio-molecules carry out the chemical or physical functions that exist in a living system. The highest honor awarded in physiology is the Nobel Prize in Physiology or...
joint formed by an articulation of the anterior scapula
Scapula
In anatomy, the scapula , omo, or shoulder blade, is the bone that connects the humerus with the clavicle ....
and the posterior thoracic rib cage
Rib cage
The rib cage is an arrangement of bones in the thorax of animals. It is formed by the vertebral column, ribs and sternum and encloses the heart and lungs....
. It is musculotendinous in nature and is formed predominantly by the trapezius
Trapezius muscle
In human anatomy, the trapezius is a large superficial muscle that extends longitudinally from the occipital bone to the lower thoracic vertebrae and laterally to the spine of the scapula...
, rhomboids
Rhomboid muscle
The rhomboid muscles, often simply called the rhomboids, are rhombus-shaped muscles associated with the scapula and are chiefly responsible for its retraction. There are two rhomboid muscles:* Rhomboid major muscle* Rhomboid minor muscle...
and serratus anterior muscles. The pectoralis minor also plays a role in its movements. The gliding movements at the scapulocostal joint are elevation
Elevation
The elevation of a geographic location is its height above a fixed reference point, most commonly a reference geoid, a mathematical model of the Earth's sea level as an equipotential gravitational surface ....
, depression
Depression (kinesiology)
Depression, in kinesiology, is the anatomical term of motion for movement in an inferior direction.It is the opposite of elevation.This term is often applied to the shoulders Depression, in kinesiology, is the anatomical term of motion for movement in an inferior direction.It is the opposite of...
, retraction
Retraction (kinesiology)
Retraction is the anatomical term of motion for posterior movement of the arms at the shoulders. It is the opposite of protraction. Major muscles involved include: * Rhomboid major muscle* Rhomboid minor muscle* Trapezius muscle-External links:* *...
, protraction
Protraction
Protraction is the anatomical term of motion for anterior movement of an anatomical part of the body forward . This can include the shoulders, jaw, pelvis, etc.It is the opposite of retraction....
and superior and inferior rotation of the scapula.
Suprahumeral joint
The suprahumeral joint (also known as the subacromial joint) is a physiological joint formed by an articulation of the coracoacromial ligamentCoracoacromial ligament
The Coracoacromial Ligament is a strong triangular band, extending between the coracoid process and the acromion.It is attached, by its apex, to the summit of the acromion just in front of the articular surface for the clavicle; and by its broad base to the whole length of the lateral border of the...
and the head of the humerus. 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
Subacromial bursa
The subacromial bursa is the synovial membrane located just below the acromion.The capsule extends above the humeral head to form a bursa between the humeral head and the overlying acromial process – this is often the site of pathology in impingement of the shoulder....
and the tendon
Tendon
A tendon is a tough band of fibrous connective tissue that usually connects muscle to bone and is capable of withstanding tension. Tendons are similar to ligaments and fasciae as they are all made of collagen except that ligaments join one bone to another bone, and fasciae connect muscles to other...
of supraspinatus. 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 pectoral 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 15 centimetres (5.9 in) 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 10 to 12 cm (3.9 to 4.7 in); at its most elevated position the scapula is always tilted so that the glenoid cavity is facing superiorly. During this tilting, the scapula rotate 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 10 to 12 cm (3.9 to 4.7 in) and the lateral angle 5 to 6 cm (2 to 2.4 in).