The human shoulder is made up of three bones: 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 ...

(collarbone), 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). Like their connected bones, the scapulae are paired, with each scapula on either side ...

(shoulder blade), and 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) as well as associated muscles, ligaments and tendons. The articulations between the bones of the shoulder make up the shoulder

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. 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 ...

, also known as the glenohumeral joint, is the major joint of the shoulder, but can more broadly include 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 joint, plane synovial joint ...

. In

human anatomy Human anatomy (gr. ἀνατομία, "dissection", from ἀνά, "up", and τέμνειν, "cut") is primarily the scientific study of the morphology of the human body. Anatomy is subdivided into gross anatomy and microscopic anatomy. Gross ...

, the shoulder joint comprises the part of the body where the

attaches to the

, and the head sits in 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 ...

. The shoulder is the group of structures in the region of the joint. The

is the main joint of the shoulder. It is a

ball and socket joint 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 ...

that allows the arm to rotate in a circular fashion or to hinge out and up away from the body. The

joint capsule In anatomy, a joint capsule or articular capsule is an envelope surrounding a synovial joint.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). Like their connected bones, the scapulae are paired, with each scapula on either side ...

, and head of 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 ...

. It is lined by a thin, smooth

synovial membrane Synovial () may refer to: * Synovial fluid * Synovial joint A synovial joint, also known as diarthrosis, joins bones or cartilage with a fibrous joint capsule that is continuous with the periosteum of the joined bones, constitutes the outer bou ...

. 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 ...

is a group of four muscles that surround the shoulder joint and contribute to the shoulder's stability. The muscles of the rotator cuff are

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 ...

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 ...

, infraspinatus, and teres minor. The cuff adheres to the

glenohumeral capsule The capsule of the glenohumeral (shoulder) joint is the articular capsule of the shoulder. It completely surrounds the joint. It is attached above to the circumference of the glenoid cavity beyond the glenoidal labrum, and below to the anatomi ...

and attaches to the humeral head. The shoulder must be mobile enough for the wide range actions of the arms and hands, but stable enough to allow for actions such as lifting, pushing, and pulling.

Structure

The shoulder consists of a ball-and-socket joint formed by the humerus and scapula and their surrounding structures -

ligament A ligament is a type of fibrous connective tissue in the body that connects bones to other bones. It also connects flight feathers to bones, in dinosaurs and birds. All 30,000 species of amniotes (land animals with internal bones) have liga ...

muscle Muscle is a soft tissue, one of the four basic types of animal tissue. There are three types of muscle tissue in vertebrates: skeletal muscle, cardiac muscle, and smooth muscle. Muscle tissue gives skeletal muscles the ability to muscle contra ...

tendon A tendon or sinew is a tough band of fibrous connective tissue, dense fibrous connective tissue that connects skeletal muscle, muscle to bone. It sends the mechanical forces of muscle contraction to the skeletal system, while withstanding tensi ...

s - which support the bones and maintain the relationship of one to another. These supporting structures attach to the

, and

, the latter providing the

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 ...

and

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 ...

es. The main

of the shoulder is the

(or glenohumeral joint), between the humerus and the glenoid process of the scapular. The

and sternoclavicular joint also play a role in shoulder movements. White

hyaline cartilage Hyaline cartilage is the glass-like (hyaline) and translucent cartilage found on many joint surfaces. It is also most commonly found in the ribs, nose, larynx, and trachea. Hyaline cartilage is pearl-gray in color, with a firm consistency and has ...

on the ends of the bones (called articular cartilage) allows the bones to glide and move on each other, and the joint space is surrounded by a synovial membrane. Around the joint space are muscles - the rotator cuff, which directly surrounds and attaches to the shoulder joint - and other muscles that help provide stability and facilitate movement. Two filmy sac-like structures called

bursa Bursa () is a city in northwestern Turkey and the administrative center of Bursa Province. The fourth-most populous city in Turkey and second-most populous in the Marmara Region, Bursa is one of the industrial centers of the country. Most of ...

e permit smooth gliding between bone, muscle, and tendon. They cushion and protect the rotator cuff from the bony arch of the acromion. The

glenoid labrum The glenoid labrum (glenoid ligament) is a fibrocartilaginous (but not fibrocartilage, as previously thought) structure attached around the rim of the glenoid cavity on the shoulder blade. The shoulder joint is considered a ball-and-socket joint ...

is the second kind of cartilage in the shoulder which is distinctly different from the articular cartilage. This cartilage is more fibrous or rigid than the cartilage on the ends of the ball and socket. Also, this cartilage is also found only around the socket where it is attached.

Joint

The

(also known as the glenohumeral joint) is the main joint of the shoulder. It is a

that allows the arm to rotate in a circular fashion or to hinge out and up away from the body. It is formed by the articulation between the head of the humerus and the lateral scapula (specifically-the glenoid cavity of the scapula). The "ball" of the joint is the rounded, medial anterior surface of the humerus and the "socket" is formed by the glenoid cavity, the dish-shaped portion of the lateral scapula. The shallowness of the cavity and relatively loose connections between the shoulder and the rest of the body allows the arm to have tremendous mobility, at the expense of being much easier to dislocate than most other joints in the body. There is an approximately 4-to-1 disproportion in size between the large head of the humerus and the shallow glenoid cavity.The glenoid cavity is made deeper by the addition of the fibrocartilaginous ring of the

. The capsule is a soft tissue envelope that encircles the glenohumeral joint and attaches to the scapula, humerus, and head of the

. It is lined by a thin, smooth

. This capsule is strengthened by the coracohumeral ligament which attaches the coracoid process of the scapula to the greater tubercle of the humerus. There are also three other ligaments attaching the lesser tubercle of the humerus to lateral scapula and are collectively called the glenohumeral ligaments. The

transverse humeral ligament The transverse humeral ligament (Brodie's ligament) forms a broad band bridging the lesser and greater tubercle of the humerus. Its attachments are limited superior to the epiphysial line. By enclosing the canal of the bicipital groove (inte ...

, which passes from the

lesser tubercle The lesser tubercle of 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 thre ...

to the greater tubercle of humerus, covers the intertubercular groove, in which the long head of

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 ...

travels.

Rotator cuff

The

is an anatomical term given to the group of four muscles and their tendons that act to stabilize the shoulder. These muscles are the

, infraspinatus, teres minor and

and that hold the head of the humerus in the glenoid cavity during movement. The cuff adheres to the glenohumeral capsule and attaches to the

head of 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 and ulna, and consists of three sections. The humeral upper extremity consists o ...

. Together, these keep the humeral head in the

, preventing upward migration of the humeral head caused by the pull of the

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 ...

at the beginning of arm elevation. The infraspinatus and the teres minor, along with the anterior fibers of the deltoid muscle, are responsible for external rotation of the arm. The four

s of these muscles converge to form the rotator cuff tendon. This tendon, along with the articular capsule, the coracohumeral ligament, and the glenohumeral ligament complex, blend into a confluent sheet before insertion into the humeral tuberosities. The infraspinatus and teres minor fuse near their musculotendinous junctions, while the

and

tendons join as a sheath that surrounds the biceps tendon at the entrance of the bicipital groove.

Other muscles

Muscles from the shoulder region In addition to the four muscles of the rotator cuff, the

and

teres major muscle The teres major muscle is a muscle of the upper limb. It attaches to the scapula and the humerus and is one of the seven scapulohumeral muscles. It is a thick but somewhat flattened muscle. The teres major muscle (from Latin ''teres'', meanin ...

s arise and exist in the shoulder region itself. The deltoid muscle covers the shoulder joint on three sides, arising from the front upper third of the clavicle, the acromion, and the spine of the scapula, and travelling to insert on the deltoid tubercle of the humerus. Contraction of each part of the deltoid assists in different movements of the shoulder - flexion (clavicular part), abduction (middle part) and extension (scapular part). The teres major attaches to the outer part of the back of the scapula, beneath the teres minor, and attaches to the upper part of the humerus. It helps with medial rotation of the humerus. Muscles from the front Muscles from the chest wall that contribute to the shoulder are: Muscles from the back

Armpit

The armpit () is formed by the space between the muscles of the shoulder. The nerves and blood vessels of the arm travel through the armpit, and it possesses several sets of lymph nodes that are able to be examined. The armpit is formed by the pectoralis major and minor muscles at the front, the latissimus dorsi and teres major muscles at the back, the serratus anterior muscle on its inner surface, and the intertubercular groove of the humerus on the outer side.

Nerve supply and passage

The skin around the shoulder is supplied by C2-C4 (upper), and C7 and T2 (lower area). The brachial plexus emerges as nerve roots from the cervical vertebrae C5-T1. Branches of the plexus, in particular from C5-C6, supply the majority of the muscles of the shoulder.

Blood vessels

The subclavian artery arises from the brachiocephalic trunk on the right and directly from the aorta from the left. This becomes the

axillary artery In human anatomy, the axillary artery is a large blood vessel that conveys oxygenated blood to the lateral aspect of the thorax, the axilla (armpit) and the upper limb. Its origin is at the lateral margin of the first rib, before which it is c ...

as it passes beyond the first rib. The axillary artery also supplies blood to the arm, and is one of the major sources of blood to the shoulder region. The other major sources are the

transverse cervical artery The transverse cervical artery (transverse artery of neck or transversa colli artery) is an artery in the neck and a branch of the thyrocervical trunk, running at a higher level than the suprascapular artery. Structure It passes transversely ...

and the

suprascapular artery The suprascapular artery is a branch of the thyrocervical trunk on the neck. Structure At first, it passes downward and laterally across the scalenus anterior and phrenic nerve, being covered by the sternocleidomastoid muscle; it then crosses t ...

, both branches of the thyrocervical trunk which itself is a branch of the subclavian artery. The blood vessels form a network (anastamosis) behind the shoulder that helps to supply blood to the arm even when the axillary artery is compromised.

Function

The muscles and joints of the shoulder allow it to move through a remarkable

range of motion Range of motion (or ROM) is the linear or angular distance that a moving object may normally travel while properly attached to another. In biomechanics and strength training, ROM refers to the angular distance and direction a joint can move be ...

, making it one of the most mobile joints in the human body. The shoulder can abduct,

adduct In chemistry, an adduct (; alternatively, a contraction of "addition product") is a product of a direct addition of two or more distinct molecules, resulting in a single reaction product containing all atoms of all components. The resultant is ...

, rotate, be raised in front of and behind the

torso The torso or trunk is an anatomical terminology, anatomical term for the central part, or the core (anatomy), core, of the body (biology), body of many animals (including human beings), from which the head, neck, limb (anatomy), limbs, tail an ...

and move through a full 360° in the

sagittal plane The sagittal plane (; also known as the longitudinal plane) is an anatomical plane that divides the body into right and left sections. It is perpendicular to the transverse and coronal planes. The plane may be in the center of the body and divi ...

. This tremendous range of motion also makes the shoulder extremely unstable, far more prone to

dislocation In materials science, a dislocation or Taylor's dislocation is a linear crystallographic defect or irregularity within a crystal structure that contains an abrupt change in the arrangement of atoms. The movement of dislocations allow atoms to sli ...

and injury than other joints The following describes the terms used for different movements of the shoulder:

Development

Puberty

Under the influence of

testosterone Testosterone is the primary male sex hormone and androgen in Male, males. In humans, testosterone plays a key role in the development of Male reproductive system, male reproductive tissues such as testicles and prostate, as well as promoting se ...

and

growth hormone Growth hormone (GH) or somatotropin, also known as human growth hormone (hGH or HGH) in its human form, is a peptide hormone that stimulates growth, cell reproduction, and cell regeneration in humans and other animals. It is thus important in ...

, the shoulders broaden in males during

puberty Puberty is the process of physical changes through which a child's body matures into an adult body capable of sexual reproduction. It is initiated by hormonal signals from the brain to the gonads: the ovaries in a female, the testicles i ...

Clinical significance

Leonardo da Vinci - Anatomical studies of the shoulder - WGA12824

The shoulder is the most mobile and potentially unstable joint in the body. Due to this, it is often prone to problems.

Fracture

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 ...

s of shoulder bones can include clavicular fractures, scapular fractures, and fractures of the upper humerus.

Pain

Shoulder problems, including

pain Pain is a distressing feeling often caused by intense or damaging Stimulus (physiology), stimuli. The International Association for the Study of Pain defines pain as "an unpleasant sense, sensory and emotional experience associated with, or res ...

, are common and can relate to any of the structures within the shoulder. The primary cause of shoulder pain is a rotator cuff tear. The

is most commonly involved in a rotator cuff tear, but other parts of the rotator cuff may also be involved. There are different severities of a rotator cuff tear, which range from a partial tear to a full-thickness tear. A partial tear is when the tendon is thinned, but still connected to the bone. Full-thickness tears can be separated into two classes: a full-thickness incomplete tear or a full-thickness complete tear. The incomplete tear is characterized by having only a portion of the tendon disconnected from the bone, where the complete tear has the tendon completely separated off the bone. For all forms of rotator cuff tears, depending on the severity of the injury, possible treatments include rest, an arm sling, physical therapy, steroid injections, and non-steroidal anti-inflammatory drugs, or surgery. When this type of cartilage starts to wear out (a process called

arthritis Arthritis is a general medical term used to describe a disorder that affects joints. Symptoms generally include joint pain and stiffness. Other symptoms may include redness, warmth, Joint effusion, swelling, and decreased range of motion of ...

), the joint becomes painful and stiff. *

Arthritis Arthritis is a general medical term used to describe a disorder that affects joints. Symptoms generally include joint pain and stiffness. Other symptoms may include redness, warmth, Joint effusion, swelling, and decreased range of motion of ...

Frozen shoulder Adhesive capsulitis, also known as frozen shoulder, is a condition associated with shoulder pain and stiffness. It is a common shoulder ailment that is marked by pain and a loss of range of motion, particularly in external rotation. There is ...

* Impingement syndrome * Shoulder dislocation * Nerve entrapment syndrome

Imaging

Imaging of the shoulder includes ultrasound, X-ray and MRI, and is guided by the suspected diagnosis and presenting symptoms. Conventional x-rays and ultrasonography are the primary tools used to confirm a diagnosis of injuries sustained to the rotator cuff. For extended clinical questions, imaging through Magnetic Resonance with or without intraarticular contrast agent is indicated. Hodler et al. recommend starting scanning with conventional x-rays taken from at least two planes, since this method gives a wide first impression and even has the chance of exposing any frequent shoulder pathologies, i.e., decompensated rotator cuff tears, tendinitis calcarea, dislocations, fractures, usures, and/or osteophytes. Furthermore, x-rays are required for the planning of an optimal CT or MR image. The conventional invasive arthrography is nowadays being replaced by the non-invasive MRI and ultrasound, and is used as an imaging reserve for patients who are contraindicated for MRI, for example pacemaker-carriers with an unclear and unsure ultrasonography.

X-ray

Projectional radiography Projectional radiography, also known as conventional radiography, is a form of radiography and medical imaging that produces two-dimensional images by X-ray radiation. The image acquisition is generally performed by radiographers, and the images a ...

views of the shoulder include: ;AP-projection 40° posterior oblique after Grashey The body has to be rotated about 30 to 45 degrees towards the shoulder to be imaged, and the standing or sitting patient lets the arm hang. This method reveals the joint gap and the vertical alignment towards the socket. ;Transaxillary projection The arm should be abducted 80 to 100 degrees. This method reveals: * The horizontal alignment of the humerus head in respect to the socket and the lateral clavicle in respect to the acromion * Lesions of the anterior and posterior socket border, or of the tuberculum minus * The eventual non-closure of the acromial apophysis * The coraco-humeral interval ;Y-projection The lateral contour of the shoulder should be positioned in front of the film in a way that the longitudinal axis of the scapula continues parallel to the path of the rays. This method reveals: * The horizontal centralization of the humerus head and socket * The osseous margins of the coraco-acromial arch and hence the supraspinatus outlet canal * The shape of the acromion This projection has a low tolerance for errors and, accordingly, needs proper execution. The Y-projection can be traced back to Wijnblath’s 1933 published cavitas-en-face projection. CR. shoulder x-ray, a.p..jpg, CR. shoulay film. Transaxillary CR shoulder.jpg, alt=A Transaxillary conventional radiography, Transaxillary conventional radiography Y CR shoulder.jpg, alt=A Y-projection conventional radiography, Y-projection conventional radiography

Ultrasound

There are several advantages of ultrasound. It is relatively cheap, does not emit any radiation, is accessible, is capable of visualizing tissue function in real time, and allows the performance of provocative maneuvers in order to replicate the patient’s pain. Those benefits have helped ultrasound become a common initial choice for assessing tendons and soft tissues. Limitations include, for example, the high degree of operator dependence and the inability to define pathologies in bones. One also has to have an extensive anatomical knowledge of the examined region and keep an open mind to normal variations and artifacts created during the scan. Although musculoskeletal ultrasound training, like medical training in general, is a lifelong process, Kissin ''et al.'' suggests that rheumatologists who taught themselves how to manipulate ultrasound can use it just as well as international musculo-skeletal ultrasound experts to diagnose common rheumatic conditions. After the introduction of high-frequency transducers in the mid-1980s, ultrasound has become a conventional tool for taking accurate and precise images of the shoulder to support diagnosis. Adequate for the examination are high-resolution, high-frequency transducers with a transmission frequency of 5, 7.5, and 10 MHz. To improve the focus on structures close to the skin an additional "water start-up length" is advisable. During the examination the patient is asked to be seated, the affected arm is then adducted and the elbow is bent to 90 degrees. Slow and cautious passive lateral and/or medial rotations have the effect of being able to visualize different sections of the shoulder. In order to also demonstrate those parts which are hidden under the acromion in the neutral position, a maximum medial rotation with hyperextension behind the back is required. To avoid the different tendon echogenicities caused by different instrument settings, Middleton compared the tendon’s echogenicity with that of the deltoid muscle, which is still lege artis. Usually the echogenicity compared to the deltoid muscle is homogeneous intensified without dorsal echo extinction. Variability with reduced or intensified echo has also been found in healthy tendons. Bilateral comparison is very helpful when distinguishing and setting boundaries between physiological variants and a possible pathological finding. Degenerative changes at the rotator cuff often are found on both sides of the body. Consequently, unilateral differences rather point to a pathological source and bilateral changes rather to a physiological variation. In addition, a dynamic examination can help to differentiate between an ultrasound artifact and a real pathology. To accurately evaluate the echogenicity of an ultrasound, one has to take into account the physical laws of reflection, absorption and dispersion. It is at all times important to acknowledge that the structures in the joint of the shoulder are not aligned in the transversal, coronal or sagittal plane, and that therefore during imaging of the shoulder the transducer head has to be held perpendicularly or parallel to the structures of interest. Otherwise the appearing echogenicity may not be evaluated.Katthagen BD. et al.. Schultersonographie. Stuttgart.

MRI

Orthopedics established the MRI early on as the tool of choice for joint- and soft tissue-imaging because of its non-invasiveness, lack of radiation exposure, multi planar slicing possibilities and the high soft tissue contrast. MRIs can provide joint details to the treating orthopedist, helping them to diagnose and decide the next appropriate therapeutic step. To examine the shoulder, the patient should lay down with the concerned arm is in lateral rotation. For signal detection it is recommended to use a surface-coil. To find pathologies of the rotator cuff in the basic diagnostic investigation, T2-weighted sequences with fat-suppression or STIR sequences have proven value. In general, the examination should occur in the following three main planes: axial, oblique coronal and sagittal. Most morphological changes and injuries are sustained to the supraspinatus tendon. Traumatic rotator cuff changes are often located antero-superior, meanwhile degenerative changes more likely are supero-posterior.
Tendons are predominantly composed of dense collagen fiber bundles. Because of their extreme short T2-relaxation time they appear typically signal-weak, respectively, dark. Degenerative changes, inflammations and also partial and complete tears cause loss of the original tendon structure. Fatty deposits, mucous degeneration and hemorrhages lead to an increased intratendinal T1-image. Edema formations, inflammatory changes and ruptures increase the signals in a T2-weighted image.

MRA

While using MRI, true lesions at the rotator interval region between the parts of the supraspinatus and subscapularis are all but impossible to distinguish from normal synovium and capsule. In 1999, Weishaupt D. et al. reached through two readers a significant better visibility of pulley lesions at the rotator interval and the expected location of the reflection pulley of the long biceps and subscapularis tendon on parasagittal (reader1/reader2 sensitivity: 86%/100%; specificity: 90%/70%) and axial (reader1/reader2 sensitivity: 86%/93%; specificity: 90%/80%) MRA images. When examining the rotator cuff, the MRA has a couple of advantages compared to the native MRI. Through a fat suppressed T2-weighted spin echo, MRA can reproduce an extreme high fat-water-contrast, which helps to detect water-deposits with better damage diagnosis in structurally changed collagen fiber bundles.

Other animals

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 ...

forelimbs are characterised by a high degree of mobility in the shoulder-thorax connection. Lacking a solid skeletal connection between the shoulder girdle and the vertebral column, the forelimb's attachment to the trunk is instead mainly controlled by

serratus lateralis 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 thora ...

and levator scapulae. Depending on locomotor style, a bone connects the shoulder girdle to the trunk in some animals; 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 ...

bone in reptiles and birds, and the

primate Primates is an order (biology), order of mammals, which is further divided into the Strepsirrhini, strepsirrhines, which include lemurs, galagos, and Lorisidae, lorisids; and the Haplorhini, haplorhines, which include Tarsiiformes, tarsiers a ...

s and

bat Bats are flying mammals of the order Chiroptera (). With their forelimbs adapted as wings, they are the only mammals capable of true and sustained flight. Bats are more agile in flight than most birds, flying with their very long spread-out ...

s. In primates, the shoulder shows characteristics that differ from other mammals, including a well developed clavicle, a dorsally shifted scapula with prominent acromion and spine, and a humerus featuring a straight shaft and a spherical head.

Additional images

File:Gray326.png, The left shoulder and acromioclavicular joints, and the proper ligaments of the scapula

References

External links

Video of the shoulder carriage in motion

{{Authority control Upper limb anatomy