Dominic Stone
The pectoral region is located on the anterior chest wall and contains four muscles: the pectoralis major, pectoralis minor, serratus anterior, and subclavius. These muscles connect the front walls of the chest with the bones of the upper arm and shoulder. The pectoralis major is the largest muscle in the pectoral region and lies underneath the breast. It is thick and fan-shaped, with two heads: the clavicular and sternocostal. The pectoralis minor is a thin, triangular muscle located beneath the pectoralis major. It attaches to the ribs and stabilizes the scapula. The serratus anterior is located laterally in the chest wall and forms the medial border of the axilla region. The subclavius is a small muscle located underneath the clavicle.
| Characteristics | Values |
|---|---|
| Location | Front walls of the chest |
| Connection | Bones of the upper arm and shoulder |
| Number | Two on each side of the human body |
| Muscle types | Pectoralis major, pectoralis minor, serratus anterior, subclavius |
| Pectoralis major shape | Thick, fan-shaped or triangular convergent |
| Pectoralis major location | Underneath the breast tissue |
| Pectoralis minor shape | Thin, triangular |
| Pectoralis minor location | Beneath the pectoralis major |
| Pectoralis minor function | Stabilises the scapula |
| Serratus anterior location | Laterally in the chest wall |
| Serratus anterior function | Holds the scapula against the ribcage |
| Subclavius location | Underneath the clavicle |
| Subclavius function | Anchors and depresses the clavicle |
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Pectoralis major
The pectoralis major is the superior most and largest muscle of the anterior chest wall. It is a thick, fan-shaped muscle that lies underneath the breast tissue and forms the anterior wall of the axilla. The pectoralis major is the most superficial muscle in the pectoral region. It is also referred to as ""pecs", "pectoral muscle", or "chest muscle".
The pectoralis major has two heads: the clavicular and the sternocostal, which reference their area of origin. The clavicular head originates from the anterior surface of the medial half of the clavicle. The sternocostal head, which is the larger of the two heads, originates from the anterior surface of the sternum, the superior six costal cartilages, and the aponeurosis of the external oblique muscle. The upper and lower fibres of the pectoralis major insert into the crest of the greater tubercle of the humerus.
The pectoralis major has several functions, including flexion, adduction, and internal rotation of the humerus, the long bone of the upper arm. It also assists in breathing when the diaphragm and intercostal muscles are insufficient. Additionally, it is responsible for keeping the arm attached to the trunk of the body.
The pectoralis major is highly vascularised and has been used in flap repairs in neck surgery. It is also utilised in soft tissue reconstruction of the neck and face following trauma or tumour-related operations. However, certain surgical procedures, such as breast reconstruction and augmentation, can pose risks to the nervous supply of the pectoralis major, potentially leading to denervation or atrophy.
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Pectoralis minor
The pectoralis minor is a thin, triangular muscle located in the pectoral region of the anterior chest wall. It is one of four muscles in this region, along with the pectoralis major, serratus anterior, and subclavius. The pectoralis minor lies underneath the pectoralis major, its larger counterpart.
The pectoralis minor originates from the third to fifth ribs and inserts into the coracoid process of the scapula. It serves to stabilise the scapula, the large bone of the shoulder, by drawing it towards the thorax. This movement is important in reaching the arm forward. The pectoralis minor also acts as an accessory muscle of respiration, elevating the ribs during deep inspiration.
The medial and lateral pectoral nerves penetrate the pectoralis minor muscle to innervate it. The primary nerve supply to the muscle comes from the medial pectoral nerve, a minor branch of the brachial plexus that arises from the cervical portion of the spinal cord. The vascular supply to the pectoralis minor comes from several sources, including the thoracoacromial artery, superior thoracic artery, and lateral thoracic artery.
The pectoralis minor is clinically important and used as a surgical landmark due to the structures that lie below it, including the nerves and blood supply to the upper limb. It also forms a bridge for structures passing into the upper limb from the thorax. Dysfunction of the pectoralis minor can result in pain and restriction in neck and upper back movements, as well as an increased risk of rotator cuff issues.
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Serratus anterior
The serratus anterior is a fan-shaped muscle that originates on the superolateral surfaces of the first to eighth or ninth ribs at the lateral wall of the thorax. Its main part lies deep under the scapula and the pectoral muscles. It is easily palpable between the pectoralis major and latissimus dorsi muscles.
The serratus anterior is divided into three parts: the superior, intermediate, and inferior. The superior part involves the first and second ribs attaching to the superior angle of the scapula. The intermediate part involves the second and third ribs attaching to the medial border of the scapula. The inferior part involves the fourth to eighth or ninth ribs attaching to the medial border and inferior angle of the scapula. The lowest attachment may extend to the tenth rib in approximately 10% of the population. The inferior part is the most prominent and powerful.
The serratus anterior acts on the scapula and is the prime mover in scapular protraction and scapular upward rotation. It is a key scapular stabiliser, keeping the shoulder blades against the rib cage when at rest and during movement. When the shoulder girdle is fixed, all three parts of the serratus anterior muscle work together to lift the ribs, assisting with respiration.
The innervation of the serratus anterior is supplied by the long thoracic nerve, a branch of the brachial plexus. The vascular supply comes from the superior and lateral thoracic arteries and branches from the thoracodorsal artery.
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Subclavius
The subclavius is a small, triangular muscle in the pectoral region of the anterior thorax, located directly underneath the clavicle. It originates from the junction of the first rib and its costal cartilage, inserting on the inferior surface of the middle third of the clavicle. The muscle is enclosed by the clavipectoral fascia, a fibrous connective tissue that spans the interval between the clavicle and pectoralis minor muscle.
The subclavius muscle is innervated by the nerve to the subclavius, a branch of the brachial plexus. It receives arterial blood from the clavicular branch of the thoracoacromial artery, with contributions from the suprascapular artery. The main function of the subclavius is to actively stabilize the clavicle at the sternoclavicular joint during movements of the shoulder and arm. It also helps to prevent dislocation of the clavicle and protects the neurovascular structures beneath it in the event of a fracture.
The subclavius is important in preventing injuries to the adjacent subclavian blood vessels and the superior trunk of the brachial plexus in the case of a fractured clavicle. The vessels and nerves running behind the subclavius can sometimes become entrapped between the clavicle and the first rib, resulting in costoclavicular syndrome, one of the types of thoracic outlet syndromes.
The subclavius may also contribute to controlling the position of the scapula in relation to the thorax through its lateral aponeurotic extension and fascial connections. This regulation of kinetic energy output during upper extremity movement is hypothesized to be one of its primary functions.
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Sternocleidomastoid
The sternocleidomastoid muscle (SCM) is a paired superficial muscle in the anterior portion of the neck. It is an important landmark in the neck, dividing it into an anterior and a posterior triangle. The SCM binds the skull to the sternum and clavicle, protecting the vertical neurovascular bundle of the neck, branches of the cervical plexus, deep cervical lymph nodes, and soft tissues of the neck from damage.
The SCM is one of over 20 pairs of muscles acting on the neck and has dual innervation and multiple functions. It is innervated by the accessory nerve of the same side, which supplies only motor fibres. The cervical plexus supplies sensation, including proprioception, from the ventral primary rami of C2 and C3. The SCM acts in concert with the entire muscular group of the cervicofacial region, responding and aiding in various complex physiological movements beyond its principal function as a lateral neck flexor.
The SCM is a unique muscle in terms of variations at its origin. It has a variable innervation arrangement, with the "classical anastomotic pattern" being observed in 50% of cases. The clavicular origin of the SCM varies greatly: in some cases, the clavicular head may be as narrow as the sternal, while in others, it may be as much as 7.5 millimetres in breadth. When the clavicular origin is broad, it is occasionally subdivided into several slips, separated by narrow intervals.
Contraction of the SCM results in the rotation of the head to the opposite side or the oblique rotation of the head. It also flexes the neck. When acting together, the muscles of both sides flex the neck and extend the head. When acting alone, it rotates to the opposite side and slightly flexes laterally to the same side. It also acts as an accessory muscle of inspiration.
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Frequently asked questions
A pectoral muscle is any of the muscles that connect the front walls of the chest with the bones of the upper arm and shoulder.
There are two pectoral muscles on each side of the sternum (breastbone) in the human body.
The two pectoral muscles are the pectoralis major and the pectoralis minor.
The pectoralis major is the superior most, largest, and most superficial muscle of the anterior chest wall. It is thick, fan-shaped, and lies underneath the breast tissue.
The pectoralis minor is a thin, triangular muscle located beneath the pectoralis major. It originates from the middle ribs and attaches to the scapula (shoulder blade).
