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The chest is primarily made up of two muscles: the pectoralis major and the pectoralis minor, also known as the pecs. The pectoralis major is the larger of the two and is fan-shaped, stretching from the armpit to the collarbone and down across the lower chest on both sides of the body. The pectoralis minor is a thin, triangular muscle that lies underneath the pectoralis major and runs along the upper ribs. Other muscles in the chest include the serratus anterior, which is located on the front of the chest, and the subclavius, a small muscle located directly underneath the clavicle.
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What You'll Learn
The pectoralis major
There are two heads of the pectoralis major, the clavicular and the sternocostal, which reference their area of origin. The sternocostal head is the larger of the two heads. The clavicular head allows for the flexion of the humerus, as seen when lifting a glass in a toast. The muscle fibres attaching to the sternocostal head permit the horizontal and vertical adduction, extension, and internal rotation of the upper extremity. The pectoralis major is active in deep or forced inspiration, but not expiration. When the diaphragm and intercostal muscles are insufficient for proper breathing, the pectoralis major can assist in inspiration.
Pectoralis major tendon rupture is a rare shoulder injury, most commonly seen in weight lifters. This injury is becoming more common due to the increased emphasis on healthy lifestyles. Other injuries to the pectoralis major are graded from I to III, with a complete tear being a Grade III injury. Poland syndrome is a rare congenital condition in which the whole muscle is missing, usually on one side of the body.
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The pectoralis minor
The primary nerve supply to the pectoralis minor comes from the medial pectoral nerve (C8, T1), a minor branch of the brachial plexus that arises from the cervical portion of the spinal cord. It may also be innervated by the lateral pectoral nerve via a communicating branch called the 'ansa pectoralis'. The vascular supply to the pectoralis minor comes from the thoracoacromial artery, a branch of the axillary artery.
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The serratus anterior
The main function of the serratus anterior is to move the scapula forward around the torso, as when throwing a punch. This movement is called scapular protraction. It also assists in scapular upward rotation, allowing for overhead lifting. When the shoulder girdle is fixed, the serratus anterior acts as an accessory inspiratory muscle, lifting the ribs and supporting breathing.
Weakness in the serratus anterior muscle can lead to a condition called scapula winging, where the scapula is no longer held firmly against the ribcage and protrudes out of the back. This can occur due to damage to the long thoracic nerve, resulting in paralysis of the serratus anterior muscle.
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The subclavius
The chest is made up of three muscles in the pectoral region: pectoralis major, pectoralis minor, and serratus anterior. The subclavius is a small, triangular muscle in the pectoral region of the thorax, located directly underneath the clavicle. It is enclosed by the clavipectoral fascia, a fibrous connective tissue that spans the interval between the clavicle and pectoralis minor muscle. The pectoralis major muscle courses superficially to the subclavius and clavipectoral fascia.
The main function of the subclavius is to actively stabilize the clavicle at the sternoclavicular joint during movements of the shoulder and arm. Its contraction leads to a depression of the sternal end of the clavicle and subsequent elevation of the first rib. This helps to protect the underlying neurovascular structures, such as in cases of clavicular fracture or other trauma.
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Intercostal muscles
The intercostal muscles are a group of intrinsic rib cage muscles that occupy the 11 intercostal spaces between the ribs. They help form and move the chest wall and are crucial for breathing. There are three layers of intercostal muscles: external, internal, and innermost.
The external intercostal muscles, or intercostalis externus, are the outermost layer of intercostal muscles. They originate on ribs 1–11 and insert on ribs 2–12. During inhalation, they elevate the ribs and bend them open, expanding the chest cavity. The muscle fibres are directed downwards, forwards, and medially in the anterior part. The blood supply to the external intercostals comes from the anterior and posterior intercostal arteries.
The internal intercostal muscles, or intercostalis internus, form the middle layer of intercostal musculature. They originate from the costal groove of one rib and insert into the superior border of the rib below. They are responsible for depressing the ribs and bending them inward, thus decreasing the size of the chest cavity. The internal intercostals aid in forced expiration.
The innermost intercostal muscles, or intercostalis intimus, are the deepest layer of intercostal muscles. They originate from the costal groove of one rib, posteriorly to the origin of the internal intercostals, and insert into the superior border of the rib below. They assist the internal and external intercostals in their functions.
Intercostal muscle strains can occur due to weakened muscles, overexertion, direct trauma, or repetitive torso twisting. Symptoms of a strain include severe pain in the upper back or rib cage, muscle tension and stiffness, and difficulty breathing due to pain.
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Frequently asked questions
The pectoralis major is the major muscle in the chest.
The pectoralis major is a large, fan-shaped muscle.
The pectoralis major stretches from the armpit up to the collarbone and down across the lower chest region on both sides of the chest.
The pectoralis major moves each shoulder joint in four distinct directions and keeps the arms attached to the body.
The pectoralis minor is located underneath the pectoralis major.
