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The pectoralis major is a large fan-shaped muscle that lies under each breast, covering the ribs. This muscle stretches from the armpit up to the collarbone and down across the lower chest region on both sides of the chest. The pectoralis minor is a thin, triangle-shaped muscle that lies under the pectoralis major. The latissimus dorsi muscle is also located near the breast, extending from the lower back to the upper arm.
| Characteristics | Values |
|---|---|
| Muscle under the breast | Pectoralis major muscle |
| Muscle under the pectoralis major muscle | Pectoralis minor muscle |
| Muscle at the back | Latissimus dorsi muscle |
| Muscle that the pectoralis major muscle connects to | Sternum or breastbone |
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What You'll Learn
The pectoralis major muscle
The pectoralis major is a thick, fan-shaped or triangular convergent muscle of the human chest. It is the largest and most superficial muscle in the chest area, lying under the breast. The pectoralis major arises from parts of the clavicle and sternum, costal cartilages of the true ribs, and the aponeurosis of the abdominal external oblique muscle. It is responsible for the movement of the shoulder joint and keeping the arm attached to the trunk of the body. The muscle has two distinct parts, the clavicular and the sternocostal, which are responsible for different actions. The clavicular part is close to the deltoid muscle and contributes to flexion, horizontal adduction, and inward rotation of the humerus. The sternocostal part, on the other hand, is antagonistic to the clavicular part, contributing to the downward and forward movement of the arm and inward rotation during adduction.
The pectoralis major receives dual innervation from the medial pectoral nerve and the lateral pectoral nerve, which originate from the medial and lateral cords, respectively. It maintains its blood supply through the pectoral branch of the thoracoacromial artery. The primary functions of the pectoralis major include flexion, adduction, and internal rotation of the humerus, with a secondary function of humeral flexion when the arm is in a position of extension posterior to the thorax's coronal plane.
Injuries to the pectoralis major are uncommon but have become more prevalent due to increased participation in weight lifting. These injuries typically present as tears, resulting in localized swelling, muscular deformity, thinning of the anterior axillary fold, and weakness in adduction and internal rotation of the arm. Treatment options include surgical reconstruction using a semitendinosus graft.
The pectoralis major can be targeted through various exercises such as the barbell bench press, dumbbell bench press, and machine bench press. These exercises induce high activation in the sternocostal region, leading to hypertrophy and increased muscle functionality.
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The pectoralis minor muscle
The medial and lateral pectoral nerves penetrate the pectoralis minor muscle to innervate it. The vascular supply to this muscle comes from several sources, including the thoracoacromial artery, superior thoracic artery, and lateral thoracic artery. The primary nerve supply to the pectoralis minor muscle is provided by the medial pectoral nerve (C8, T1), a minor branch of the brachial plexus arising from the cervical portion of the spinal cord.
The pectoralis minor is clinically significant and serves as a surgical landmark due to the structures beneath it and its tendon. It is used as a reference point for the three divisions of the axillary artery. Additionally, the pectoralis minor provides a pathway for structures passing into the upper limb from the thorax, forming a "bridge" by attaching to the coracoid process.
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The latissimus dorsi muscle
While there are no muscles in the breast, muscles do lie under each breast and cover the ribs. One such muscle is the latissimus dorsi, commonly known as the "lats", especially among bodybuilders. The latissimus dorsi is a large, flat muscle on the back that stretches to the sides, behind the arm, and is partly covered by the trapezius on the back near the midline. The word "latissimus dorsi" comes from Latin and means "broadest of the back", derived from "latissimus" (Latin for broadest) and "dorsum" (Latin for back).
The latissimus dorsi is responsible for extension, adduction, transverse extension (also known as horizontal abduction or horizontal extension), flexion from an extended position, and medial internal rotation of the shoulder joint. It also plays a role in the extension and lateral flexion of the lumbar spine. The muscle's primary function is in the upper extremity, but it is also considered a respiratory accessory muscle. It works with the levator scapulae, trapezius, and rhomboid muscles as part of the superficial layer of extrinsic back muscles.
The latissimus dorsi is attached to the spine and the humerus, and its action on moving the arms can influence the movement of the scapulae. It can pull the inferior angle of the scapula in various directions, producing movements on the shoulder joint. It also assists in holding the scapula against the thorax during upper limb movements. In activities such as walking with crutches, the latissimus dorsi can pull the trunk forward relative to the arms, with a lifting of the pelvis.
The latissimus dorsi is supplied predominantly by the thoracodorsal artery, a continuation of the subscapular artery, which is a branch of the third part of the axillary artery. In addition to the thoracodorsal artery, the muscle also receives blood supply from the dorsal perforating branches of the inferior three posterior intercostal arteries and the superior three lumbar arteries. The thoracodorsal nerve, a branch of the posterior cord of the brachial plexus, provides innervation to the latissimus dorsi.
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The chest wall
The outermost layer of the chest wall is the skin, which covers and protects the underlying structures. Beneath the skin lies a layer of subcutaneous fat, which provides insulation and cushioning to the chest. Following this, we find the muscular layer, consisting of both extrinsic and intrinsic muscles. The extrinsic muscles, such as the pectoralis major and latissimus dorsi, are attached to the upper limbs and contribute to movements of the shoulder girdle and upper arms. The intrinsic muscles are associated with the ribs and include three layers of intercostal muscles, which assist in respiratory functions.
Deep to the muscles lies the endothoracic fascia, a thin layer of connective tissue that surrounds the thoracic cavity. This fascia is continuous with the parietal pleura, a delicate membrane lining the inside of the thoracic cavity and surrounding the lungs. These structures provide support and protection to the lungs and assist in respiratory mechanics.
The bony skeletal component of the chest wall is the rib cage, consisting of twelve thoracic vertebrae that give rise to ribs encircling the lateral and anterior thoracic cavity. The ribs provide structural support, protection to the internal organs, and attachment sites for the muscles. At the centre of the chest wall is the sternum, a flat, elongated bone consisting of three parts: the manubrium, sternal body, and xiphoid process. These components are connected by flexible cartilaginous joints, contributing to the structural integrity and mobility of the chest wall.
An understanding of chest wall anatomy is crucial in surgical procedures involving the chest or breast. The chest wall provides stability and allows for respiratory movements, making it an essential structure for respiratory mechanics and protection of vital organs.
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The ribs
The pectoralis minor muscle resides under the pectoralis major. This thin, triangle-shaped muscle runs along the upper ribs. The ribs provide support for the breasts, with the chest (thoracic cavity) sloping outwards from the thoracic inlet (atop the breastbone) to the lowest ribs that support the breasts. The inframammary fold (IMF), where the lower portion of the breast meets the chest, is an anatomical feature created by the adherence of the breast skin and the underlying connective tissues of the chest.
In summary, the ribs provide the foundation for the breasts, with the pectoralis major and minor muscles attaching to the chest wall and providing mobility and support for the breast tissue. The shape and position of the breasts are influenced by the underlying muscle and bone structures of the chest, including the ribs, which contribute to the overall anatomy and function of the breasts.
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Frequently asked questions
The pectoralis major muscle lies beneath the breast, extending from the armpit to the collarbone and down across the lower chest region.
The pectoralis major muscle moves each shoulder joint in four distinct ways and keeps the arms attached to the body.
The pectoralis minor muscle resides under the pectoralis major. It is a thin, triangle-shaped muscle that runs along the upper ribs.
No, there are no muscles in the breast. However, muscles lie under each breast and cover the ribs.
