Logan Steele
The pectoralis major is a thick, fan-shaped muscle that makes up the bulk of the chest muscles and lies under the breast. The pectoralis major receives its blood supply from branches of the internal mammary and thoracoacromial arteries. The breast is anchored to the pectoralis major by suspensory ligaments, which allow the breast to maintain its natural mobility. The pectoralis major is frequently used in reconstructive plastic surgery due to its ability to provide good muscle coverage for a breast implant.
| Characteristics | Values |
|---|---|
| Muscle under breast | Pectoralis major |
| Location | Underneath the breast |
| Muscle type | Thick, fan-shaped or triangular convergent muscle |
| Muscle origin | Lateral sternum and clavicle |
| Muscle insertion | Humeral head |
| Muscle innervation | Dual motor innervation by the medial pectoral nerve and the lateral pectoral nerve |
| Blood supply | Branches of the internal mammary and thoracoacromial arteries |
| Function | Flexion, adduction, and internal rotation of the humerus |
| Additional notes | The pectoralis major muscle may develop intramuscular lipomas, which are benign tumors |
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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 innervated by the medial and lateral pectoral nerves, which originate from the medial and lateral cords, respectively. The muscle fibres converge laterally, crossing over the long head of the biceps tendon and inserting onto the lateral lip of the intertubercular groove.
The pectoralis major has several important functions. Its primary functions are flexion, adduction, and internal rotation of the humerus. It is also responsible for keeping the arm attached to the trunk of the body. The muscle has two distinct parts, each responsible for different actions. The clavicular part contributes to flexion, horizontal adduction, and inward rotation of the humerus, while the sternocostal part contributes to downward and forward movement of the arm, along with inward rotation during adduction.
Injuries to the pectoralis major are uncommon but have become more prevalent due to increased participation in weight lifting. Tears of the pectoralis major typically affect healthy and athletic individuals, particularly in high-impact contact sports such as powerlifting. The classic patient is a muscular male between 20 to 40 years old, presenting with symptoms such as localized swelling, muscular deformity, and weakness in adduction and internal rotation of the arm.
Poland syndrome is a rare congenital condition where the pectoralis major muscle is completely absent, usually on one side of the body. This condition can be accompanied by the absence of the breast in females. Surgical implants are available to modify aesthetic contours, mass, and asymmetry in individuals with Poland syndrome.
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The pectoralis minor muscle
The pectoralis minor is a thin, triangular muscle located in the chest, underneath the pectoralis major muscle. It is one of the most superficial muscles on the anterior aspect of the chest or thoracic wall. The pectoralis minor muscle is important clinically and as a surgical landmark. This is due to the structures that lie below or deep to the muscle and its tendon. Running deep to the pectoralis minor muscle are the nerves and blood supply to the upper limb: the posterior, lateral, and medial cords of the brachial plexus.
The primary nerve supply to the pectoralis minor muscle comes via the medial pectoral nerve (C8, T1), one of the minor branches of the brachial plexus that arises from the cervical portion of the spinal cord. The medial and lateral pectoral nerves penetrate the pectoralis minor muscle to innervate it. The vascular supply to the pectoralis minor comes from several sources, including the thoracoacromial artery, superior thoracic artery, and lateral thoracic artery.
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The breast's fatty tissue
The breasts are made up of fat, connective tissue, glands, and ducts. The amount of fat in the breast determines its size, with breasts containing more fat appearing larger. Breasts with more fat are also associated with a decreased risk of breast cancer.
Breast density is a measure of how much fibrous and glandular tissue there is in the breast compared to fat tissue. Breasts with more fibrous and glandular tissue are denser, while breasts with more fat tissue are less dense. The density of breast tissue is not related to breast size or firmness, and it is common for women to have dense breast tissue. However, breasts typically become less dense with age.
There are four categories of breast density: Category A breasts are composed of almost entirely fatty tissue, while Category D breasts are extremely dense, with very little fat. The density of breast tissue can impact the effectiveness of mammograms in detecting cancer. Dense breast tissue appears white on a mammogram, the same color as breast masses and cancers, making it harder to identify abnormalities. In contrast, fatty tissue appears almost black, making it easier to detect tumors in breasts with higher fat content.
The amount of fat in the breast can vary between women and even between a woman's two breasts. The size and shape of breasts can also be influenced by estrogen and progesterone, which cause changes in breast tissues during each menstrual cycle. After menopause, when estrogen and progesterone levels decrease, breasts typically undergo fewer changes.
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The nipple's muscle fibres
The nipple is located at the centre of the areola and has an opening to release milk. The nipples contain muscle fibres, and when these fibres contract, the nipple becomes erect and points outward. The areola is the circular area around the nipple on the surface of the breast. It contains small glands that secrete an oily substance that acts as a lubricant for the nipple and areola.
The areola muscle is continuous with the smooth muscle surrounding the lactiferous ducts in the nipple. Contraction of the muscle allows for the secretion of milk produced by the lactiferous glands. The smooth musculature of the nipple-areola complex (NAC) is divided into outer, intermediate, and inner layers from the areola to the nipple. The skin of the nipple sits on areolar smooth muscle fibres that are arranged radially (Meyerholz muscle) and circularly (Sappey muscle).
The inner muscular fibres continue into the stronger intermediate layer, which is covered by an outer layer of smooth musculature situated around the ducts of the Montgomery glands. There are muscles surrounding the lactiferous ducts. Nitric oxide may mediate nipple erection.
The main function of a woman's breasts is to make, store and release milk to feed a baby. After giving birth, hormones in a woman's body stimulate the glands in the lobules throughout the breast to make milk. The ducts carry the milk to the nipple. Milk passes from the nipple to the baby during breastfeeding.
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The chest wall veins
The internal thoracic vein begins at the end of the superior epigastric vein, which is responsible for draining blood and waste products from the abdominal wall. It runs alongside the internal thoracic artery, situated near the sternum (breastbone). In some individuals, the left internal thoracic vein splits into two veins between the third and fourth ribs, while the right internal thoracic vein may be a single vein or divide between the second and fourth ribs.
The internal thoracic vein plays a crucial role in the circulatory system by collecting blood from the chest wall and breasts and returning it to the heart for reoxygenation. This vein works in conjunction with other veins to ensure the proper flow of blood back to the heart. Any damage to the internal thoracic vein, such as through a chest injury or during certain medical procedures, can lead to health complications.
Chest wall veins can sometimes be indicative of underlying health issues. Superior vena cava syndrome, for instance, can be caused by a tumour or blood clot blocking the vein. Other potential causes include breast cancer, thyroid cancer, and chest infections. If chest wall veins are present, it is recommended to seek medical advice to identify and address any underlying causes and improve vascular health. Treatments are available to address vascular weakness and reduce the appearance of chest wall veins, including sclerotherapy, endovenous laser therapy, and microphlebectomy.
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Frequently asked questions
The pectoralis major muscle is a thick, fan-shaped or triangular convergent muscle that makes up the bulk of the chest muscles and lies under the breast.
The pectoralis major muscle's primary functions are flexion, adduction, and internal rotation of the humerus. It is also responsible for the movement of the shoulder joint.
Beneath the pectoralis major muscle is the pectoralis minor muscle.
The pectoralis major muscle receives most of its blood supply from branches of the internal mammary and thoracoacromial arteries.
