Thoracic Wall Anatomy
The skin covering the thorax receives its nerve supply from lower cervical and upper thoracic spinal nerves. Above the level of the manubriosternal joint, C4 gives cutaneous innervation, while tho- racic nerves T2–T11 provide the dermatomes for the remainder of the thoracic wall. The first thoracic nerve does not contribute to the cutaneous nerve supply of the thorax but innervates some of the skin of the upper limb (Figs 1.35 & 3.6).
The breast (Fig. 2.8) consists of glandular tissue and a quantity of fat embedded in the subcutaneous tissue of the anterior chest wall. In the male and immature female, the gland is rudimentary. Although the size and shape of the breast in the adult female vary, the base (the part lying on the deep fascia covering pectoralis major, serratus anterior and rectus abdominis) is constant in posi- tion. In the adult female, the base is roughly circular and extends between the second and sixth ribs. Medially, the gland overlies the lateral border of the sternum. Part of the breast extends upwards and laterally and reaches the anterior fold of the axilla.
This is the axillary tail (process) and is the only part of the breast to penetrate beneath the deep fascia. During clinical palpation of the breast it is essential that the axillary tail is included as part of the physical examination.
The glandular elements consist of 15–20 lobes arranged radially, each draining into a lactiferous duct. These ducts open independently onto the surface of the nipple. The nipple is surrounded by an area of pink skin, the areola, which may develop brown pigmentation during pregnancy.
The gland is traversed by fibrous septa (ligaments of Astley Cooper) (Fig. 2.8), which subdivide the lobes and loosely attach the skin of the breast to the deep fascia covering the chest wall. In certain types of breast carcinoma, these fibrous septa may produce characteristic dimpling of the skin over the lesion. Normally, the breast is freely mobile over the underlying muscles. However, lack of mobility when pectoralis major is contracted indicates that breast pathology has fixed the gland to the underlying chest wall muscles.
The fat and glandular elements of the breast receive blood from arteries that also supply the deeper structures of the chest wall. These vessels include perforating branches from the internal tho- racic artery (internal mammary artery) and the second, third and fourth intercostal arteries. The lateral thoracic and thoracoacromial arteries arising from the axillary artery also supply the breast. The gland is drained by veins that accompany the arteries.
Within the substance of the breast, the lymphatic vessels form a system of interconnecting channels that collect lymph from all parts of the organ. The superior and lateral aspects of the breast usually drain into central and apical axillary nodes via infraclavicular and pectoral nodes. It is therefore important to palpate axillary lymph nodes in suspected cases of malignant breast disease. The medial and inferior parts of the breast drain deeply into glands along the internal thoracic vessels and thence via the bronchomediastinal lymph trunk into the confluence of lymphatic vessels in the root of the neck (p. 330). Lymphatics may also cross the midline to communicate with vessels in the opposite breast.
The outer surfaces of the ribs, costal cartilages and sternum give attachment to muscles involved in movements of the upper limb and the scapula, namely pectoralis major, pectoralis minor and serratus anterior. In addition, the external surfaces of the lower ribs provide attachment for rectus abdominis and the external oblique muscles of the anterior abdominal wall (pp 143, 144).
This large fan-shaped muscle (Fig. 2.9) attaches to the clavicle, sternum and upper costal cartilages and forms the bulk of the anterior wall of the axilla. The clavicular head is attached to the anterior surface of the medial half of the clavicle. The sternocostal head is anchored to the manubrium and body of the sternum, and to the upper six costal cartilages. Laterally, both parts of the muscle attach to the humerus along the lateral lip of the intertubercular sulcus (p. 77).
Pectoralis major is supplied by the medial and lateral pectoral nerves from the brachial plexus. Functionally, it is a powerful adductor and flexor of the arm at the shoulder joint and also produces medial rotation of the humerus. When the upper limb is fixed, the sternocostal part may act as an accessory muscle of inspiration by elevating the ribs.
This small muscle (Fig. 2.10) lies deep to pectoralis major and is usually attached to the third, fourth and fifth ribs. The muscle converges on the medial border of the coracoid process of the scapula. Pectoralis minor is supplied by the medial and lateral pectoral nerves and assists in movements of protraction and rotation of the scapula.
This large muscle lies between the scapula and chest wall and attaches to the lateral aspects of the upper eight ribs (Fig. 2.11), forming part of the medial wall of the axilla. The muscle fibres from the upper four ribs attach to the superior angle and to the costal surface of the medial border of the scapula. The fibres from ribs 5–8 converge on the costal surface of the inferior angle of the scapula.
Innervation is provided by the long thoracic nerve arising in the neck from the upper three roots (C5, C6 & C7) of the brachial plexus. The muscle is a powerful protractor of the scapula and assists trapezius in producing scapular rotation during abduction of the upper limb. In addition, the muscle helps to stabilize the scapula during movements of the upper limb.
The interval between two adjacent ribs is called an intercostal space. On each side of the thorax there are 11 such spaces, numbered from above and occupied by muscles, membranes, nerves and vessels. The number given to each intercostal space and its neurovascular structures corresponds to that of the rib which limits the space superiorly. The nerves and vessels immediately inferior to the twelfth ribs are termed the subcostal nerves and vessels. The intercostal nerves and vessels supply the intercostal muscles and the parietal pleura deep to each space. Branches from these vessels also supply the overlying muscles of the body wall, the superficial fascia and skin. Most intercostal nerves have cutaneous branches that supply the skin covering the chest and abdominal walls.
There are three layers of intercostal muscles, which lie superficial, intermediate and deep. These are named the external, the internal and the innermost intercostal muscles.
The fibres of the external intercostal muscles slope downwards and forwards from the lower border of one rib to the upper border of the subjacent rib (Fig. 2.12). The muscle extends from the tuber- cle of the rib posteriorly to the junction of the rib and its costal cartilage anteriorly. Between costal cartilages the muscle fibres are replaced by a thin fascial sheet, the external intercostal membrane, which reaches the lateral border of the sternum (Fig. 2.13).
Internal intercostal muscles
The internal intercostal muscles (Fig. 2.14) lie immediately deep to the external intercostal muscles. The fibres of the two muscles are mutually at right angles, those of the internal intercostal muscles running downwards and backwards from the lower border of one rib to the upper border of the subjacent rib. Ante- riorly, each muscle continues between the costal cartilages to reach the lateral border of the sternum (Fig. 2.13). Posteriorly, each muscle extends only to the angles of the ribs, where it is replaced by the internal intercostal membrane, which continues as far as the tubercles of the ribs.
These muscles lie on a plane deep to that of the internal intercostal muscles (Fig. 2.15). They form the lateral part of an incomplete layer of muscle which includes the transversus thoracis (sterno- costalis) anteriorly (Fig. 2.16) and subcostalis posteriorly. The innermost intercostal muscles connect the inner surface of each rib to that of its neighbours.
All the intercostal muscles in a particular intercostal space are supplied by the corresponding intercostal nerve.
Although the main role of the intercostal muscles is in ventilation of the lungs, it must be emphasized that during normal, quiet breathing, the muscles of the thoracic wall make only a small contribution. Inspiration is usually brought about mainly by the diaphragm, whose descent increases the vertical diameter of the thorax. The transverse and anteroposterior diameters of the thorax are increased, especially in deep inspiration, by the external intercostal muscles, which incline the ribs outwards, upwards and forwards, so that the intercostal spaces are widened. During quiet breathing, expiration is largely due to the ‘elastic’ recoil of the
lungs and thoracic wall and involves minimal activity by the intercostal muscles. Even when expiration is ‘forced’, for example during vigorous physical exertion or when coughing, the main muscular effort is provided by the muscles of the abdominal wall rather than the chest wall. However, the internal intercostal muscles contribute to forced expiration by drawing the ribs down-wards and inwards, thereby narrowing the intercostal spaces.
Intercostal vessels and nerves
Each intercostal space has a principal artery, vein and nerve, which collectively form the neurovascular bundle (Fig. 2.15). This bundle lies in the neurovascular plane between the internal and innermost intercostal muscles and runs along the upper part of the intercostal space, occupying the costal groove of the rib. Usually, the vein lies superiorly and the nerve inferiorly in the bundle. A collateral nerve and collateral vessels arise posteriorly from the neurovascular bundle and run forwards along the lower border of the intercostal space to supply the intercostal muscles.
Intercostal arteries enter from both anterior and posterior ends of the intercostal space. Anteriorly, the internal thoracic arteries (internal mammary arteries) (Fig. 2.16) arising from the subclavian arteries in the root of the neck (p. 330) provide branches that run laterally to supply the upper six pairs of intercostal spaces. On each side, the lower five spaces receive anterior intercostal arteries from the musculophrenic artery, one of the terminal branches of the internal thoracic artery. These anterior arteries anastomose end-to-end with the posterior intercostal arteries.
Posterior intercostal arteries to the lower nine intercostal spaces arise as direct branches from the descending thoracic aorta (Fig. 2.64). For the first and second spaces, the posterior intercostal arteries are derived from the intercostal branch of the costocervical trunk. This trunk arises from the subclavian artery (p. 331) and its intercostal branch enters the thorax by crossing the neck of the first rib. The anastomoses between anterior and posterior intercostal arteries in the lower nine intercostal spaces and between the scapular arteries and posterior intercostals are important because they enable blood to reach the descending aorta when the aortic arch is abnormally narrowed (coarctation of the aorta).
Anteriorly, the intercostal veins from the lower five intercostal spaces drain into the musculophrenic veins. The upper six intercostal veins and the musculophrenic veins drain into the internal thoracic veins, which themselves are tributaries of the brachiocephalic veins in the root of the neck. Posteriorly, the intercostal veins drain into the azygos venous system. On the right, those in the lower eight spaces terminate directly in the azygos vein (Fig. 2.64). The veins from the second and third spaces combine into a single vessel, the right superior intercostal vein, which drains into the arch of the azygos vein. The first posterior intercostal vein (supreme intercostal vein) leaves the thorax to terminate in the root of the neck, usually in the right vertebral vein.
On the left, the lower eight posterior intercostal veins enter either the hemiazygos or accessory hemiazygos veins (Fig. 2.65).
The left superior intercostal vein drains the second and third spaces and crosses the left side of the arch of the aorta to terminate in the left brachiocephalic vein (Fig. 2.57). As on the right the first posterior intercostal vein (supreme intercostal vein) leaves the thorax to terminate usually in the vertebral, but occasionally in the brachiocephalic, vein.
The intercostal nerves comprise the anterior rami of the upper 11 thoracic spinal nerves. Each intercostal nerve enters the neurovascular plane posteriorly (Fig. 2.65) and gives a collateral branch that supplies the intercostal muscles of the space. Except for the first, each intercostal nerve gives off a lateral cutaneous branch near the midaxillary line, which pierces the overlying muscle (Fig. 1.36). This cutaneous nerve divides into anterior and posterior branches, which supply the adjacent skin. The intercostal nerves of the second to the sixth spaces enter the superficial fascia near the lateral border of the sternum and divide into medial and lateral cutaneous branches.
Most of the fibres of the anterior ramus of the first thoracic spinal nerve join the brachial plexus for distribution to the upper limb (p. 80). The small first intercostal nerve is the collateral branch and supplies only the muscles of the intercostal space, not the overlying skin.
The intercostal nerves of the lower five spaces continue in the neurovascular plane beyond the costal margin to supply the muscles and skin of the abdominal wall (p. 145).