Atlas of Anatomy

5 Thoracic Wall

Thoracic Skeleton

image The thoracic skeleton consists of 12 thoracic vertebrae (p. 8), 12 pairs of ribs with costal cartilages, and the sternum. In addition to participating in respiratory movements, it provides a measure of protection to vital organs. The female thorax is generally narrower and shorter than the male equivalent.

Fig. 5.1   Thoracic skeleton

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Fig. 5.2 Structure of a thoracic segment
Superior view of 6th rib pair.

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Fig. 5.3   Types of ribs
Left lateral view.

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Sternum & Ribs

Fig. 5.4   Sternum
The sternum is a bladelike bone consisting of the manubrium, body, and xiphoid process. The junction of the manubrium and body (the sternal angle) is typically elevated and marks the articulation of the second rib. The sternal angle is an important landmark for internal structures.

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Fig. 5.5   Ribs Right ribs, superior view. See pp. 258259 for joints of the shoulder.

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Joints of the Thoracic Cage

image The diaphragm is the chief muscle for quiet respiration (see p. 52). The muscles of the thoracic wall (see p. 50) contribute to deep (forced) inspiration.

Fig. 5.6   Rib cage movement
Full inspiration (red); full expiration (blue). In deep inspiration, there is an increase in transverse and sagittal thoracic diameters, as well as the infrasternal angle. The descent of the diaphragm further increases the volume of the thoracic cavity.

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Fig. 5.7   Sternocostal joints
Anterior view with right half of sternum sectioned frontally. True joints are generally found only at ribs 2 to 5; ribs 1, 6, and 7 attach to the sternum by synchondroses.

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Fig. 5.8   Costovertebral joints
Two synovial joints make up the costovertebral articulation of each rib. The costal tubercle of each rib articulates with the costal facet of its accompanying vertebra (A). The head of most ribs articulates with the vertebra of its own number and the vertebra immediately superior. Ribs 1, 11, and 12 typically articulate only with their own vertebrae.

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Thoracic Wall Muscle Facts

image The muscles of the thoracic wall are primarily responsible for chest respiration, although other muscles aid in deep inspiration: the pectoralis major and serratus anterior are discussed with the shoulder (see pp. 264267), and the serratus posterior is discussed with the back (see p. 30).

Fig. 5.9   Muscles of the thoracic wall

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Fig. 5.10   Muscles of the thoracic wall
Anterior view. The external intercostal muscles are replaced anteriorly by the external intercostal membrane. The internal intercostal muscles are replaced posteriorly by the internal intercostal membrane (removed in Fig. 5.11).

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Fig. 5.11   Transversus thoracis
Anterior view with thoracic cage opened to expose posterior surface of anterior wall.

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Diaphragm

Fig. 5.12   Diaphragm
The diaphragm, which separates the thorax from the abdomen, has two asymmetric domes and three apertures (for the aorta, vena cava, and esophagus; see Fig. 5.13B).

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Fig. 5.13   Diaphragm in situ

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Neurovasculature of the Diaphragm

Fig. 5.14   Neurovasculature of the diaphragm
Anterior view of opened thoracic cage.

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Fig. 5.15   Innervation of the diaphragm
Anterior view. The phrenic nerve lies on the lateral surface of the fibrous pericardium together with the pericardiacophrenic arteries and veins. Note: The phrenic nerve also innervates the pericardium.

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Fig. 5.16   Arteries and nerves of the diaphragm
Note: The margins of the diaphragm receive sensory innervation from the lowest intercostal nerves.

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Arteries & Veins of the Thoracic Wall

image The posterior intercostal arteries anastomose with the anterior intercostal arteries to supply the structures of the thoracic wall. The posterior intercostal arteries branch from the thoracic aorta, with the exception of the 1st and 2nd, which arise from the superior intercostal artery (a branch of the costocervical trunk).

Fig. 5.17   Arteries of the thoracic wall
Anterior view.

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Table 5.5 Arteries of the thoracic wall

Origin

Branch

Axillary a.

Lateral thoracic a.

Thoracoacromial a.

Subclavian a.

Posterior intercostal aa.
(1st and 2nd; see p. 34)

Superior thoracic a.

Thoracic aorta

Posterior intercostal aa.
(3rd through 12th)

Internal thoracic a.

Anterior intercostal aa.

Musculophrenic a.

Superior epigastric a.

Fig. 5.18   Branches of the posterior intercostal arteries
Superior view.

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image The intercostal veins drain primarily into the azygos system, but also into the internal thoracic vein. This blood ultimately returns to the heart via the superior vena cava. The intercostal veins follow a similar course to their arterial counterparts. However, the veins of the vertebral column form an external vertebral venous plexus that traverses the entire length of the spine (see p. 35).

Fig. 5.19   Veins of the thoracic wall
Anterior view.

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Fig. 5.20   Superficial veins
Anterior view. The thoracoepigastric veins are a potential superficial collateral venous drainage route in the event of superior or inferior vena cava obstruction.

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Nerves of the Thoracic Wall

Fig. 5.21   Intercostal nerves
Anterior view. The 1st rib has been removed to reveal the 1st and 2nd intercostal nerves.

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Fig. 5.22   Thoracic wall: Peripheral sensory cutaneous innervation

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Fig. 5.23   Spinal nerve branches
Superior view. Formed by the union of the posterior (sensory) and anterior (motor) roots, the at-most 1 cm-long spinal nerve courses through the intervertebral foramen and exits the vertebral canal. Its posterior ramus innervates the skin and intrinsic muscles of the back; its anterior ramus forms the intercostal nerves. See p. 36 for more details.

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Fig. 5.24   Course of the intercostal nerves
Coronal section, anterior view.

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Fig. 5.25   Thoracic wall: Dermatomes
Landmarks: T4 generally includes the nipple; T6 innervates the skin over the xiphoid.

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Neurovascular Topography of the Thoracic Wall

Fig. 5.26   Anterior structures
Anterior view (see pp. 3439 for neurovasculature of the back).

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imageClinical

Insertion of a chest tube

Abnormal fluid collection in the pleural space (e.g., pleural effusion due to bronchial carcinoma) may necessitate the insertion of a chest tube. Generally, the optimal puncture site in a sitting patient is at the level of the 7th or 8th intercostal space on the posterior axillary line. The drain should always be introduced at the upper margin of a rib to avoid injuring the intercostal vein, artery, and nerve. See p. 113 for details on collapsed lungs.

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Fig. 5.27   Intercostal structures in cross section
Transverse section, anterosuperior view.

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Female Breast

image The female breast, a modified sweat gland in the subcutaneous tissue layer, consists of glandular tissue, fibrous stroma, and fat. The breast extends from the 2nd to the 6th rib and is loosely attached to the pectoral, axillary, and superficial abdominal fascia by connective tissue. The breast is additionally supported by suspensory ligaments. An extension of the breast tissue into the axilla, the axillary tail, is often present.

Fig. 5.28   Female breast
Right breast, anterior view.

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Fig. 5.29   Mammary ridges
Rudimentary mammary glands form in both sexes along the mammary ridges. Occasionally, these may persist in humans to form accessory nipples (polythelia), although only the thoracic pair normally remains.

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Fig. 5.30   Blood supply to the breast

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Fig. 5.31   Sensory innervation of the breast

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imageThe glandular tissue is composed of 10 to 20 individual lobes, each with its own lactiferous duct. The gland ducts open on the elevated nipple at the center of the pigmented areola. Just proximal to the duct opening is a dilated portion called the lactiferous sinus. Areolar elevations are the openings of the areolar glands (sebaceous). The glands and lactiferous ducts are surrounded by firm, fibrofatty tissue with a rich blood supply.

Fig. 5.32   Structures of the breast

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Lymphatics of the Female Breast

image The lymphatic vessels of the breast (not shown) are divided into three systems: superficial, subcutaneous, and deep. These drain primarily into the axillary lymph nodes, which are classified based on their relationship to the pectoralis minor (Table 5.7). The medial portion of the breast is drained by the parasternal lymph nodes, which are associated with the internal thoracic vessels.

Fig. 5.33   Axillary lymph nodes

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Table 5.7 Levels of axillary lymph nodes

Level

Position

Lymph nodes (l.n.)

I

Lower axillary group

Lateral to pectoralis minor

Pectoral axillary l.n.
Subscapular axillary l.n.
Humeral axillary l.n.
Central l.n.

II

Middle axillary group

Along pectoralis minor

Interpectoral axillary l.n.

III

Upper infraclavicular group

Medial to pectoralis minor

Apical axillary l.n.

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Breast cancer

Stem cells in the intralobular connective tissue give rise to tremendous cell growth, necessary for duct system proliferation and acini differentiation. This makes the terminal duct lobular unit (TDLU) the most common site of origin of malignant breast tumors.

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Tumors originating in the breast spread via the lymphatic vessels. The deep system of lymphatic drainage (level III) is of particular importance, although the parasternal lymph nodes provide a route by which tumor cells may spread across the midline. The survival rate in breast cancer correlates most strongly with the number of lymph nodes involved at the axillary nodal level. Metastatic involvement is gauged through scintigraphic mapping with radiolabeled colloids (technetium [Tc] 99m sulfur microcolloid). The downstream sentinel node is the first to receive lymphatic drainage from the tumor and is therefore the first to be visualized with radiolabeling. Once identified, it can then be removed (via sentinel lymphadenectomy) and histologically examined for tumor cells. This method is 98% accurate in predicting the level of axillary nodal involvement.

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