6 Thoracic Wall

A bony and muscular cage, which communicates superiorly with the neck and inferiorly with the abdomen, encloses and protects the thoracic contents. A superficial layer of extrinsic muscles overlies the thoracic cage, although these muscles act primarily on the upper limb. The breast, a derivative of the epidermis (outermost layer of skin), is a prominent superficial structure of the thoracic wall.

6.1 The Breast

General Features

The breast is made up of the mammary gland, a modified sweat gland, and its supporting fat and fibrous tissue (Figs. 6.1 and 6.2). Breasts remain rudimentary in males but are prominent struc-tures of the female pectoral region.

—Female breasts extend from the lateral border of the sternum to the midaxillary line and overlie the 2nd through 6th ribs.

—The breast is embedded in the subcutaneous layer of the skin overlying the deep fascia of the pectoralis major and serratus anterior muscles.

—The retromammary space is a plane of loose connective tissue that separates the breast from underlying fascia of the pectoralis major muscle and allows movement of the breasts on the thoracic wall.

—The highest prominence of the breast, the nipple, is located at the center of the areola. Circularly arranged smooth muscle fibers cause erection of the nipple in response to cold or tactile stimulation. In men and young women, the nipple is at the T4 vertebral level, but in older women this location varies considerably.

—The areola, the pigmented skin surrounding the nipple, contains sebaceous glands whose oily secretions lubricate the area during nursing.

—An axillary tail, or small finger of glandular tissue, may extend into the axilla (armpit) along the lower edge of the pectoralis muscle.

—The volume of fat, not the volume of glandular tissue, largely determines the differences in breast size among women.

Fig. 6.1 Surface anatomy of the breast

Right breast, anterior view. (From Schuenke M, Schulte E, Schumacher U. THIEME Atlas of Anatomy, Vol 1. Illustrations by Voll M and Wesker K. 3rd ed. New York: Thieme Publishers; 2020.)

Mammary Gland

The mammary gland has two components:

—The parenchyma, or milk-producing part of the gland (see Fig. 6.2)

•The parenchyma consists of lobes divided into 15 to 20 lobules, which contain grapelike clusters of alveoli, hollow balls lined by secretory cells.

•Lactiferous ducts in the parenchyma drain each lobule and open at the nipple. Deep to the areola, each duct has a small, dilated portion called the lactiferous sinus where, in lactating females, a small amount of milk is stored.

—The stroma, or fibrous framework, of the gland that separates the lobules and supports the lobes

•Stroma is attached to the overlying dermis of the skin by suspensory (Cooper) ligaments, which are particularly strong on the superior surface of the breast.

Neurovasculature of the Breast

—Anterior intercostal branches and medial mammary branches (from perforating branches) of the internal thoracic artery, lateral thoracic and thoracoacromial branches of the axillary artery, and the 2nd, 3rd, and 4th posterior intercostal arteries supply the breast (see Sections 5.2 and 6.4).

—Venous blood drains primarily to the axillary vein but also to the internal thoracic vein.

—Most lymph (> 75%) from the breast drains (particularly from the lateral

the upper limb. The thoracic cage includes the sternum, 12 pairs of ribs, and the 12 thoracic vertebrae (Fig. 6.4).

The Sternum

The sternum is a flat, elongated bone that has three parts:

1.The manubrium, which articulates laterally with the costal cartilages (cartilage that attaches the ribs to the sternum) of the 1st and 2nd ribs. A deep jugular notch separates the right and left sternoclavicular joints, where the manubrium articulates with the clavicles.

2.The body, which is fused superiorly with the manubrium at the manubriosternal joint. Laterally, the body articulates with costal cartilages of the 2nd to 7th ribs.

3.The xiphoid process, the lowest part of the sternum, which joins superiorly

with the body of the sternum at the xiphis-ternal joint.

•The sternal angle is an important surface landmark on the anterior thoracic wall that provides orientation to the internal anatomy of the thorax. It is a palpable ridge that marks the fusion of the manubrium and body of the sternum. A horizontal plane through the sternal angle intersects

◦the articulation between the sternum and the costal cartilages of the 2nd ribs,

◦the division of the mediastinum into superior and inferior regions,

◦the origin and termination of the aortic arch,

◦the bifurcation of the trachea, and

◦the T4–T5 intervertebral disk.

Fig. 6.4 Thoracic skeleton

Anterior view. (From Schuenke M, Schulte E, Schumacher U. THIEME Atlas of Anatomy, Vol 1. Illustrations by Voll M and Wesker K. 3rd ed. New York: Thieme Publishers; 2020.)

The Ribs

—The ribs are numbered 1 to 12 from superior to inferior, and each rib articulates with a thoracic vertebra of the same number.

—The ribs hang obliquely downward from their articulation with the vertebral column. Their anterior ends may be two to five vertebral levels lower than their posterior attachment.

—Ribs 1 to 10 articulate anteriorly with a cartilaginous segment called the costal cartilage.

—The ribs are classified according to the articulation of their costal cartilage with the sternum (Fig. 6.5):

•True ribs (1 to 7) articulate directly with the sternum via individual costal cartilages.

•False ribs (8 to 10) articulate indirectly with the sternum through costal cartilages that are connected to the cartilage superior to it.

•Floating ribs (11 and 12) have no connection to the sternum.

—Most ribs articulate at three joints (Fig. 6.6):

1. Costochondral joints between the bony segments of ribs 1 to 10 and their respective costal cartilages

Fig. 6.6 Structure of a thoracic segment

Superior view of 6th rib pair. (From Schuenke M, Schulte E, Schumacher U. THIEME Atlas of Anatomy, Vol 1. Illustrations by Voll M and Wesker K. 3rd ed. New York: Thieme Publishers; 2020.)

6.3 Muscles of the Thorax

Muscles of the Thoracic Wall (Fig. 6.7; Table 6.1)

—Extrinsic muscles of the upper limb, including the pectoralis major, pectoralis minor, and serratus anterior, cover the thorax. Although they mainly move or stabilize the upper limb, these muscles also assist in movements of the ribs during deep inspiration. (See Chapter 18, Overview of the Upper Limb.)

—The anterior scalene, middle scalene, and posterior scalene muscles originate on the transverse processes of the cervical vertebrae and insert onto the 1st and 2nd ribs. They assist the intrinsic thoracic muscles during inspiration and are consid-ered extrinsic muscles of respiration.

—Intrinsic muscles of the thoracic wall are the chief muscles that move the ribs during respiration.

•The intercostal muscles occupy the intercostal spaces between the ribs. They extend from the inferior border of one rib to the superior border of

the next inferior rib. The intercostal muscles move the ribs primarily during forced respiration. During quiet respiration, they stabilize the thoracic wall. They include

Fig. 6.7 Transversus thoracis

Anterior view with thoracic cage opened to expose posterior surface of anterior wall. The external and internal intercostal membranes have been removed. (From Gilroy AM, MacPherson BR, Wikenheiser JC. Atlas of Anatomy. Illustrations by Voll M and Wesker K. 4th ed. New York: Thieme Publishers; 2020.)

◦the external intercostal muscles, with fibers directed inferoanteriorly, which make up the most superficial layer

◦the internal intercostal and innermost intercostal muscles, which occupy the middle and deepest layers of the thoracic wall, respectively, with their fibers directed inferoposteriorly

•The subcostal muscles are most prominent along the lower thoracic wall, where they cross the inner surface of one or two intercostal spaces.

•The transversus thoracic muscles consist of four of five thin slips that extend superiorly and laterally from the posterior surface of the sternum to the ribs.

The Thoracic Diaphragm

The thoracic diaphragm (or simply, the diaphragm), a musculotendinous sheet that separates the thorax from the abdomen, is the principal muscle of respiration. The diaphragm forms the floor of the thorax, the roof of the abdomen, and a portion of the posterior abdominal wall (Fig. 6.8).

—The skeletal muscle fibers of the diaphragm originate along the costal margin, the vertebral bodies of L1–L3, the medial and lateral arcuate ligaments, and the xiphoid process. They insert on the diaphragm’s central tendon.

—Right and left crura, extensions of the posterior diaphragm, attach to

Illustrations by Voll M and Wesker K. 4th ed. New York: Thieme Publishers; 2020.)

Fig. 6.9 Projection of the diaphragm onto the trunk

Anterior view. Position of the diaphragm in inspiration and expiration. (From Schuenke M, Schulte E, Schumacher U. THIEME Atlas of Anatomy, Vol 2. Illustrations by Voll M and Wesker K. 3rd ed. New York: Thieme Publishers; 2020.)

—The domes on the right and left sides of the diaphragm are asymmetric; the right hemidiaphragm is generally higher than the left.

—During full expiration the diaphragm is 4 to 6 cm higher than during full inspiration. During expiration it ascends to the level of the 4th or 5th rib on the right and slightly lower on the left, although this varies with respiration, posture, and body type (Fig. 6.9).

—The diaphragm has three openings to allow the passage of structures between the thorax and abdomen (Fig. 6.10):

1.The caval opening, which is a passage for the inferior vena cava through the central tendon at the T8 vertebral level.

2.The esophageal hiatus, which is an opening at the T10 vertebral level for the esophagus, the anterior and poste-rior vagal trunks, and left gastric artery and vein.It is usually formed by the right crus of the diaphragm (occasionally by the right and left crura) that, when contracted, forms a sphincter around the esophagus.

3.The aortic hiatus, which is a passageway for the aorta between the right

and left crura as it passes behind the diaphragm at T12. The thoracic duct, and often the azygos and hemiazygos veins, also passes through this aperture.

—The inferior phrenic arteries, branches of the abdominal aorta (or celiac trunk), are the primary blood supply to the diaphragm. The superior phrenic, pericardiacophrenic, and musculophrenic arteries make additional contributions (Fig. 6.11).

—Venous blood drains to the azygos system through posterior intercostal and superior phrenic veins.

—The phrenic nerve (C3–C5) provides all of the motor and most of the sensory innervation to the diaphragm. The subcostal and lower intercostal nerves transmit sensory information from the periphery of the diaphragm

(Fig. 6.12).

Fig. 6.10 Diaphragmatic apertures

Fig. 6.11 Neurovasculature of the diaphragm

Fig. 6.12 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. (From Gilroy AM, MacPherson BR, Wikenheiser JC. Atlas of Anatomy. Illustrations by Voll M and Wesker K. 4th ed. New York: Thieme Publishers; 2020.)

6.4 Neurovasculature of the Thoracic Wall

—Intercostal neurovascular bundles course along the inferior surface of the ribs within the costal groove (Figs. 6.13, 6.14, 6.15, 6.16).

•Anterior intercostal arteries (branches of the internal thoracic arteries) and posterior intercostal arteries (branches of the thoracic aorta and subclavian arteries) supply the muscles and skin of the thoracic wall

(Fig. 6.14).

•Intercostal veins drain primarily to the azygos system but also into the brachiocephalic and internal thoracic veins, which join the superior vena

Schulte E, Schumacher U. THIEME Atlas of Anatomy, Vol 1. Illustrations by Voll M and Wesker K. 3rd ed. New York: Thieme Publishers; 2020.)

Fig. 6.16 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. (From Schuenke M, Schulte E, Schumacher U. THIEME Atlas of Anatomy, Vol 1. Illustrations by Voll M and Wesker K. 3rd ed. New York: Thieme Publishers; 2020.)

—The 7th through 11th intercostal nerves continue anteriorly from the intercostal space to innervate the anterior abdomi-nal wall.

—Landmark dermatomes on the thoracic wall include T4 at the nipple and T6 over the xiphoid process.

—Intercostal nerves and vessels are protected within the costal groove on the deep inferior edge of the ribs (see Box 6.2). Within this bundle of neurovascular structures, the nerve runs inferior to its accompanying vessels.

Anatomy, Vol 1. Illustrations by Voll M and Wesker K. 3rd ed. New York: Thieme Publishers; 2020.)

BOX 6.2: CLINICAL CORRELATION

INSERTION OF A CHEST TUBE

Abnormal fluid collection in the pleural space (e.g., pleural effu-sion 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 fourth and fifth intercostal space in the mid to anterior axillary line, immediately behind the lateral edge of the pectoralis major. The drain should always be intro-duced at the upper margin of a rib to avoid injuring the intercostal vein, artery, and nerve.

Coronal section, anterior view

(From Gilroy AM, MacPherson BR, Wikenheiser JC. Atlas of Anatomy. Illustrations by Voll M and Wesker K. 4th Edition. New York: Thieme Publishers;