Thorax: overview and surface anatomy

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CHAPTER 53 Thorax: overview and surface anatomy

The thorax is the upper part of the trunk. It consists of an external musculoskeletal cage, the thoracic wall, and an internal cavity that contains the heart, lungs, oesophagus, trachea, thymus, the vagus and phrenic nerves and the right and left sympathetic trunks, the thoracic duct and major systemic and pulmonary blood vessels. Inferiorly the thorax is separated from the abdominal cavity by the diaphragm, superiorly it communicates with the neck and the upper limbs. The thoracic wall also offers protection to some of the abdominal viscera: the greater part of the liver lies under the right dome of the diaphragm; the stomach and spleen lie under the left dome of the diaphragm; the posterior aspects of the superior poles of the kidneys lie on the diaphragm and are anterior to the twelfth rib on the right, and to the eleventh and twelfth ribs on the left (Fig. 53.1).

image

Fig. 53.1 Thoracic wall and cavity.

(From Drake, Vogl and Mitchell 2005.)

Variations in thoracic dimensions and proportions are partly individual and also linked to age, sex and race. At birth, the transverse diameter is relatively less than it is in the adult, but adult proportions develop as walking begins. Thoracic capacity is less in females than it is in males, both absolutely and proportionately: the female sternum is shorter, the thoracic inlet more oblique, and the suprasternal notch is level with the third thoracic vertebra (whereas it is level with the second in males). In all individuals, the size of the thoracic cavity changes continuously, according to the movements of the ribs and diaphragm during respiration (see Ch. 58), and the degree of distension of the abdominal viscera.

MUSCULOSKELETAL FRAMEWORK

BONES AND JOINTS

The thoracic skeleton consists of twelve thoracic vertebrae and their intervening intervertebral discs (midline, posterior), twelve pairs of ribs and their costal cartilages (predominantly lateral) and the sternum (midline, anterior). When articulated, they form an irregularly shaped osteocartilaginous cylinder, reniform in horizontal section, which is narrow above, broad below, flattened anteroposteriorly and longer behind (see Fig. 54.7). Laterally the thoracic cage is convex and is formed by the ribs, and anteriorly it is slightly convex and is formed by the sternum and the distal parts of the ribs and their costal cartilages. The first seven pairs of ribs are connected to the sternum by costal cartilages, the costal cartilages of the eighth to tenth ribs usually join the superjacent cartilage, and the eleventh and twelfth ribs are free (floating) at their anterior ends. The posterolateral curvature of the ribs, from their vertebral ends to their angles, produces a deep internal groove, the paravertebral gutter, on either side of the vertebral column. The ribs and costal cartilages are separated by intercostal spaces, which are deeper anteriorly and between the upper ribs. Each space is occupied by three layers of flat muscles and their aponeuroses, neurovascular bundles and lymphatic channels.

The narrow thoracic inlet (superior thoracic aperture) typically measures 5 cm anteroposteriorly and 10 cm transversely. It is bounded by the first thoracic vertebral body posteriorly, the medial border of the first ribs on each side, and the superior border of the manubrium sterni anteriorly. It slopes down and forwards, so that the apex of the lung extends upwards into the neck behind the anterior end of the first rib: structures that pass between the thorax and the upper limb therefore pass over the first rib and the apices of the lungs and the apical pleurae.

The broad thoracic outlet (inferior thoracic aperture) is limited posteriorly by the twelfth thoracic vertebral body, posterolaterally by the twelfth rib and the distal end of the eleventh rib, anterolaterally by the distal cartilaginous ends of the seventh to tenth ribs (which unite and ascend to form the costal margin), and anteriorly by the xiphoid process. It is wider in the transverse plane than in the sagittal plane and slopes obliquely downward and backward, so that the thoracic cavity is deeper posteriorly than anteriorly.

The horizontal plane that passes through the intervertebral disc between the fourth and fifth thoracic vertebrae posteriorly and the manubriosternal joint (sternal angle or angle of Louis) at the level of the second costal cartilage anteriorly is a useful aid to orientation in the thorax. It separates the superior mediastinum from the inferior mediastinum and also marks the positions of the superior limits of the pericardium and of the pulmonary trunk, the origin of the arch of the aorta, the level at which the trachea bifurcates into right and left main bronchi, and the site where the superior vena cava penetrates the pericardium to enter the right atrium.

MUSCLES

Intrinsic and extrinsic muscles

The intrinsic muscles of the chest wall are the intercostal muscles, subcostalis, transversus thoracis, levatores costarum, serratus posterior superior and serratus posterior inferior. The intercostal muscles occupy each of the intercostal spaces and are named according to their surface relations, i.e. external, internal and innermost. All except levatores costarum are innervated by the adjacent intercostal nerves derived from the ventral rami of the thoracic spinal nerves; levatores costarum are innervated by the dorsal rami of the thoracic spinal nerves. The intrinsic muscles can elevate or depress the ribs, and are active during respiration, particularly forced respiration: their primary action is believed to be to stiffen the chest wall, preventing paradoxical movement during inspiration (see Ch. 58).

The skeletal framework of the thoracic wall provides extensive attachment sites for muscles associated functionally with the neck, abdomen, back, and upper limbs. Some of them (scalenes, infrahyoid strap muscles, sternocleidomastoid, serratus anterior, pectoralis major and minor, external and internal obliques, and rectus abdominis) function as accessory muscles of respiration and are usually active only during forced respiration; scalenus medius is active in quiet inspiration. Scalenus anterior, medius and posterior are described in Chapter 28. Trapezius, latissimus dorsi, rhomboid major, rhomboid minor, levator scapulae, pectoralis major, pectoralis minor, subclavius, serratus anterior, deltoid, subscapularis, supraspinatus, infraspinatus, teres minor and teres major are described in Chapter 46. Rectus abdominis, external oblique and internal oblique are described in Chapter 61.

Diaphragm

The diaphragm is a curved musculotendinous sheet attached to the circumference of the thoracic outlet and to the upper lumbar vertebrae. It forms the floor of the thoracic cavity, and separates it from the abdominal cavity (see Ch. 58). The diaphragm is relatively flat centrally and domed peripherally, rising higher on the right side than on the left, an asymmetry that reflects the relative densities of the underlying liver and gastric fundus respectively. From its highest point on each side the diaphragm slopes downward to its costal and vertebral attachments: this slope is most marked posteriorly, where the space between the diaphragm and the posterior wall of the thorax is very narrow.

THORACIC CAVITY

PLEURAL CAVITIES

The right and left pleural cavities are separate compartments on either side of the mediastinum. Each encloses a lung and its associated bronchial tree and vessels, nerves and lymphatics (see Ch. 57). The walls are formed by a serous membrane, the pleura, arranged as a closed sac. The outer layer of the sac, the parietal pleura, lines the corresponding half of the thoracic wall and covers much of the diaphragm and structures occupying the middle region of the thorax. The inner or visceral layer is more delicate and adheres closely to the pulmonary surface, following the interlobar fissures. The two layers are continuous with each other around the structures at the hila of the lungs. They remain in close, though sliding, contact at all phases of respiration. The potential space between them is the pleural cavity, which is maintained at a negative pressure by the inward elastic recoil of the lung and the outward pull of the chest wall. The lungs do not fill this space in quiet respiration, but move into recesses such as the costodiaphragmatic recess, which separates the costal and diaphragmatic pleura, in deep breathing.

The left pleural cavity is the smaller of the two pleural cavities because the heart extends further to the left.

MEDIASTINUM

The mediastinum lies between the right and left pleural sacs in and near the median sagittal plane of the chest (see Ch. 55). It extends from the sternum anteriorly to the vertebral column behind. A horizontal plane passing through the manubriosternal joint and the intervertebral disc between the fourth and fifth thoracic vertebrae separates the mediastinum into superior and inferior portions.