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).
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.
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.
Diaphragmatic openings
The inferior vena cava passes through an opening in the central tendon of the diaphragm to enter the right side of the mediastinum at the level of the eighth thoracic vertebra; the oesophagus passes through the muscular part of the diaphragm and enters the abdomen just to the left of the midline at the level of the tenth thoracic vertebra; and the descending thoracic aorta passes posterior to the diaphragm in the midline at the level of the twelfth thoracic vertebra (see Fig. 58.1A,B). Other structures which pass between the thorax and abdomen include the right and left vagi, phrenic and subcostal nerves, the thoracic duct, the sympathetic trunks and the thoracic splanchnic nerves, the right and left superior epigastric arteries and the hemiazygos and accessory hemiazygos veins.
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.
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.
