The trachea (Figs 1, 2)

The trachea commences in the neck at the level of the lower border of the cricoid cartilage (C6) and runs vertically downwards to end below the level of the sternal angle of Louis (T4/5), just to the right of the midline, by dividing to form the right and left main bronchi. In the erect position and in full inspiration the level of bifurcation is at T6.

The pleura (Figs 2, 3)

The cervical pleura can be marked out on the surface by a curved line drawn from the sternoclavicular joint to the junction of the medial and middle thirds of the clavicle; the apex of the pleura is approximately 2.5 cm (1 in) above the clavicle. This fact is easily explained by the oblique slope of the first rib. It is important because the pleura can be wounded (with consequent pneumothorax) by a stab wound – and this includes the surgeon’s knife and the anaesthetist’s needle – above the clavicle, or, in an attempted subclavian vein catheterization, below the clavicle. The lines of pleural reflexion pass from behind the sternoclavicular joint on each side to meet in the midline at the 2nd costal cartilage (the angle of Louis). The right pleural edge then passes vertically downwards to the 6th costal cartilage and then crosses:

• the 8th rib in the midclavicular line;
  
• the 10th rib in the midaxillary line;
  
• the 12th rib at the lateral border of the erector spinae.

On the left side the pleural edge arches laterally at the 4th costal cartilage and descends lateral to the border of the sternum, owing, of course, to its lateral displacement by the heart; apart from this, its relationships are those of the right side.

The pleura actually descends just below the 12th rib margin at its medial extremity – or even below the edge of the 11th rib if the 12th is unusually short; obviously, in this situation, the pleura may be opened accidentally in making a loin incision to expose the kidney, perform an adrenalectomy or drain a subphrenic abscess.

The lungs (Figs 2, 3)

The surface projection of the lung is somewhat less extensive than that of the parietal pleura as outlined previously, and in addition it varies quite considerably with the phase of respiration. The apex of the lung closely follows the line of the cervical pleura and the surface marking of the anterior border of the right lung corresponds to that of the right mediastinal pleura. On the left side, however, the anterior border has a distinct notch (the cardiac notch) that passes behind the 5th and 6th costal cartilages. The lower border of the lung has an excursion of as much as 5–8 cm (2–3 in) in the extremes of respiration, but in the neutral position (midway between inspiration and expiration) it lies along a line which crosses the 6th rib in the midclavicular line, the 8th rib in the midaxillary line and reaches the 10th rib adjacent to the vertebral column posteriorly.

The oblique fissure, which divides the lung into upper and lower lobes, is indicated on the surface by a line drawn obliquely downwards and outwards from 2.5 cm (1 in) lateral to the spine of the 3rd thoracic vertebra along the 5th intercostal space to the 6th costal cartilage approximately 4 cm (1.5 in) from the midline. This can be represented approximately by abducting the shoulder to its full extent; the line of the oblique fissure then corresponds to the position of the medial border of the scapula.

The surface markings of the transverse fissure (separating the middle and upper lobes of the right lung) is a line drawn horizontally along the 4th costal cartilage and meeting the oblique fissure where the latter crosses the 5th rib.

The heart (Fig. 4)

The outline of the heart can be represented on the surface by an irregular quadrangle bounded by the following four points (Fig. 4):

1. the 2nd left costal cartilage 1.25 cm (0.5 in) from the edge of the sternum;
   
2. the 3rd right costal cartilage 1.25 cm (0.5 in) from the sternal edge;
   
3. the 6th right costal cartilage 1.25 cm (0.5 in) from the sternum;
   
4. the 5th left intercostal space 9 cm (3.5 in) from the midline (corresponding to the apex beat).

The left border of the heart (indicated by the curved line joining points 1 and 4) is formed almost entirely by the left ventricle (the auricular appendage of the left atrium peeping around this border superiorly); the lower border (the horizontal line joining points 3 and 4) corresponds to the right ventricle and the apical part of the left ventricle; the right border (marked by the line joining points 2 and 3) is formed by the right atrium (see Fig. 24a).

The thoracic vertebrae

See ‘The vertebral column’, page 347. See also page 350 and Fig. 228.

The ribs

The greater part of the thoracic cage is formed by the twelve pairs of ribs. Of these, the first seven are connected anteriorly by way of their costal cartilages to the sternum, the cartilages of the 8th, 9th and 10th articulate each with the cartilage of the rib above and the last two ribs are free anteriorly (‘floating ribs’).

Each typical rib (Fig. 5) has a head bearing two articular facets, for articulation with the upper demifacet on the side of the body of the numerically corresponding thoracic vertebra and the lower demifacet of the vertebra above (see Fig. 228). Thus, the head of the third rib articulates with its own third vertebral body and the one above. The head continues as a stout neck, which gives attachment to the costotransverse ligaments, a tubercle with a rough non‐articular portion and a smooth facet, for articulation with the transverse process of the corresponding vertebra, and a long shaft flattened from side to side and divided into two parts by the ‘angle’ of the rib. The angle demarcates the lateral limit of attachment of the erector spinae muscle.

The following are the significant features of the ‘atypical’ ribs.

The 1st rib (Fig. 6) is flattened from above downwards. It is not only the flattest but also the shortest and most highly curved of all the ribs. It has a prominent tubercle on the inner border of its upper surface for the insertion of scalenus anterior. In front of this tubercle, the subclavian vein crosses the rib; behind the tubercle is the subclavian groove, where the subclavian artery and lowest trunk of the brachial plexus lie in relation to the bone. This is one of the sites where the anaesthetist can infiltrate the plexus with local anaesthetic.

Crossing the front of the neck of the first rib vertically and lying from medial to lateral are the sympathetic trunk, the superior intercostal artery (from the costocervical trunk) and the large branch of the first thoracic nerve to the brachial plexus.

The 2nd rib is much less curved than the 1st and approximately twice as long.

The 10th rib has only one articular facet on the head.

The 11th and 12th ribs (the ‘floating ribs’) are short, have no tubercles and only a single facet on the head. The 11th rib has a slight angle and a shallow subcostal groove; the 12th has neither of these features.

The costal cartilages

These bars of hyaline cartilage serve to connect the upper seven ribs directly to the side of the sternum and the 8th, 9th and 10th ribs to the cartilage immediately above. The cartilages of the 11th and 12th ribs merely join the tapered extremities of these ribs and end in the abdominal musculature.

The sternum

This dagger‐shaped bone, which forms the anterior part of the thoracic cage, consists of three parts. The manubrium is roughly triangular in outline and provides articulation for the clavicles and for the first and upper part of the 2nd costal cartilages on either side. It is situated opposite the 3rd and 4th thoracic vertebrae. Opposite the disc between T4 and T5 it articulates at an oblique angle at the manubriosternal joint (the angle of Louis) with the body of the sternum (placed opposite T5–T8). This is composed of four parts or ‘sternebrae’, which fuse between puberty and 25 years of age. Its lateral border is notched to receive part of the 2nd and the 3rd to the 7th costal cartilages. The xiphoid process is the smallest part of the sternum and usually remains cartilaginous well into adult life. The cartilaginous manubriosternal joint and that between the xiphoid and the body of the sternum may also become ossified after the age of 30.

The intercostal spaces

There are slight variations between the different intercostal spaces, but typically each space contains three muscles, comparable to those of the abdominal wall, and an associated neurovascular bundle (Fig. 8). The muscles are:

1. the external intercostal, the fibres of which pass downwards and forwards from the rib above to the rib below and reach from the vertebrae behind to the costochondral junction in front, where muscle is replaced by the anterior intercostal membrane;
   
2. the internal intercostal, which runs downwards and backwards from the sternum to the angles of the ribs where it becomes the posterior intercostal membrane;
   
3. the innermost intercostal, which is only incompletely separated from the internal intercostal muscle by the neurovascular bundle. The fibres of this sheet cross more than one intercostal space and it may be incomplete. Anteriorly it has a more distinct portion that is fan‐like in shape, termed the transversus thoracis (or sternocostalis), which spreads upwards from the posterior aspect of the lower sternum to insert onto the inner surfaces of the 2nd to the 6th costal cartilages.

Just as in the abdomen, the nerves and vessels of the thoracic wall lie between the middle and innermost layers of muscles. This neurovascular bundle consists, from above downwards, of vein, artery and nerve, the vein lying in a groove on the undersurface of the corresponding rib (remember: v,a,n).

The vessels comprise the posterior and anterior intercostal arteries and veins.

The posterior intercostal arteries of the lower nine spaces are direct branches of the descending thoracic aorta, while the first two are derived from the superior intercostal branch of the costocervical trunk, the only branch of the second part of the subclavian artery. Each runs forward in the subcostal groove to anastomose with the anterior intercostal artery. Each has a number of branches to adjacent muscles, to the skin and to the spinal cord. The corresponding veins are mostly tributaries of the azygos and hemiazygos veins. The first posterior intercostal vein drains into the brachiocephalic or vertebral vein. On the left, the 2nd and 3rd veins often join to form a superior intercostal vein, which crosses the aortic arch to drain into the left brachiocephalic vein.

The anterior intercostal arteries are branches of the internal thoracic artery (1st–6th space) or of its musculophrenic branch (7th–9th spaces). The lowest two spaces have only posterior arteries. Perforating branches pierce the upper five or six intercostal spaces; those of the 2nd–4th spaces are large in the female and supply the breast.

The intercostal nerves are the anterior primary rami of the thoracic nerves, each of which gives off a collateral muscular branch and lateral and anterior cutaneous branches for the innervation of the thoracic and abdominal walls (Fig. 9).

The diaphragm

The diaphragm is the dome‐shaped septum dividing the thoracic from the abdominal cavity. It is present only in mammals. It comprises two portions: a peripheral muscular part that arises from the margins of the inferior aperture of the thoracic cage (termed by anatomists as the ‘thoracic outlet’) and a centrally placed aponeurosis (Fig. 10).

The muscular fibres are arranged in three parts:

1. A vertebral part from the crura and from the arcuate ligaments. The right crus arises from the front of the bodies of the upper three lumbar vertebrae and intervertebral discs; the left crus is attached to only the first two vertebrae. The arcuate ligaments are a series of fibrous arches, the medial being a thickening of the fascia covering psoas major and the lateral of the fascia overlying quadratus lumborum. The tendinous medial borders of the two crura join each other in front of the aorta to form the median arcuate ligament.
   
2. A costal part is attached to the inner aspect of the lower six ribs and their costal cartilages.
   
3. A sternal portion consists of two small slips from the deep surface of the xiphisternum.

The central tendon, into which the muscular fibres are inserted, is trefoil in shape and is partially fused with the undersurface of the pericardium.

The diaphragm receives its entire motor supply from the phrenic nerve (C3, C4, C5), whose long course from the neck follows the embryological migration of the muscle of the diaphragm from the cervical region (see ‘The development of the diaphragm and the anatomy of diaphragmatic herniae’). Injury or operative transection of this nerve results in paralysis and permanent elevation of the ipsilateral half of the diaphragm.

Radiographically, paralysis of the diaphragm is recognized by its elevation and paradoxical movement; instead of descending on inspiration, it is forced upwards by pressure from the abdominal viscera.

The sensory nerve fibres from the central part of the diaphragm also run in the phrenic nerve; hence, irritation of the diaphragmatic pleura (in pleurisy) or of the peritoneum on the undersurface of the diaphragm by subphrenic collections of pus or blood produces referred pain in the corresponding cutaneous area, the shoulder‐tip.

The peripheral part of the diaphragm, including the crura, receives sensory (proprioceptive) fibres from the lower intercostal nerves.

Openings in the diaphragm

The three main openings in the diaphragm (Figs 10, 11) are:

1. The aortic (at the level of T12), which transmits the abdominal aorta, the thoracic duct and often the azygos vein. The aortic opening lies in the midline.
   
2. The oesophageal (T10), which is situated within the muscular fibres of the left and right crura of the diaphragm and transmits, in addition to the oesophagus, branches of the left gastric artery and vein and the two vagi. The oesophageal opening lies slightly to the left of the midline.
   
3. The opening for the inferior vena cava (T8), which is situated in the central tendon and also transmits the right phrenic nerve. The vena caval opening lies slightly to the right of the midline.

In addition to these structures, the greater and lesser splanchnic nerves (see page 52) pierce the crura and the sympathetic chain passes behind the diaphragm deep to the medial arcuate ligament to reach the posterior abdominal wall.

The development of the diaphragm and the anatomy of diaphragmatic herniae

The diaphragm is formed (Fig. 12) by fusion in the embryo of:

1. the septum transversum (forming the central tendon);
   
2. the dorsal oesophageal mesentery;
   
3. a peripheral rim derived from the body wall;
   
4. the pleuroperitoneal membranes, which close the fetal communication between the pleural and peritoneal cavities.

The septum transversum is the mesoderm which, in early development, lies in front of the head end of the embryo. With the folding off of the head, this mesodermal mass is carried ventrally and caudally, to lie in its definitive position at the anterior part of the diaphragm. During this migration, the cervical myotomes and nerves contribute muscle and nerve supply respectively, thus accounting for the long course of the phrenic nerve (C3, C4 and C5) from the neck to the diaphragm.

With such a complex embryological story, one may be surprised to know that congenital abnormalities of the diaphragm are unusual.

However, a number of defects can occur, giving rise to a variety of congenital herniae through the diaphragm. These may be:

1. through the foramen of Morgagni – anteriorly between the xiphoid and costal origins;
   
2. through the foramen of Bochdalek – the pleuroperitoneal canal – lying posteriorly;
   
3. through a deficiency of the whole central tendon (occasionally such a hernia may be traumatic in origin);
   
4. through a congenitally large oesophageal hiatus.

Far more common are the acquired hiatus herniae (subdivided into sliding and rolling herniae). These are found in patients usually of middle age in whom weakening and widening of the oesophageal hiatus has occurred (Fig. 13).

In the sliding hernia the upper stomach and lower oesophagus slide upwards into the chest through the lax hiatus when the patient lies down or bends over; the competence of the cardia is often disturbed and peptic juice can therefore regurgitate into the gullet in lying down or bending over. This may be followed by oesophagitis with consequent heartburn, bleeding and, eventually, stricture formation.

In the rolling hernia (which is far less common) the cardia remains in its normal position and the cardio‐oesophageal junction is intact, but the fundus of the stomach rolls up through the hiatus in front of the oesophagus; hence, the alternative term of para‐oesophageal hernia. In such a case there may be epigastric discomfort, flatulence and even dysphagia, but no regurgitation because the cardiac mechanism is undisturbed.