The thorax constitutes the upper part of the trunk. It is like a truncated cone which is flattened from before backwards. In the adult, the shape of the thorax in transverse section is reniform whereas it is more rounded in the infant. Thus, the human thorax contrasts sharply from that of ordinary mammals in which it is more or less spindle-shaped in transverse section. The shape of the human chest is the outcome of man’s arboreal ancestry. The bony thoracic cage is made up of 12 thoracic vertebrae, 12 pairs of ribs and their costal cartilages, and the sternum. The sternum has three parts: (1) the manubrium lying opposite the third and fourth thoracic vertebrae; (2) the body of the sternum opposite the fifth, sixth, seventh and eighth thoracic vertebrae; and (3) the xiphoid process. The junction between the manubrium and the body of the sternum is the sternal angle, a palpable landmark at the level of the second sternocostal joint. This corresponds to the lower border of the body of the fourth thoracic vertebra. The manubrium and the body of the sternum are joined together by fibrocartilage to form a secondary cartilaginous joint which permits only a restricted amount of movement during respiration. However, the sternocostal joints, with the exception of the first, are of the synovial type in which a greater freedom of movement is possible. The thorax has a superior aperture of the thorax, which is formed by the upper end of the manubrium of the sternum, the first pair of ribs and the first thoracic vertebra. Through this aperture, various structures pass from the neck into the thorax and vice versa. The inferior aperture of the thorax is formed by the twelfth thoracic vertebra, the lower six costal cartilages and the xiphisternal joint. It is closed by a fibromuscular partition, the diaphragm, which is an important muscle of respiration. The cavity of the thorax has three subdivisions a single midportion called the mediastinum containing the heart, the large vessels, the trachea, the oesophagus and nerves; while the two lateral portions, one on either side, contain the lungs with their coverings.

THORACIC WALL AND LUNGS

1. With the body on its back, remove any remaining fascia and the remnants of the pectoral, serratus anterior and latissimus dorsi muscles. Note the lateral cutaneous branches of the intercostal nerves in the midaxillary line as you remove the fascia and muscles. After noting the upper three slips of origin of the obliquus externus abdominis from ribs 5, 6, 7 detach them so that the superficial layer of intercostal muscles, namely the external intercostals, can be viewed in the upper six spaces. Observe that the fibres of these muscles run downwards and forwards. Note that each external intercostal muscle is replaced anteriorly by the external intercostal membrane medial to the costochondral junction.

2. Remove the external intercostal muscles of the upper two or three intercostal spaces. Define the intermediate layer formed by internal intercostal muscles running downwards and backwards, i.e., in a direction opposite to that of the external intercostals. The internal intercostals are replaced posteriorly by the internal intercostal membranes medial to the angles of the ribs.

3. Remove the internal intercostal muscles of the upper two or three intercostal spaces and examine the innermost layer of intercostal muscles which do not form a continuous sheet. They consist of three parts:

(a)transversus thoracis passing between the lower third of the posterior surface of the sternum and the deep surfaces of the costal cartilages 2–6.

(b) intercostalis intimi occupying the middle two-fourths of the intercostal spaces; and

(c) subcostals covering the lower intercostal spaces close to the vertebral bodies. Only the intercostalis intimi and part of the transversus thoracis can be seen at this stage.

4. Find the internal thoracic artery running 1–2 cm lateral to the margin of the sternum in front of the transversus thoracis muscle. Note that the artery divides into the superior epigastric and musculophrenic branches in the sixth intercostal space. Find the anterior intercostal arteries arising from the internal thoracic artery and anastomosing with the posterior intercostal arteries.

5. In the upper part of the intercostal space, search for the nerve and vessels lying in the costal groove close to the lower margin of the rib. The pleura lies deep to the innermost intercostal muscles. Secure the main intercostal nerve and note that it has lateral cutaneous, anterior cutaneous, muscular and collateral branches. The collateral branch runs along the upper border of the rib below. All the intercostal as well as the innermost layer of muscles are supplied by the adjacent thoracic nerves.

6. Note that the posterior intercostal vein, posterior intercostal artery and intercostal nerve lie in that order; from above downwards in the costal groove. The posterior intercostal arteries anastomose anteriorly with the anterior intercostal arteries from the internal thoracic artery. Note that the neurovascular plane lies between the internal and innermost layers of intercostal muscles.

7. Next remove the anterior part of the thoracic cage. With a small saw, cut transversely through the middle of the manubrium of the sternum. Then with a scalpel, cut backwards along the first intercostal space as far as the posterior axillary line, all the time taking care not to cut through the parietal pleura which is adherent to the innermost layer of thoracic muscles and ribs. Next, with a pair of shears, cut through ribs 2, 3, 4, 5, 6, 7, 8 vertically along the posterior axillary line. Finally make an oblique cut forwards and upwards to the xiphisternal junction which is then cut transversely using a scalpel or bone shears as necessary. As far as is possible, try not to damage the parietal pleura. Now remove the anterior part of the thoracic cage.

8. Next examine the internal aspect of the part of the thoracic cage that has been removed. Identify the transversus thoracis and the internal thoracic artery and look for one or two intercostal nerves. Whenever necessary replace this part of the thoracic cage for reference.

9. Before you examine the pleura, note that each lung is invested by a double-layered serous membrane. The outer part of the membrane, the parietal pleura, lines the inner surface of the corresponding half of the chest wall, a large part of the upper surface of the diaphragm and the mediastinum (the structures occupying the middle part of the thorax). The inner part of the membrane, the visceral (pulmonary) pleura, covers the surface of the lung and lines the fissures between the lobes. Different regions of the parietal pleura have distinctive names: the part ascending into the neck over the summit of the lung is the cervical pleura; the part lining the internal aspect of the thoracic cage and the sides of the vertebral bodies is the costal pleura; that covering the thoracic surface of the diaphragm is the diaphragmatic pleura; and that applied to the mediastinum is the mediastinal pleura.

10. Make a cruciform incision in the parietal pleura and reflect the flaps to expose the lungs covered by visceral pleura. You are now in the pleural cavity which is between the two layers of the pleura. Pass your hand over the lungs and locate its apex, base (diaphragmatic surface), mediastinal and costal surfaces. On the mediastinal surface palpate the root of the lung and the pulmonary ligament. The latter is a fold of pleura immediately below the root of the lung. Explore the costodiaphragmatic recess by passing your hand between the diaphragm and thoracic cage. This recess allows for expansion of the lungs during inspiration.

1. Examine the levels of reflection of the parietal pleurae on the two sides. Note that the cervical parietal pleura on both sides extends into the root of the neck for about 1–2 cm above the medial third of the clavicle. From here, trace the parietal pleurae downwards until they almost meet each other in the midline behind the sternum at the level of the sternal angle. The two layers then continue to lie near the midline till they reach the level of the fourth costal cartilage, where the left layer diverges so that it is about 1cm away from the lateral margin of the sternum at the level of the sixth costal cartilage. Here it curves laterally as the inferior margin reaching the eighth rib at the midclavicular line, tenth rib at the midaxillary line and twelfth rib at the paravertebral line. The levels of reflection of the right parietal pleura are the same as those of the left, except that the right layer continues to run close to the midline behind the sternum down to the sixth costal cartilage.

2. Now examine the limits of the lungs. The lower limits of the lungs are two ribs above those of the parietal pleura at the midclavicular, midaxillary and paravertebral lines, i.e. at the level of the sixth, eighth and tenth ribs respectively. The upper limits of the lungs follow more or less the outline of the pleurae.

3. Identify the phrenic nerves covered by mediastinal pleura and lying on either side of the pericardium, which encloses the heart, and in front of the root of each lung. Trace these nerves to the diaphragm.

4. Remove the lungs by dividing the roots and pulmonary ligaments. Note that the root of each lung contains a bronchus, pulmonary artery and a superior and inferior pulmonary vein. These structures enter and leave the hilum of the lung.

5. Examine each lung in turn and identify its apex; and the diaphragmatic; costal and mediastinal surfaces; and its anterior, posterior and inferior borders. Note that the anterior and inferior margins are sharp while the posterior border is rounded.

6. Left Lung: Identify the cardiac notch along the lower part of the anterior border of the lung. This notch corresponds to the area where the left pleura recedes away from the midline between the fourth and sixth costal cartilages. Observe the oblique fissure passing from the posterior border to the inferior margin thus dividing this lung into superior and inferior lobes. Note the tongue shaped process of the superior lobe situated between the cardiac notch and the oblique fissure. This is known as the lingula. Observe the following impressions on the mediastinal surface:

(a)cardiac impression in front of and below the hilum;

(b) groove for the thoracic aorta behind the hilum and in front of the posterior border;

(c) groove for the arch of aorta situated above the hilum; and

(d) grooves for the left subclavian and common carotid arteries passing upwards from the groove for the arch of the aorta; the impression for the common carotid artery is in front of that for the subclavian artery.

Examine the arrangement of structures passing through the hilus: from before backwards these are the superior pulmonary vein, pulmonary artery and bronchus.

7. Right Lung: Identify the oblique fissure, as in the left lung. From about the middle of the oblique fissure, observe the horizontal fissure passing forwards to the anterior border. These two fissures divide the right lung into superior, middle and inferior lobes. Now identify the impressions seen on the mediastinal surface:

(a)cardiac impression in front of and below the hilum;

(b) groove for the azygos vein in front of the posterior border and arching forwards above the hilum;

(c) groove for the superior vena cava passing upwards from the anterior end of the groove for the azygos vein;

(d) groove for the inferior vena cava in front of the pulmonary ligament; and

(e) groove for the oesophagus behind the hilum and in front of the groove for azygos vein. Examine the arrangement of structures passing through the hilum: from before backwards these are the superior pulmonary vein, pulmonary artery and bronchus. Note that there is also a bronchus above the pulmonary artery.

8. Observe that the trachea divides into two main bronchi, which in turn subdivide into lobar bronchi; two on the left and three on the right. These are on the left the left superior and the left inferior lobar bronchus; and on the right the right superior, the middle and the right inferior lobar bronchus. Note the presence of bronchopulmonary lymph nodes in relation to the lobar bronchi at the hilum and that the bronchial arteries accompany the lobar bronchi of the lungs. These arteries usually arise from the thoracic aorta. Trace the lobar bronchi further into the lungs and note their subdivision into segmental bronchi; each segmental bronchus supplies a pyramidal shaped area of the lung termed the bronchopulmonary segment. Note that each segmental bronchus is accompanied by a branch of the pulmonary artery whereas the corresponding vein runs in between segments and therefore drains adjacent bronchopulmonary segments.

Examine museum specimens and your atlas showing the ten bronchopulmonary segments in the right and in the left lung.

Summary

The intercostal nerves are the anterior rami of the thoracic spinal nerves. They supply the adjacent intercostal muscles and skin. The lower six nerves pass into the abdominal wall after leaving the intercostal spaces. The intercostal nerve is a segmental nerve and it also supplies the adjacent parietal pleura, which is sensitive to pain. However, the visceral pleura is devoid of such innervation and is consequently insensitive to pain. The lungs which occupy the lateral portions of the thoracic cavity are lateral outgrowths from the foregut. Each lung invaginates the pleural cavity from its medial side. As a result, the roots of the lungs and pulmonary ligaments are found on the mediastinal surface of the lung. During respiration, the roots of the lungs move downwards, forwards and outwards so that there is room for expansion of the posterior parts of the lungs. In addition, there is possibly differential expansion between the lobes of the lungs during respiration and this may be assisted by the fissures which allow freedom of movement between the lobes. The right lung is shorter and heavier than the left lung, and its inferior surface is pushed upwards by the right lobe of the liver and overlying dome of the diaphragm. Each lung is divisible into ten bronchopulmonary segments. This enables the surgical removal of a diseased segment of the lung. The levels of reflection of pleura and lungs must be comprehended. For example, you should know that the cervical pleura is about 1–2 cm above the medial third of the clavicle. This is due to the fact that the first rib slopes downwards and forwards from its vertebral towards its sternal end. As a result, an accidental puncture through this region could produce a pneumothorax. This will also explain why breath sounds from the apex of the lungs are better heard over the supraclavicular region than from behind. Moreover, note that the pleural cavity overlaps the upper abdominal organs, so an injury through the lower part of the pleural cavity could involve the organs occupying the upper part of the abdomen.