Last's Anatomy, International Edition Regional and Applied PDF

Chapter |4| Thorax inferior epigastric arteries supply the ventral midline tis- Part one: Body wall sues. From all these arteries...

Chapter |4| Thorax inferior epigastric arteries supply the ventral midline tis- Part one: Body wall sues. From all these arteries cutaneous branches pass to the superficial fat and skin. The venous return from The wall of the thorax and the wall of the abdomen are the subcutaneous tissue does not follow the arteries. The one, topographically and developmentally, the essential blood is collected by an anastomosing network of veins difference being the presence of ribs in the part primarily that radiate away from the umbilicus. Below this level concerned with respiration. they pass to the great saphenous vein in the groin; above The skin varies in texture, tending to be thin in front and the umbilicus they run up to the lateral thoracic vein thick behind. Distribution of hair varies with sex, age and and so to the axillary vein. From the umbilicus a few para- race. The tension lines run almost horizontally around the umbilical veins accompany the ligamentum teres and body wall, except over and above the breast (Fig. 1.2, p. 2). drain to the left branch of the portal vein; they may dis- In the subcutaneous tissue over the dilatable part of tend in portal obstruction, giving rise, if the distension the body wall, namely the anterior abdominal wall and spreads to the subcutaneous veins, to a pattern of dilated lower part of the thoracic wall in front of the midaxillary veins around the umbilicus, the caput Medusae. A longi- lines, the fibrous septa of the subcutaneous tissue are tudinal channel, the thoracoepigastric vein uniting the condensed beneath the fat into a thin but strong mem- lateral thoracic vein with the superficial epigastric vein branous layer of superficial fascia, the fascia of Scarpa. above the inguinal ligament, provides a communication This fascia allows the fatty layer of superficial fascia, the between superior and inferior venae cavae and often fascia of Camper, to slide freely over the underlying tho- becomes prominent in cases of obstruction of the inferior racic wall, rectus sheath and external oblique aponeurosis. vena cava. It fades out over the upper thoracic wall and along the midaxillary lines. The fascia of Scarpa is continued over the penis and scrotum as the superficial fascia of the penis (Buck’s fascia) and the superficial perineal fascia (Colles’ Lymph drainage fascia; see pp. 318 and 319). Below, over the thighs, it is Lymphatic channels from the subcutaneous tissue and attached to the fascia lata along the flexure skin crease of skin follow the veins to axillary and superficial inguinal the hip, extending laterally from the pubic tubercle just nodes. From above the level of the umbilicus, lymph from below the inguinal ligament (Fig. 4.1). the front of the body goes to the anterior (pectoral) group and from the back of the body to the posterior (scapular) group of axillary nodes. From the umbilicus downwards Blood supply lymph from the anterior aspect of the abdominal wall The intercostal, subcostal and lumbar arteries pass for- and perineum goes to the medial group and from the ward in the neurovascular plane (see p. 11) to supply lateral and posterior aspects of the abdominal wall to the flanks; the internal thoracic and the superior and the lateral group of superficial inguinal nodes. © 2011 Elsevier Ltd. 179  Last’s anatomy A B Figure 4.1 Bilateral damage to the inferior epigastric artery—during the introduction of instruments through both iliac fossae for minimal access surgery—has resulted in blood leaking at the port sites and tracking down in a plane deep to Scarpa’s fascia: A to the penis and scrotum and B to the upper thigh as far as the level of the flexure skin crease of the hip. Nerve supply stomach and both kidneys lie in the abdominal cavity wholly or partly covered by ribs. Above the second rib and the manubriosternal joint the At the back the ribs articulate with the vertebral column skin is supplied by supraclavicular branches of the cervical in two places: by their heads (joints of costal heads) and plexus (C4; see Fig. 1.7, p. 12). Below this level a midline by their tubercles (costotransverse joints). Collectively and paramedian strip of skin is supplied by the anterior these form the costovertebral joints. At the front the ribs cutaneous branches of the spinal nerves from T2 to L1; join their costal cartilages (costochondral joints). The the skin in the upper epigastric region is supplied by T7, upper seven costal cartilages articulate with the sternum in the umbilical region by T10 and suprapubic skin by at the sternocostal joints, the next three articulate with L1. A broad lateral strip is supplied by the lateral cutane- each other (interchondral joints) and the eighth articu- ous branches of the spinal nerves from T2 or 3 to L1; lates with the seventh costal cartilage to complete the cos- these branches emerge in the midaxillary line. The lateral tal margin; the last two costal cartilages have free ends. cutaneous branches of T12 and the iliohypogastric nerve The manubriosternal joint is a symphysis between the descend over the iliac crest to also supply the skin of the manubrium and body of the sternum, and the xiphister- buttock. The ilioinguinal nerve has no lateral cutaneous nal joint is a symphysis between the body and the xiphoid branch; it is the collateral branch of the iliohypogastric, process. both coming from L1 nerve. A posterior strip of skin is innervated by the posterior rami of spinal nerves, by their medial branches in the upper thoracic and their lateral Joints of costal heads branches in the lower thoracic and lumbosacral parts (see Fig. 1.6, p. 12). The head of a typical rib possesses two articular facets that slope away from each other, separated by a ridge. Each facet makes a small synovial joint with a demifacet Part two: Thoracic wall and of a vertebral body; the lower rib facet with the upper diaphragm costal facet of its own vertebra, and the upper facet with the lower costal facet of the vertebra above (Fig. 4.2). The ridge between the two is attached to the intervertebral The skeleton of the thoracic wall consists of the 12 tho- disc by the intra-articular ligament. The front of the joint racic vertebrae, the 12 pairs of ribs and costal cartilages capsule is reinforced by the radiate ligament which consists and the sternum. The thoracic cavity is roofed in above of three bands. The upper band passes to the body of the the lung apices by the suprapleural membrane and is vertebra above, the lower band to the vertebra below, floored by the diaphragm. The floor is highly convex while the central band runs horizontally, deep to the ante- (domes of the diaphragm), so that when the diaphragm rior longitudinal ligament, and blends with the interverte- is relaxed in expiration the volume of the thoracic cavity bral disc. The first rib articulates with T1 vertebra only, is much less than inspection of the bony cage would sug- never coming into contact with C7, and the last two ribs gest. The liver and spleen and the upper parts of the also articulate only with their own vertebrae. 180 Chapter |4| Thorax The next six costal cartilages each articulate with the sternum by a synovial joint—a single cavity except in the Anterior case of the second, which articulates with the manubrium Superior longitudinal costotransverse ligament and body (Fig. 4.3). ligament Manubriosternal joint Intervertebral Radiate ligament At this symphysis between the manubrium and body of disc the sternum, the surfaces are covered by hyaline cartilage Figure 4.2 Joints of the heads of the ribs from the front. and there is an intervening disc of fibrocartilage which The upper vertebra shows the articular facet on the may become ossified in the elderly. Sometimes (30%) transverse process for the costotransverse joint with the cavitation appears in the disc so that the joint may appear tubercle of a rib. The radiate ligament on the right has been to be synovial, but this is simply a degenerative change divided. that does not alter the fact that the joint is a symphysis. Costotransverse joints Xiphisternal joint The tubercle of a typical rib (p. 218) has two facets. The This is another symphysis, between the body of the ster- medial facet, covered with hyaline cartilage, articulates num and the xiphoid process. Ossification from middle with a facet near the tip of the transverse process of its age onwards is common. own vertebra at a small synovial joint. The lateral facet (non-articular) gives attachment to the lateral costotrans- Thoracic muscles verse ligament which runs to the tip of the transverse pro- cess and is one of three ligaments helping to stabilize the joint. The other two are the costotransverse ligament, The muscles of the thoracic wall lie in the same three mor- which occupies the space between the back of the neck phological layers as those of the abdominal wall (see of the rib and the front of the transverse process, and p. 221), but in the thoracic region they have become the superior costotransverse ligament, which passes as two divided up by the presence of ribs. They are innervated laminae from the crest of the neck of the rib to the under- segmentally by anterior rami. In the outer thoracic layer surface of the transverse process of the vertebra above the external intercostal muscles correspond to the external (Fig. 4.2). The anterior lamina is continuous with the oblique in the abdomen and in the middle layer the inter- posterior intercostal membrane; the posterior is in the nal intercostals correspond to the internal oblique. The same plane as the external intercostal muscle. The fibres inner layer is broken up into three muscles, the subcos- of these two laminae are at right angles to each other, in tals, innermost intercostals and the transversus thoracis. a similar manner to the fibres of the intercostal muscles. This incomplete layer corresponds to the transversus The lower two ribs do not possess tubercles and make abdominis. Between it and the middle layer is the neuro- no synovial joints with transverse processes. vascular plane, continuous with that of the abdominal wall; in it run intercostal vessels and nerves, with their collateral branches (see Fig. 1.5, p. 11). Costochondral joints Every rib makes with its costal cartilage a primary cartilag- Outer layer inous joint. The costal cartilage represents no more than Two small muscles of the external layer have migrated the unossified anterior part of a rib. posteriorly, and lie on the surface of the erector spinae mass. They are the posterior serratus muscles and are Interchondral joints supplied by anterior rami. Each arises from four spinous processes, two in the thorax and two beyond it, and each Adjacent surfaces of costal cartilages 6 and 7, 7 and 8, and is inserted into four ribs. 8 and 9 are joined to each other by small synovial joints; Serratus posterior superior arises from the spinous 9 and 10 are connected by ligamentous fibres. processes of the lowest two cervical and the upper two thoracic vertebrae and is inserted just lateral to the angles of ribs 2–5. Many tendinous fibres in the sheet of muscle Sternocostal joints give it a characteristic glistening appearance which pro- The first costal cartilage articulates with the manubrium vides a useful landmark in exposures of this region. The by a primary cartilaginous joint. Thus the manubrium and dorsal scapular nerve and vessels run down on the poste- the first ribs are fixed to each other and move together rior surface of the muscle, between it and the rhomboids as one. (see Fig. 2.5, p. 41). 181  Last’s anatomy Groove for subclavian artery and first thoracic nerve Scalene tubercle Groove for Jugular notch subclavian vein External intercostal muscle First sternocostal joint Manubrium of sternum Manubriosternal Sternocostal joint articulations 2 and 3 Body of sternum Internal intercostal muscle Central tendon of diaphragm Xiphoid process Interchondral articulation Median and medial arcuate ligaments Costal part of diaphragm Left crus Right crus Costal margin Eleventh rib Figure 4.3 Chest wall, including diaphragm: anterior aspect. Serratus posterior inferior arises from the lower two it is replaced by the anterior intercostal membrane (see thoracic and the upper two lumbar spinous processes and Fig. 1.5, p. 11). This extends to the side of the sternum. is inserted just lateral to the angles of the lowest four ribs. Between the bony ribs is muscle; between the costal The serratus posterior muscles are weak muscles of cartilages is membrane. respiration. The superior muscle elevates the upper ribs (inspiration) while the inferior muscle depresses the lower ribs (expiration). Middle layer Although morphologically associated with the muscles This consists of the internal intercostal muscles. The of the back (see p. 428), the levator costae muscles are fibres run downwards and backwards, from the costal functionally classified as thoracic muscles. Each one of groove to the upper border of the rib below (Fig. 4.4). Each each of the 12 pairs is fan-shaped, spreading down from muscle, unlike an external intercostal, extends as far for- the tip of a transverse process (from C7 to T11 vertebra) wards as the side of the sternum; it is replaced posteriorly to be inserted into the upper border of the rib below, lat- by the posterior intercostal membrane, which extends from eral to its tubercle (see Fig. 6.82, p. 429), and presumably the angle of the rib to the superior costotransverse ligament helps to elevate it. They are supplied by the posterior rami at the posterior limit of the space (see Fig. 1.5, p. 11). of spinal nerves (from C8 to T11). The chief muscles of the outer layer are the external intercostals. The fibres of the external intercostal muscles Inner layer pass obliquely downwards and forwards from the sharp lower border of the rib above to the smooth upper border Of the three groups of muscles in this layer the innermost of the rib below. Each muscle extends from the superior intercostals line the rib cage at the side, while the subcos- costotransverse ligament at the back of the intercostal tals are at the back, and the transversus thoracis at the space as far forwards as the costochondral junction; here front. They cross more than one intercostal space. 182 Chapter |4| Thorax Intercostal nerves The mixed spinal nerve, having emerged from the inter- Vein vertebral foramen and given off its posterior ramus, passes around in the neurovascular plane, between the internal Artery External intercostal intercostal and the transversus thoracis group of muscles Nerve (see Fig. 1.5, p. 11). This intercostal nerve gives off a Internal intercostal collateral and a lateral cutaneous branch before it reaches Transversus group the costal angle. The collateral branch runs along the inferior border of its space and supplies the muscles of the space, the parietal pleura and the periosteum of the ribs. The lateral cutaneous branch pierces the intercostal muscles and the overlying muscles of the body wall along the midaxillary line, and divides into an anterior and pos- terior branch to supply skin over the space. The intercostal Figure 4.4 Vertical section through an intercostal space. The nerve ends as an anterior cutaneous nerve, which in the neurovascular structures (vein, artery, nerve from above upper six spaces passes anterior to the internal thoracic downwards) lie between the internal intercostals and the artery and pierces the intercostal muscles to reach the transversus group. The small collateral branches (not shown) skin. In its course around the space the intercostal nerve are in the same plane just above the lower rib. lies below the vein and artery, but in its course around the body wall the main nerve lies in a wider circle that Lying in the paravertebral gutter are the subcostal embraces the narrower circle of the vessels. Hence, at the muscles attached to the inner surfaces of ribs. They are back of the intercostal space where the nerve crosses separated from the posterior border of the innermost the intercostal artery, the nerve lies behind the artery; intercostals by a space across which the intercostal nerves at the front of the space the nerve crosses in front of the and vessels are in contact with the parietal pleura. internal thoracic artery. The innermost intercostal muscles (intercostales The lower five intercostal nerves and their collateral intimi) are attached to the inner surfaces of ribs on the branches slope downwards behind the costal margin into lateral part of the thoracic wall. the neurovascular plane of the abdominal wall, which Transversus thoracis arises from the posterior surface they supply (see p. 224). of the lower part of the sternum, whence digitations The anterior ramus of T1 ascends across the neck of the diverge on each side to the second to the sixth costal first rib to enter the brachial plexus. Before it does so it cartilages. This muscle was formerly called sternocostalis, gives off the small first intercostal nerve, which courses which was a more exact name. The transversus thoracis around beneath the flat inferior surface of the first rib group is the best inclusive name for all three muscles of (see Fig. 6.9B, p. 347) and supplies the intercostal muscles the inner layer because it conforms with the transversus of the first space, the adjacent pleura and rib periosteum. abdominis muscle. The lateral cutaneous branch of the second intercostal nerve crosses the axilla to the medial side of the arm as the intercostobrachial nerve (see p. 61). Intercostal spaces The subcostal nerve (twelfth thoracic), although arising in the thorax, quickly leaves by passing behind the lateral The intercostal spaces (between the ribs) are filled in by arcuate ligament of the diaphragm into the abdomen, the muscles of the three layers described above. Running below the subcostal artery and vein. in the plane between the intermediate and inner layers are the intercostal nerves and vessels (Fig. 4.4). The vein, Intercostal arteries artery and nerve lie in that order from above downwards, under cover of the downward projection of the lower Arteries enter intercostal spaces at the back and front. border of the rib. Thus a needle or trocar for pleural drain- At the back the upper two spaces are supplied by the age (see p. 213) is inserted just above the rib that forms superior intercostal artery. This is the descending branch the lower boundary of the space, in order to avoid the of the costocervical trunk, which comes off from the sec- main nerve and vessels that are at its upper boundary. ond part of the subclavian artery behind scalenus anterior. The collateral branches of nerve and vessels that run along It enters the thorax by passing across the front of the neck the upper border of a rib are small and can be ignored. of the first rib; here it has the sympathetic trunk on its A needle introduced to deliver an anaesthetic solution medial side, while the first thoracic nerve passes laterally on an intercostal nerve is inserted just below the lower across the first rib to join the brachial plexus (Fig. 6.9B). border of a rib. The small first posterior intercostal vein lies between the 183  Last’s anatomy artery and the sympathetic trunk. At this point the first highest costal fibres of the diaphragm to enter the rectus thoracic sympathetic ganglion is frequently fused with sheath behind the muscle. The latter passes along the costo- the inferior cervical ganglion to form the cervicothoracic diaphragmatic gutter and gives off two anterior intercostal (stellate) ganglion. arteries in each space till it ends by piercing the diaphragm The remaining nine intercostal spaces are supplied each in the ninth space to ramify on its abdominal surface. with a separate branch of the descending thoracic aorta; The internal thoracic artery is accompanied by two venae the nine right branches cross the front of the vertebrae comitantes that empty into the brachiocephalic vein. and anterior longitudinal ligaments, behind the hemiazy- The artery gives off a pericardiacophrenic branch that runs gos veins and thoracic duct. All 11 pairs of arteries consti- with the phrenic nerve and supplies branches to the nerve tute the posterior intercostal arteries. Each artery gives itself, pleura and fibrous and parietal pericardium. off a small collateral branch, which passes around in the Perforating branches emerge towards the surface from neurovascular plane at a lower level than the main trunk. each intercostal space. They are especially large in the sec- At the front of the intercostal space the internal thoracic ond, third and fourth spaces of the female for supply of artery in the upper six spaces and the musculophrenic the breast. Thus the internal thoracic artery supplies the artery in the seventh, eighth and ninth spaces give off anterior body wall from the clavicle to the umbilicus. two anterior intercostal arteries that pass backwards and anastomose with the posterior vessels. There are no anterior intercostal arteries in the last two spaces. Suprapleural membrane This is a rather dense fascial layer (Sibson’s fascia) attached to the inner border of the first rib and costal car- Intercostal veins tilage and the transverse process of C7 vertebra (Fig. 4.5). In each space there are one posterior and two anterior It is not attached to the neck of the first rib. It has the cer- intercostal veins, accompanying the arteries of the same vical dome of the pleura attached to its undersurface, and names. The anterior veins drain into the musculophrenic when traced medially it is found to thin out and disappear and internal thoracic veins. The posterior veins are not reg- into the mediastinal pleura. It lies in the oblique plane of ular. In the lower eight spaces they drain into the azygos the thoracic inlet, and the subclavian vessels arch upwards system: the azygos vein on the right and the hemiazygos and laterally over it. The membrane gives rigidity to the and accessory hemiazygos on the left. The first posterior thoracic inlet and prevents the neck structures being intercostal vein opens either into the vertebral vein or the ‘puffed’ up and down during respiration. Damage to the brachiocephalic vein of its own side. The second and third suprapleural membrane during surgical procedures at the posterior intercostal veins, and sometimes the fourth, root of the neck will usually result in the development form a single trunk on each side, the superior intercostal of a pneumothorax on the same side. vein. That on the right drains simply into the azygos vein (Fig. 4.12). That on the left runs forward over the arch of the aorta, lateral to the vagus nerve and medial to the Diaphragm phrenic nerve, to empty into the left brachiocephalic vein (Fig. 4.13). The diaphragm is a domed fibromuscular sheet that sepa- rates the thoracic and abdominal cavities. Its purpose is Lymph drainage essentially for inspiration. Morphologically the diaphragm is a derivative of the The lymph vessels of the intercostal spaces follow the inner (transversus) layer of the muscles of the body wall, arteries. From the front of the space vessels pass to the and its fibres arise in continuity with those of transversus anterior intercostal (parasternal) nodes that lie along abdominis from within the costal margin. It is completed the internal thoracic artery; from the back of the space behind the costal origin by fibres that arise from the arcu- they drain to posterior intercostal nodes. ate ligaments and the crura. From the circumference of this oval origin the fibres arch upwards into a pair of domes and then descend to a central tendon which lies Internal thoracic artery at the level of the xiphisternal joint. Viewed from in front From the first part of the subclavian artery, the internal the diaphragm curves up into right and left domes. The thoracic artery (formerly the internal mammary) passes right dome of the diaphragm is higher than the left, vertically downwards about 1 cm lateral to the border of ascending in full expiration as high as the nipple (fourth the sternum (Fig. 4.5). It gives off two anterior intercostal space), while the left dome reaches the fifth rib. arteries in each intercostal space. At the costal margin it Viewed from the side the profile of the diaphragm divides into the superior epigastric and musculophrenic resembles an inverted J, the long limb extending up from arteries. The former passes between the xiphisternal and the crura (upper lumbar vertebrae) and the short limb 184 Chapter |4| Thorax Anterior longitudinal ligament Carotid tubercle Sympathetic chain Foramen of transverse Neck of first rib process (C7) Transverse C8 anterior ramus process (T1) Costocervical trunk Neck of first rib T1 anterior ramus Suprapleural membrane Subclavian artery Groove for subclavian artery Scalenus anterior Scalene tubercle Lower trunk of brachial plexus Second rib Subclavian vein Internal thoracic artery Groove for subclavian vein Figure 4.5 Thoracic inlet and the suprapleural membrane. The membrane on the right and the subclavian vessels on the left are not shown. attached to the xiphisternum (level of T8 vertebra). across to the middle of the lower margin of the twelfth Viewed from above the outline is kidney shaped, in rib; it is a thickening in the anterior layer of the lumbar conformity with the oval outline of the body wall fascia on the front of quadratus lumborum. Further which is indented posteriorly by the vertebral column laterally a digitation arises from the internal surfaces of (see Fig. 1.5, p. 11). each of the lower six costal cartilages and ribs, interdigi- The crura are strong tendons attached to the anterolat- tating with the slips of origin of transversus abdominis. eral surfaces of the bodies of the upper lumbar vertebrae. In front, the diaphragm arises from the back of the The right crus is fixed to the upper three lumbar vertebrae xiphisternum. and the intervening discs; the left crus likewise to the The central tendon has a trefoil shape, having an ante- upper two lumbar vertebrae. Muscle fibres radiate from riorly situated middle leaf fused on each side to lateral each crus, overlap, and pass vertically upwards before leaves that extend back towards the paravertebral gutters. curving forwards into the central tendon. Some of the The tendon is inseparable from the fibrous pericardium, fibres of the right crus pass up on the abdominal surface both having the same embryological origin. of fibres from the left crus and surround the oesophageal orifice in a sling-like loop (Fig. 4.6; see also Fig. 5.26, Openings in the diaphragm p. 251). Tendinous fibres from the medial edge of each crus unite with one another in front of the aorta at the For the passage of structures between thorax and abdo- level of T12 vertebra to form the median arcuate ligament. men there are three large openings in the diaphragm The medial arcuate ligament is a thickening in the psoas and several smaller ones. fascia. It extends from the side of the body of L1 or L2 The aortic opening is opposite T12 vertebra, in the vertebra to a ridge on the anterior surface of the trans- midline, behind the median arcuate ligament. It transmits verse process of L1 vertebra, at the lateral margin of the aorta with the azygos vein to the right and the thoracic psoas. From here the lateral arcuate ligament extends duct leading up from the cisterna chyli between them. 185  Last’s anatomy Xiphoid process Sternal part Oesophagus (cut) surrounded by muscle fibres from right crus Middle leaf of central tendon Vena caval opening in tendon Right leaf of Left leaf of central tendon central tendon Median arcuate ligament Aorta Medial arcuate ligament Quadratus Lateral arcuate lumborum ligament Psoas major Right crus Left crus Figure 4.6 Diaphragm: abdominal aspect. The oesophageal opening is opposite T10 vertebra, lesser, and least splanchnic nerves pierce each crus. The usually 2.5 cm to the left of the midline behind the sev- sympathetic trunk passes behind the medial arcuate liga- enth left costal cartilage. It lies in the fibres of the left crus, ment. The subcostal nerve and vessels pass behind the lateral but a sling of fibres from the right crus loop around it. arcuate ligament. The left phrenic nerve pierces the muscle The transversalis fascia on the undersurface of the dia- of the left dome. The neurovascular bundles of the seventh phragm extends up through the opening, blends with to the eleventh intercostal spaces pass between the digita- the endothoracic fascia above the diaphragm, and is tions of the diaphragm and transversus abdominis into attached to the oesophagus about 2–3 cm above the oeso- the neurovascular plane of the abdominal wall. The supe- phagogastric junction. This fascial cone is the phreno- rior epigastric vessels pass between the xiphisternal and oesophageal ligament. It becomes stretched in the ‘sliding’ costal (seventh) fibres of the diaphragm. Extraperitoneal type of hiatus hernia (see p. 187). The vagal trunks and lymph vessels on the abdominal surface pass through the the oesophageal branches of the left gastric artery, veins diaphragm to lymph nodes lying on its thoracic surface, and lymphatics accompany the oesophagus as it passes mainly in the posterior mediastinum. through the diaphragm. The venous drainage from this At the posterior part of the diaphragm there may be a site passes caudally to the portal venous system and crani- gap between the lowest costal fibres and those arising ally to the azygos venous system, constituting a major site from the lateral arcuate ligament. The posterior surface of portal–systemic anastomosis. of the kidney and its perirenal fascial covering is then The vena caval foramen is opposite T8 vertebra just separated from the pleura only by areolar tissue of the to the right of the midline, behind the sixth right costal endothoracic fascia. cartilage. It lies between the middle and right leaves of the central tendon, the fibres of which fuse firmly with the Blood supply adventitial wall of the inferior vena cava (Fig. 4.12). The right phrenic nerve passes through the central tendon The costal margin of the diaphragm is supplied by the alongside the inferior vena cava at this opening. lower five intercostal and the subcostal arteries. The main Other structures make their own smaller openings. The mass of fibres rising up from the crura are supplied on hemiazygos vein passes through the left crus. The greater, their abdominal surface by right and left inferior phrenic 186 Chapter |4| Thorax arteries from the abdominal aorta (see Fig. 5.43, p. 275). Abdominal straining. For evacuation of a pelvic efflu- The pericardiacophrenic and musculophrenic branches ent (defecation, micturition, parturition) diaphragmatic of the internal thoracic artery and the superior phrenic contraction aids that of the abdominal wall in raising branches of the thoracic aorta make small contributions intra-abdominal pressure. It is much weaker than the to the blood supply of the diaphragm. powerful obliques, transversus and recti, so for maxi- mum pressure a deep breath is taken, the glottis is closed and the diaphragm is prevented from undue elevation Nerve supply by being held down by a cushion of compressed air. Forcible escape of some of this air causes the characteristic The motor nerve supply is solely from the phrenics (C3, grunt. 4, 5 but predominantly C4). Each half of the diaphragm During heavy lifting in the stooping position abdomi- is supplied by its own phrenic nerve and the fibres of nal straining is beneficial. With the breath held and intra- the right crus that loop to the left around the oesophageal coelomic pressure raised as above, the vertebral column opening are supplied by the left phrenic. The lower inter- cannot easily flex; it is as though an inflated football filled costal nerves give proprioceptive fibres to the periphery of the body from pelvic brim to thoracic inlet. The weight the diaphragm, proprioceptive supply to the central part of the stooping trunk is supported on the football, free- coming from the phrenics. On reaching the abdominal ing erector spinae to use all its power to lift the weight. surface of the diaphragm, both nerves divide into ante- Such acts are similarly accompanied on occasion by the rior, lateral and posterior branches which run radially, characteristic grunt. giving off branches that enter the muscle from below. Associated with a lifetime of constant activity, about 55% of diaphragmatic muscle fibres (and 65% of inter- Development costal muscle fibres) are of the slow twitch fatigue- resistant variety. The diaphragm develops from four sources. The septum transversum (p. 24) gives rise to most of the central ten- don. Prior to its descent from the neck the transverse Actions septum becomes invaded by muscle cells derived from the third, fourth and fifth cervical myotomes. The muscle The major role of the diaphragm is inspiratory, but it is cells carry their own nerve supply with them, hence the used also in abdominal straining. motor supply from the phrenic nerves. Mesodermal folds, Inspiration. When the fibres contract in tranquil inspira- the pleuroperitoneal membranes, which close the connec- tion only the domes descend; this sucks down the lung tion between the thoracic and abdominal parts of the bases and does not disturb the mediastinum. In a deeper coelom, and the oesophageal mesentery also contribute to breath further descent of the domes, below the level of the development of the diaphragm. Failure of pleuro- the central tendon, can depress the central tendon from peritoneal membrane development is the most common T8 to T9 level. This stretches the mediastinum (traction cause of congenital diaphragmatic hernia. The defect on pericardium and great vessels) and no further descent (Bochdalek’s foramen) is posteriorly placed and clinically of the tendon is possible. Further contraction of the manifests more often on the left side, probably due to the muscle (maximum inspiration) now everts the ribs of presence of the liver on the right side. Another possible the costal margin in a ‘bucket handle’-like movement with but smaller hernial site is at the junction of the costal widening of the subcostal angle. and xiphoid origins—Morgagni’s foramen. As the diaphragm contracts intra-abdominal pressure tends to rise, and the vena caval foramen (in the central tendon) is pulled widely open to assist venous return via Diaphragmatic hernia the inferior vena cava. The oesophageal opening is held closed by the pinch-like action resulting from contrac- The congenital types of diaphragmatic hernia have just tion of the muscle sling of the right crus, to discourage been mentioned. Of the acquired varieties, the most regurgitation of stomach contents. The aortic opening is common is the sliding type of hiatus hernia, through the unaffected. oesophageal opening. Here the oesophagogastric junc- Hiccup is a (repeated) spasmodic contraction of the tion rises up into the thorax; this encourages gastro- diaphragm, its contraction being followed immediately oesophageal reflux. In the much rarer paraoesophageal by closure of the glottis and subsequent release of the (rolling) type, the oesophagogastric junction remains in trapped air to produce the characteristic sound. the abdomen, but a pouch of peritoneum containing a Expiration. Whether expiration is tranquil or forced part of the stomach projects upwards alongside the (coughing, sneezing, blowing, etc.) the diaphragm is oesophagus; this compresses the lower oesophagus and wholly passive, its relaxed fibres being elongated by the blood supply of the herniated stomach may be pressure from below. compromised. 187  Last’s anatomy Thoracic movements and respiration The diaphragm is the main muscle of inspiration. The external intercostals are most active in inspiration and the internal intercostals in expiration, but the intercostals are more important for stiffening of the chest wall and preventing paradoxical movement of the interspaces. In expiration, elastic recoil of the lungs assisted by con- traction of the muscles of the abdominal wall makes the relaxed diaphragm regain its domed form. During maximal inspiratory efforts, other muscles too are active. The scalene muscles and sternocleidomastoid elevate the first rib and manubrium. The twelfth rib is fixed by quadratus lumborum and may even descend. The erector spinae extend the spine, and muscles con- necting the trunk to the upper limbs, such as pectoralis major, contribute to chest expansion when the arms are fixed. In forced expiratory efforts, latissimus dorsi con- tracts, compressing the lower ribs. All three diameters of the thorax—anteroposterior, transverse and vertical—are increased during inspira- tion. The anteroposterior diameter increases because the sternum moves forwards as the ribs are raised since their anterior ends are at a lower level than their heads (see Figure 4.7 Radiograph of the thorax in extreme expiration, in Fig. 4.33, p. 218). This sternal movement is facilitated a healthy male aged 21 years. by the hinge movement (up to about 7! ) at the manu- briosternal joint. If this joint becomes ankylosed, thoracic expansion due to sternocostal movement is virtually lost (as in emphysema) and only diaphragmatic respira- tion is possible. As each rib forms the arc of a circle, which is larger than that formed by the rib above, elevation of the ribs during inspiration increases the transverse diameter of the thorax. Change in the vertical extent of the thoracic cavity is due to diaphragmatic movement (Figs 4.7 and 4.8). Movements of the abdominal wall Since the volume of the abdominal cavity remains constant, the abdominal wall moves in accordance with changes in the thoracic cavity. Diaphragmatic inspira- tion and rib inspiration occur simultaneously, but each in itself produces opposite movements in the abdominal wall. In purely diaphragmatic breathing, with the ribs motionless, descent of the diaphragm is accompanied by passive protrusion of the relaxed abdominal wall. Ascent of the diaphragm is accompanied by retraction of the abdominal wall; indeed, it is the active con- traction of the abdominal wall muscles that forces the relaxed diaphragm up. This to-and-fro movement of the abdominal wall is usually called ‘abdominal Figure 4.8 Radiograph of the same thorax as in Figure 4.7 respiration’. taken in full inspiration. The anterior ends of the ribs are In ‘thoracic respiration’ the movements of the abdomi- elevated and the lower ribs everted. The descent of the domes nal wall are purely passive. When the ribs are elevated in of the diaphragm is greater than that of the central tendon. 188 Chapter |4| Thorax inspiration the diaphragm is elevated with the up-going Divisions of the mediastinum costal margin and the abdominal wall is sucked in. With descent of the costal margin in expiration the abdominal There is a plane of division to which the whole topogra- wall moves forwards again. phy of the mediastinum can be related, namely a plane The ordinary simultaneous rib and diaphragm move- passing horizontally through the sternal angle (of Louis), ments can be so balanced that the abdominal wall does i.e. the manubriosternal joint (Fig. 4.9). From the second not move at all. Thus respiration may function quite well costal cartilages, this plane passes backwards to the lower in tight corsets, plaster casts, etc. In children and many border of T4 vertebra. Above, between it and the thoracic women thoracic movement is greater than diaphragmatic inlet, lies the superior mediastinum. Below the plane, the movement. In men diaphragmatic movement is greater, inferior mediastinum is divided into three compartments especially as the years go by. by the fibrous pericardium: a part in front, the anterior mediastinum; a part behind, the posterior mediastinum; and the middle mediastinum in between containing the pericardium and heart together with the adjoining parts Thoracotomy of the great vessels and the lung roots. The anterior and Surgical access to thoracic structures involves some kind posterior mediastina are in direct continuity with the of thoracotomy. In the anterolateral or posterolateral type, superior mediastinum; their separation from it is purely skin and underlying muscles are incised along the line descriptive, not anatomical. The plane passes through of the chosen rib (often fifth or sixth) and the periosteum the bifurcation of the trachea, the concavity of the arch stripped off the rib (keeping away from the intercostal of the aorta, and just above the bifurcation of the pulmo- nerve and vessels which run immediately below a rib). nary trunk. On the plane the azygos vein enters the supe- With or without resection of the rib, the periosteal bed rior vena cava, and the thoracic duct reaches the left of the rib is then incised to enter the pleural cavity. The side of the oesophagus in its passage upwards from the nerve to serratus anterior may have to be sacrificed when abdomen. Also lying in the plane are the ligamentum incising that muscle in order to obtain the necessary arteriosum, with the left recurrent laryngeal nerve recur- exposure. ving below it, tracheobronchial lymph nodes, and the In anterior thoracotomy, or median sternotomy, the whole superficial and deep parts of the cardiac plexus. length of the sternum is split vertically in the midline, the The prevertebral and pretracheal fasciae extend from the sternal origin of the diaphragm detached, and the tissues neck into the superior mediastinum. The former fuses with behind the sternum freed by blunt dissection. Damage to the pleural sacs is avoided, remembering that the right pleura may extend a little to the left of the midline (see p. 213). In the combined thoracoabdominal incision, the line of approach is through the eighth or ninth rib bed or inter- Investing layer costal space, dividing the cartilage at the costal margin and incising the anterior abdominal wall. The dia- Pretracheal fascia C7 phragm is incised circumferentially near its periphery or Prevertebral fascia radially towards the point of entry of the phrenic nerve, thus minimizing damage to the branches of the nerve Pretracheal space T4 (see p. 187). Anterior mediastinum Posterior mediastinum Part three: Thoracic cavity The cavity of the thorax is completely filled laterally by the lungs, each lying in its pleural cavity. The space between the pleural cavities occupying the centre of Figure 4.9 Divisions of the mediastinum, showing the the thoracic cavity is the mediastinum. It contains the continuity with the tissue spaces of the neck. The superior mediastinum is above the interrupted line passing from the heart and great blood vessels, oesophagus, trachea and sternal angle to the lower border of T4 vertebra. The anterior its bifurcation, thymus, thoracic duct, lymph nodes, mediastinum is continuous through the superior mediastinum phrenic and vagus nerves. The loose connective tissue with the pretracheal space of the neck, up to the hyoid bone. between these structures connects freely with that of The posterior mediastinum is continuous with the the neck. Mediastinitis may complicate infections in retropharyngeal and paratracheal space of the neck, up to the the neck. base of the skull. The pharynx and oesophagus are not depicted. 189  Last’s anatomy the anterior longitudinal ligament over T4 vertebra; the lat- The concavity of the arch of the aorta lies in the plane ter blends with the pericardium over the front upper part of the sternal angle, and the arch of the aorta lies wholly of the heart. Thus, neck infection in front of the pretracheal in the superior mediastinum, behind the manubrium. fascia is directed into the anterior mediastinum, while infec- It arches over the beginning of the left bronchus and tion behind the pre-vertebral fascia is imprisoned in the the bifurcation of the pulmonary trunk. The brachioce- superior mediastinum in front of the vertebral bodies phalic trunk begins as a midline branch from the arch (Fig. 4.9). From elsewhere in the neck infection may extend and diverges to the right as it ascends in front of the through the superior into the posterior mediastinum. trachea (Fig. 4.10). The two other branches of the arch, the left common carotid and left subclavian arteries, pass upwards on the left side of the trachea (Fig. 4.11). These Part four: Superior mediastinum General topography The superior mediastinum is wedge shaped (Fig. 4.9). The anterior boundary is the manubrium. The posterior boundary is much longer, due to the obliquity of the tho- racic inlet. It consists of the bodies of the first four thoracic vertebrae; this wall is concave towards the mediastinum. At the thoracic inlet (Fig. 4.10; and Fig. 6.9A, p. 347), often called clinically the thoracic outlet, the oesophagus lies against the body of T1 vertebra. The trachea lies on the oesophagus and may touch the jugular notch of the manubrium. The midline of the inlet is thus wholly occupied by these two tubes. At the inlet the apices of Figure 4.11 CT scan above the level of the aortic arch. Viewed the lungs lie laterally, separated by the trachea and from below, the oesophagus with a rather small round lumen oesophagus and by vessels and nerves passing between is seen in front of the vertebral column, and the round the neck and the superior mediastinum. Below the inlet, translucency in front of the oesophagus is the trachea. The the trachea slopes back and the manubrium slopes for- four opacities adjacent to the trachea are, from the right side ward; the brachiocephalic trunk, the left brachiocephalic of the body to the left, the superior vena cava, brachiocephalic vein and the thymus occupy the space thus provided. trunk, left common carotid artery and left subclavian artery. Oesophagus and trachea Vertebral artery and vein Sympathetic Scalenus anterior ganglion and phrenic nerve Thyrocervical Right subclavian trunk artery Right common Thoracic duct carotid artery and vagus nerve Left subclavian Internal thoracic artery artery Left internal jugular vein Brachiocephalic Left subclavian trunk vein Internal thoracic vein Left common carotid artery Superior vena cava Thymic vein Arch of Ligamentum arteriosum aorta and left recurrent laryngeal nerve Figure 4.10 Superior mediastinum and thoracic inlet, after removal of the sternum, costal cartilages and clavicles. The left brachiocephalic vein crosses in front of the three great arteries to join its fellow to form the superior vena cava. 190 Chapter |4| Thorax First rib Oesophagus Sympathetic trunk Vagus nerve Superior Trachea intercostal vein Right brachiocephalic Upper lobe vein brochus Left brachiocephalic vein Azygos vein Arch of aorta Right main Superior vena cava bronchus Upper lobe artery Rami communicantes Right pulmonary artery Phrenic nerve Pulmonary veins Pericardiophrenic artery Intercostal vein, artery and nerve Pulmonary ligament Oesophagus and vagal plexus Fibrous pericardium Greater splanchnic nerve Diaphragm Inferior vena cava Figure 4.12 Mediastinum: right aspect. great arteries keep the left vagus nerve and apex of the left pulmonary hilum (Fig. 4.12). The right phrenic nerve lung away from contact with the trachea (Fig. 4.13). On descends in contact with the lateral aspect of the right the right side there is no structure to separate the trachea brachiocephalic vein and superior vena cava. from the right vagus (Fig. 4.12) and apex of the right lung. The veins entering the superior mediastinum are the right and left brachiocephalic veins, each formed by the Great vessels confluence of the internal jugular with the subclavian vein. They lie in front of the arteries and are asymmetrical. Arch of the aorta The right brachiocephalic vein passes vertically down- wards; the left vein runs across the superior mediastinum, Emerging from the pericardium the ascending aorta above the arch of the aorta, to join the right (Fig. 4.10). approaches the manubrium and then at the level of the The confluence of the brachiocephalic veins produces manubriosternal joint becomes the arch, which passes the superior vena cava, which passes vertically downwards backwards over the left bronchus to reach the body of T4 behind the right edge of the sternum, anterior to the right vertebra just to the left of the midline. From its upper 191  Last’s anatomy convexity, which reaches as high as the midpoint of the ligamentum arteriosum to pass upwards on the right side manubrium, arise the three great arteries for the head of the arch of the aorta, in the groove between the trachea and upper limbs: the brachiocephalic trunk, and the left and oesophagus. The pulmonary trunk bifurcates into common carotid and left subclavian arteries (Figs 4.10 right and left pulmonary arteries in the concavity of the and 4.13). The arch is crossed on its left side by the arch. On the right side of the arch lie the trachea and phrenic and vagus nerves as they pass downwards in front oesophagus. of and behind the lung root respectively. Between them The adventitial layer of the arch contains baroreceptors lie the sympathetic and vagus branches to the superficial (like the carotid sinus in the wall of the internal carotid part of the cardiac plexus. The left superior intercostal vein artery, see p. 343) innervated by vagal nerve fibres, which passes forwards across the arch superficial to the vagus, are concerned with the reflex control of the heart rate. deep to the phrenic, to empty into the left brachiocephalic Under the arch in the region of the ligamentum arterio- vein. The left recurrent laryngeal nerve hooks around the sum there are some very small masses of tissue, the aortic First rib Left common carotid artery Oesophagus Left subclavian artery Trachea Thoracic duct Left brachiocephalic Recurrent laryngeal vein nerve Recurrent laryngeal nerve Communication between left Arch of aorta superior intercostal and accessory hemiazygos vein Pulmonary artery Left superior intercostal vein Vagus nerve Left main bronchus and bronchial artery Pulmonary veins Thoracic aorta Pulmonary ligament Sympathetic ganglion Phrenic nerve Rami communicantes Pericardiophrenic Greater splanchnic artery nerve Fibrous pericardium Oesophagus Diaphragm Figure 4.13 Mediastinum: left aspect. 192 Chapter |4| Thorax bodies (also supplied by vagal fibres), which like the the lower part of the muscle. Medial to scalenus anterior, carotid bodies (see p. 343) are chemoreceptors concerned these veins have joined to form the brachiocephalic vein, with respiratory reflexes. which lies in front of the first part of the subclavian artery. The brachiocephalic trunk (innominate artery) arises This part of each brachiocephalic vein thus receives tribu- in or a little to the left of the midline of the body. It slopes taries corresponding to the branches of the first part of upwards across the trachea to the back of the right sterno- the subclavian artery (vertebral, inferior thyroid, internal clavicular joint, where it divides into the right common thoracic and, on the left side only, superior intercostal). carotid and right subclavian arteries. It has no branches The right brachiocephalic vein commences behind apart from the rare thyroidea ima artery, which may arise the right sternoclavicular joint and runs downwards. At from it or directly from the arch of the aorta. The termina- its commencement it receives the right jugular, subclavian tion of the left brachiocephalic vein lies in front of the and bronchomediastinal lymph trunks separately or their artery (Fig. 4.10). confluent channel, the right lymphatic duct. The left common carotid artery arises just behind the The left brachiocephalic vein passes to the right with a brachiocephalic trunk from the upper convexity of the downward inclination, across the superior mediastinum, aortic arch. It passes straight up alongside the trachea above the arch of the aorta, behind the thymus and into the neck (Fig. 4.13). It has no branches in the the upper half of the manubrium. In the infant the left mediastinum. brachiocephalic vein projects slightly above the jugular The left subclavian artery arises just behind the left notch, and may do so in the adult if the vein is distended, common carotid; the two run upwards together. The sub- especially if the head and neck are thrown back. The vein clavian artery arches to the left over the pleura and the is then vulnerable to suprasternal incisions (e.g. for apex of the lung, which it deeply grooves. It moves away tracheotomy). The commencement of the vein receives from the left common carotid at a point directly behind the thoracic duct, which often divides into two or three the left sternoclavicular joint. It has no branches in the branches that join the vein separately. In addition to the mediastinum. vertebral and internal thoracic veins the left brachioce- phalic vein receives most of the inferior thyroid veins, the left superior intercostal vein, and a large thymic vein Ligamentum arteriosum (Figs 4.10 and 4.13). This is the fibrous remnant of the ductus arteriosus of the The pretracheal fascia (see p. 331) passes down behind fetus, a channel that short-circuited the lungs. It passes the vein and directs a retrosternal goitre into the space from the commencement of the left pulmonary artery to between the vein and the brachiocephalic trunk and the concavity of the aortic arch (Fig. 4.16), beyond the trachea. point where the left subclavian artery branches off. It lies almost horizontally. The left recurrent laryngeal nerve hooks around it. The superficial part of the cardiac plexus Superior vena cava lies anterior to it, and the deep part is on its right, This vessel commences at the lower border of the first between the aortic arch and tracheal bifurcation. right costal cartilage by confluence of the two brachio- Surgical approach. When the ductus persists after birth cephalic veins (Figs 4.10 and 4.12). It passes vertically (patent ductus arteriosus) and requires surgical inter- downwards behind the right border of the sternum and, ruption the pleura over the aortic arch is incised behind piercing the pericardium at the level of the second costal the vagus nerve and upwards towards the origin of the left cartilage, enters the upper border of the right atrium at subclavian artery. The pleural flap is reflected forwards the lower border of the third right costal cartilage. Behind with the vagus and its left recurrent laryngeal branch the sternal angle it receives the azygos vein, which has to give sufficient access to the ductus. Video-assisted arched forwards over the root of the right lung. There are thoracoscopy is utilised for patent ductus arteriosus clo- no valves in the superior vena cava, the brachiocephalic sure. The lumen of the ductus may also be obliterated veins or the azygos system of veins. by an occlusive device inserted by interventional radiol- ogy techniques. Cardiac plexus Brachiocephalic veins The cardiac plexus consists of sympathetic, parasympa- The brachiocephalic (innominate) veins are formed thetic and afferent fibres and small ganglia. It is divided behind the sternoclavicular joints by confluence of into superficial and deep parts, but functionally they are the internal jugular and subclavian veins. In the neck the one. Their branches enter the pericardium to accompany internal jugular vein lies lateral to the common carotid the coronary arteries (vasomotor) and to reach the myocar- artery, in front of the upper part of scalenus anterior. dium, in particular the SA and AV nodes (cardioinhibitor The subclavian vein lies lateral to and then in front of and cardioaccelerator). 193  Last’s anatomy The superficial part of the cardiac plexus lies in front of only 3 mm and in childhood it is about equal in milli- the ligamentum arteriosum. The deep part of the cardiac metres to the age in years. In full inspiration the trachea plexus is larger and lies to the right of the ligamentum may stretch to 15 cm and the bifurcation descend to the arteriosum, in front of the bifurcation of the trachea and level of T6 vertebra. behind the aortic arch. The cervical part of the trachea is described on page 341. The cardiac plexus receives sympathetic fibres from the The thoracic part runs through the superior media- three cervical and the upper four or five thoracic sympa- stinum in front of the oesophagus. In front of this part thetic ganglia of both sides, and parasympathetic fibres are the manubrium with sternohyoid and sternothyroid from both vagi in their cervical course and both recurrent muscles attached, remnants of the thymus, the inferior laryngeal nerves. The sympathetic fibres accelerate the thyroid and left brachiocephalic veins, and the brachioce- heart and dilate the coronary arteries; the parasympathetic phalic and left common carotid arteries as they diverge fibres slow the heart and constrict the coronary arteries. to either side (Figs 4.10 and 4.15). The right vagus is in The vagi carry afferent fibres concerned with cardiovas- contact with the right side of the trachea, which is sepa- cular reflexes. Pain fibres run with sympathetic nerves, rated from the right lung by the pleura and the arch reaching any of the cervical and upper thoracic sympa- of the azygos vein as it hooks forwards over the right thetic ganglia. The pain fibres pursue the usual pathway bronchus (Fig. 4.12). The right brachiocephalic vein and to the central nervous system, passing through the sympa- superior vena cava are anterolateral to the trachea. On thetic ganglia to the spinal nerves via white rami commu- the left, the left common carotid and subclavian arteries nicantes (see Fig. 1.13C, p. 19). The connection with (Fig. 4.13) prevent the pleura and the left vagus nerve cervical and thoracic spinal nerves presumably explains from coming into contact with the trachea; the arch of the referral of cardiac pain to the arm, chest or neck. the aorta curves backwards over the left bronchus, and the left recurrent laryngeal nerve passes upwards in the groove between trachea and oesophagus. The pulmonary trunk branches into the right and left Trachea pulmonary arteries to the left of the tracheal bifurcation, in front of the left bronchus, and the right pulmonary The trachea is the continuation of the larynx and com- artery crosses the midline (in front of the oesophagus) mences in the neck below the cricoid cartilage at the level just below the tracheal bifurcation (Fig. 4.29). of C6 vertebra, 5 cm above the jugular notch. Entering the thoracic inlet in the midline it passes downwards and backwards behind the manubrium to bifurcate into the Blood supply two principal or main bronchi a little to the right of the Branches from the inferior thyroid and bronchial arteries midline, level with the upper border of T5 vertebra form anastomotic networks in the tracheal wall. Veins (Fig. 4.14). The trachea is about 10 cm long and 2 cm in drain to the inferior thyroid vein. diameter. In the first year of life the tracheal diameter is Lymph drainage Lymphatic channels pass to pre- and paratracheal nodes and to inferior deep cervical nodes. Nerve supply The mucous membrane is supplied by afferent (including pain) fibres from the vagi and recurrent laryngeal nerves. Sympathetic fibres from upper ganglia of the sympathetic trunks supply the smooth muscle and blood vessels. Structure The patency of the trachea as an airway, its essential func- tion, is maintained by 15–20 horseshoe-shaped hyaline Figure 4.14 CT scan at the T5 vertebra level, just below the cartilages. The gaps in the rings are at the back, where there tracheal bifurcation, viewed from below: 1, superior vena cava; is smooth muscle, mostly transverse (the trachealis muscle). 2, ascending aorta; 3, left pulmonary artery; 4, left main There is a high content of elastic fibres in the submucosa bronchus; 5, thoracic aorta; 6, oesophagus; 7, azygos vein; to facilitate the necessary elastic recoil during respiration. 8, right main bronchus. The mucous membrane is of typical respiratory type, with 194 Chapter |4| Thorax Right internal Thyroid gland jugular vein Phrenic nerve Scalenus anterior Trachea and Right subclavian inferior thyroid veins artery Left common carotid artery Right subclavian vein First rib Right brachiocephalic vein Left brachiocephalic vein Superior vena cava Thymus Fibrous pericardium Upper Upper lobes of Lobes of right lung Middle left lung Lower Lower Diaphragm Figure 4.15 Thoracic contents, seen after removal of the anterior thoracic wall. The thymus lies in front of the upper pericardium and great vessels. pseudostratified columnar ciliated epithelium and goblet anterior to the vagus and runs through the mediastinum cells, mucous glands and scattered lymphoid nodules. in front of the lung root. Each nerve is in contact laterally with the mediastinal pleura throughout the whole of its course. Function The right phrenic nerve is related medially with venous The cartilaginous rings keep the airway open, mucus traps structures throughout its thoracic course (Fig. 4.12). The particles, cilia beat upwards to clear debris, and glandular right brachiocephalic vein, the superior vena cava, the secretion helps to humidify the passing air. During pericardium over the right atrium, and the inferior vena swallowing the trachea is stretched as the larynx moves cava, lie to its medial side. It reaches the undersurface of upwards (the bifurcation does not move) and elasticity the diaphragm by passing through the central tendon restores the normal position. The trachealis muscle con- alongside the inferior vena cava, piercing the tendon trols the diameter of the tube. During coughing there is fibres that fuse with the caval wall. a 30% increase in transverse diameter produced by com- The left phrenic nerve is related medially to arterial pressed air in the trachea while the vocal cords are shut, structures throughout its thoracic course (Fig. 4.13). but the trachea narrows to 10% less than the resting diam- It has the left common carotid and left subclavian arteries eter at the instant the cords open. Like the choke barrel of that arise from the arch of the aorta to its medial side. a shotgun this greatly increases the explosive force of the It crosses the arch lateral to the superior intercostal vein blast of compressed air. and in front of the vagus nerve, and then runs laterally down the pericardium over the left ventricle towards the apex of the heart. It reaches the undersurface of the dia- phragm by piercing the muscular part just to the left of Phrenic and vagus nerves the pericardium. About two-thirds of the phrenic nerve fibres are motor to the diaphragm. The rest are sensory to the diaphragm Phrenic nerve (except for the most peripheral parts which receive inter- Arising principally from C4 in the neck, the nerve passes costal afferent fibres), and to the mediastinal pleura, the down over the anterior scalene muscle across the dome fibrous pericardium, the parietal layer of serous pericar- of the pleura behind the subclavian vein. It crosses dium, and the central parts of the diaphragmatic pleura 195  Last’s anatomy and peritoneum. Pain referred from the diaphragmatic and sternothyroid muscles, the manubrium and upper peritoneum is classically felt in the shoulder tip (C4), part of the body of the sternum and their adjacent costal but pain from thoracic surfaces supplied by the phrenic cartilages. Behind it are the pericardium, the arch of the nerve (pleura, pericardium) is usually only vaguely aorta with its three large branches, the left brachiocephalic located there. vein and the trachea. Vagus nerve Blood supply In their descent through the thorax, the right vagus Small branches enter the thymus from the inferior thyroid (Fig. 4.12) is in contact with the trachea, while the left and internal thoracic arteries, and there are corresponding vagus (Fig. 4.13) is held away from that structure by great veins. Frequently a relatively large short thymic vein enters arteries that spring from the arch of the aorta. The left the left brachiocephalic vein (Fig. 4.10) and needs to nerve crosses the arch medial to the left superior intercos- be secured before the thymus is retracted after median tal vein, and the right nerve lies on the trachea medial to sternotomy. the arch of the azygos vein. Each vagus passes down behind the lung root, dividing into branches which con- tribute to the pulmonary plexuses and pass onwards to Lymph drainage form a plexus around the oesophagus. On the arch of the aorta the left vagus nerve flattens out and gives off Efferent channels drain into parasternal, tracheobronchial its recurrent laryngeal branch (Figs 4.10 and 4.13). This and brachiocephalic nodes. The thymus does not receive nerve hooks around the ligamentum arteriosum, and, any afferent lymphatics. passing up on the right side of the aortic arch, ascends in the groove between trachea and oesophagus. The right recurrent laryngeal nerve is given off at the root of the Development neck and hooks around the right subclavian artery The epithelium of the thymus develops mainly from the (Fig. 4.10). Both recurrent laryngeal nerves give branches endoderm of the third branchial pouch. Some of the epi- to the deep part of the cardiac plexus. Both recurrent thelial cells become the thymic (Hassall’s) corpuscles; laryngeal nerves supply the whole trachea and the adja- others form a network of epithelial reticular cells believed cent oesophagus (i.e. above the lung roots), and proceed to be the source of thymic hormones concerned with the to supply the larynx (see p. 396). differentiation of T lymphocytes. Connective tissue ele- The oesophagus lies against the vertebrae at the back of ments are derived from surrounding mesoderm, but the the superior mediastinum. The thoracic duct lies to its left. original colonizing lymphocytes have migrated from the Both structures pass through the posterior mediastinum; bone marrow. The developing thymus descends from they are described on pages 208 and 210. the neck into the mediastinum in front of all the major contents. It doubles its weight rapidly after birth and then maintains that level although the lymphoid content decreases with age, being replaced by fat and fibrous tis- Part five: Anterior mediastinum sue. However, the secretion of thymic hormones and its influence on lymphocytes that migrate to it continue This space, little more than a potential one, lies between throughout life. the pericardium and sternum. It is overlapped by the anterior edges of both lungs. It contains the thymus (or its remnants), sternopericardial ligaments, a few lymph Surgical approach nodes and branches of the internal thoracic vessels. Median sternotomy provides surgical access to the thymus. Thymus Part six: Middle mediastinum and heart The thymus may appear to be a single organ, but in fact it consists of right and left lobes closely applied to each other for much of their extent (Fig. 4.15). It is usually The middle mediastinum (a term not frequently used) most prominent in children, where it may extend from contains the pericardium and heart, the adjoining parts the level of the fourth costal cartilages to the lower poles of the great vessels, the lung roots, the phrenic nerves, of the thyroid gland. In front of it lie the sternohyoid and the deep part of the cardiac plexus. 196 Chapter |4| Thorax Veins: Pericardium Right common left internal jugular carotid artery left subclavian Right subclavian Fibrous pericardium artery left brachiocephalic Brachiocephalic Left subclavian Unlike the pleura and peritoneum, the pericardium has an trunk artery outer single-layered fibrous sac that encloses the heart and Right the roots of the great vessels, fusing with the adventitia of brachiocephalic Left common vein carotid artery these vessels. Its broad base overlies the central tendon of the diaphragm, with which it is inseparably blended, both Superior Arch of aorta vena cava Ligamentum being derived from the septum transversum (see p. 187). arteriosum The phrenic nerves lie on the surface of the fibrous peri- Superior Pulmonary trunk cardium and the mediastinal pleura is adherent to it, vena cava wherever the two membranes are in contact with each Left pulmonary other. The fibrous pericardium is connected to the back Transverse sinus veins of pericardium of the sternum by weak sternopericardial ligaments. It is supplied with blood by the internal thoracic arteries. Right Oblique sinus of pulmonary pericardium veins Serous pericardium A serous layer lines the inside of the fibrous pericardium, Inferior vena cava whence it is reflected around the roots of the great vessels to cover the entire surface of the heart, where it forms the epicardium. Between these parietal and visceral layers there are two sinuses: the transverse sinus and the oblique sinus of the pericardium. The transverse sinus is a passage above the heart, between the ascending aorta and pulmo- Fig 4.16 Posterior wall of serous pericardial sac: viewed from nary trunk in front and the superior vena cava, left atrium in front after removal of the heart. and pulmo