Thieme Dissector - Lungs

Unit 7 - Lungs, Trachea and Posterior MediastinumGoals

Remove the lungs.Describe the trachea, bronchi and lungs: lobes, features, segments.Describe the root of the lung: contents, arrangement of structures.Identify structures passing from the superior mediastinum to the abdominal cavity.Reveal the structures of the posterior thoracic wall.

Dissection and Identification1. Return to the root of the lung (Fig. 26.1).

Identify the phrenic nerve and pericardiophrenic artery passing over its anterior surface, between the mediastinal pleura and the fibrouspericardium.

Identify the vagus nerve passing posterior to it.

Review: Neurovasculature in the thoracic cavity

Fig. 26.1 Adapted from Gilroy et al. Atlas of Anatomy, second edition, Fig. 7.1 B.

The trachea is attached to the cricoid cartilage above at the level of the CV6 and ends at the level of TV5 by dividing into the left and rightprimary bronchi. Its upper portion receives its blood supply from the inferior thyroid arteries and its lower portion from bronchial arteries. Behind itis the esophagus. Transect the trachea below the thyroid gland and note that its cartilages are C-shaped and that the posterior portion which isrelated to the esophagus has no cartilage. Cut through the left primary bronchus and right pulmonary artery. Detach the pericardium from thepulmonary veins. Free the lungs from the parietal pleura and remove them.

Study the right primary bronchus, which is larger in diameter, shorter and more vertical than the left primary bronchus (Plates 196, 197;

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1.34A, 1.35 1.37A). Aspirated food is most apt to enter the posterior basal segmental bronchus of the lower right lobe because it is most in line withthe trachea. Note the left primary bronchus is therefore smaller, longer and less vertical than the right primary bronchus.

2. Using a scalpel, cut through the root of the lung, near the midpoint. Take care not to damage the phrenic nerve or structures lying posterior to it.Remove both lungs. It may be necessary to use a scalpel to incise adhesions between the parietal and visceral layers of pleura that extend inferiorlyas the pulmonary ligament.

Show me how: Remove the lungs

Viewing the anterior surface of the right and left removed lungs (Figs. 26.2 and 26.3), identify:

the apexthe basethe surfaces: diaphragmatic, costal, mediastinalthe borders: anterior, posterior, inferior

Review: Surface anatomy of the lungs

Study the external surface of the lungs. Find on the right lung the three lobes separated by oblique and horizontal fissures. The obliquefissure begins posteriorly above the level of the sixth rib, and descends as it comes forward to end on the diaphragmatic surface below the sixthrib. The horizontal fissure begins laterally at the oblique fissure at the mid-axillary line deep to the sixth rib, and passes forward to end at themediastinal surface behind the costal cartilage of the fourth rib. The lobes of the lung may be fused where the fissures should be found. When thehorizontal fissure is not fused, the pulmonary artery can be found at its depths. Find on the left lung the two lobes and an oblique fissure. Theupper lobe of the left lung reaches the diaphragm, that portion being called the lingula. Both lungs have costal, mediastinal and diaphragmaticsurfaces. The mediastinal surface usually has indentations corresponding to the organs or structures related to that surface

Review: Anatomy of the right lung

Review: Anatomy of the left lung

Fig. 26.2 Adapted from Gilroy et al. Atlas of Anatomy, second edition, Fig. 9.7 A, 9.7 B.

Compare the relationships at the hilum of the left and right lungs. For the right lung, the pulmonary artery enters anterior andslightly inferior to the primary bronchus. The pulmonary veins are inferior to the artery. On the left, the pulmonary artery enters the lung abovethe primary bronchus, since it must cross over the bronchus. The veins are again inferior to the artery and primary bronchus. There are severallymph nodes in the hilar region. These tend to be filled with black carbon particles. Identify the pulmonary ligament, which consists in thepleural reflection encircling the hilum of the lung that hangs inferiorly from the root







Review: Hilum of the left lung

Review: Hilum of the right lung

the impressions on the medial surface, formed by structures adjacent to the lung.On the right lung identify the following impressions: cardiac, esophageal, azygos.

Review: Impressions on the right lung

On the left lung identify the following impressions: cardiac and aortic.

Review: Impressions on the left lung







Fig. 26.3 Adapted from Gilroy et al. Atlas of Anatomy, second edition, Fig. 9.7 C, 9.7 D.

3. Either use a prosected bronchial tree (if available) OR use scissors and a probe to rake away lung tissue to demonstrate the bronchial tree in situ.It is generally necessary to remove the blood vessels to properly clear the field to allow identification of the various secondary and tertiary bronchifound in the right and left lungs. Refer to Table 1 and Fig. 26.4 to help you identify the tertiary bronchi in both lungs.

Show me how: Dissect a bronchopulmonary segment




Fig. 26.4 Adapted from Gilroy et al. Atlas of Anatomy, second edition, Fig. 9.14 B.

After completing this dissection module, you should be able to identify the following structures. For muscles you should know attachments,innervations, and actions; for neurovasculature, the course and function; and for organs, the structure, relationships, and function.

Phrenic nervePericardiophrenic arteryVagus nerveLung

ApexBaseDiaphragmatic surfaceCostal surfaceMediastinal surfaceAnterior border



Posterior borderInferior border

Right lungSuperior lobeMiddle lobeInferior lobeHorizontal fissureOblique fissureAzygos vein impression

Left lungSuperior lobeInferior lobeOblique fissureLingulaCardiac notchCardiac impressionAortic impressionImpression for the aortic arch

HilumBronchiPulmonary arteriesPulmonary veinsPulmonary ligament

Posterior MediastinumDissection and Identification

1. Use scissors and forceps (or a scalpel, but carefully!) to incise the posterior wall of the pericardial sac that remains in the middlemediastinum. Remove the remnants of the pericardial sac. Clean the vagus nerves from the neck to the esophagus, noting the difference inthe location of branching of the left and right recurrent laryngeal nerves. When the vagus nerves reach the esophagus they formthe esophageal plexus. Clean this plexus without destroying the venous drainage of the esophagus or the thoracic duct. Note thecontributions from the sympathetic trunk to the esophageal plexus.

Show me how: Window the posterior wall of the pericardial sac

Note that the esophagus is located immediately behind the pericardial sac. Gently probe its anterior surface to identify the esophagealplexus.

Verify that the right and left vagus nerves contribute fibers to this plexus.




Fig. 31.1 Adapted from Gilroy et al. Atlas of Anatomy, second edition, Fig. 8.9.

2. Remove the remaining parts of the pericardial sac using forceps and scissors (or a scalpel) and blunt dissection. Clean the structures of theposterior mediastinum that are now exposed.

Show me how: Remove the pericardial sac

Re-identify the left and right vagus nerves and follow them inferiorly toward the diaphragm where they pass through into the abdomenas the anterior and posterior vagal trunks.

Move the esophagus to the left to reveal the azygous vein and the thoracic duct. Note that the thoracic (descending) aorta is on the leftside of the esophagus.

Return to the trachea, follow it inferiorly to its bifurcation as the left and right main/primary bronchi. This occurs at the T4/5 vertebral level.Note that the right main bronchus is shorter, wider and more inline with the trachea than the left.

Review: Posterior mediastinum — Vagus nerves, thoracic duct, and the trachea

Behind and to the right of the esophagus in between the azygos vein and the thoracic aorta locate the thoracic duct. It lies on the vertebral columnand ascends to the level of TV5, crosses to the left side and continues up to the neck before arching forward over the subclavian artery to reach thejunction of the internal jugular and subclavian veins. The mid-section of the thoracic duct receives lymph vessels from the intercostal spaces, but thevessels from the lower intercostal spaces drains into the cisterna chyli. The upper segment receives intercostal lymph vessels from the left side only.






Locate in the superior mediastinum: the esophagus which lies immediately in front of the vertebral column and behind thetrachea and the descending thoracic aorta which begins at TV5 and at first lies to the left of the vertebral column and esophagus. Inferiorly,the aorta wedges its way between the vertebral column and esophagus to reach the mid-line at TV12. Because of the dome shape of thediaphragm, the esophagus passes through the fleshy part of the diaphragm in front and to the left of the aorta at the level of TV10, while theaorta leaves the thoracic cavity at the level of TV12.

Look for branches coming off the anterior surface of the descending aorta . These are small branches and supply, from abovedownward, the bronchi, esophagus, mediastinum, pericardial sac and diaphragm. Find posteriorly, the intercostal arteries which can beseen leaving the aorta and traveling to the intercostal spaces. The highest aortic intercostal arteries go to the third intercostal space, buthelp supply the second space along with the highest intercostal arteries from the costocervical trunk of the subclavian artery.




4. Examine the venous pattern of the posterior mediastinum first on the right side of the thorax (Fig. 31.4).

Observe the intercostal veins which travel with the intercostal arteries, but when they reach the posterior mediastinum and superiormediastinum, they have their own system of venous collection called the azygos system of veins. Note that the first two intercostal veinsempty into vessels in the neck, usually the subclavian veins. The third through 5th intercostal veins form the superior intercostal vein. Onthe right, the superior intercostal vein delivers blood to the azygos vein as the azygos arches over the right primary bronchus to enter thesuperior vena cava. On the left, the superior intercostal vein crosses the aortic arch to reach the left brachiocephalic vein. On the right side,the remaining intercostal veins empty into the azygos vein. The intercostal veins from the mid-thoracic left intercostal spaces form theaccessory hemiazygos vein below the superior intercostal vein and the lower intercostal veins form the hemiazygos vein. Dissectthe accessory hemiazygos and hemiazygos veins as they cross the vertebral column to empty into the azygos vein.The azygos and hemiazygos veins are upward continuations of the ascending lumbar veins in the abdominal cavity.

Show me how: Remove the parietal pleura from the left posterior thoracic wall

the accessory hemiazygos vein, crosses the midline to join the azygos vein around T8.

the hemiazygos vein crosses the midline to join the azygos vein around T9.

Review: Veins of the posterior thoracic wall






6. On the right side of the body, use your hands or forceps to remove any remaining parietal costal pleura from the posterior aspect of thethoracic cavity (Fig. 31.5). It is easier to find the following on the right side as the aorta often overhangs these structures on the left side.

Remove the parietal pleura from the right posterior thoracic wall

Identify a posterior intercostal vein, segmental veins that drain an intercostal space into the azygos system.

Identify the sympathetic trunk, running longitudinally along the posterior thoracic wall.

Identify the following branches/specializations:

gray ramus communicanswhite ramus communicanssplanchnic nervesgreater splanchnic nervesympathetic (paravertebral) ganglion chain

Locate within the medial portion of the intercostal spaces the external intercostal muscle posteriorly and the posterior intercostalmembrane which replaces the internal intercostal muscle. Dissect several intercostal nerves which can be found entering the intercostalspaces from the intervertebral foramina behind the bodies of the corresponding vertebrae. They join the vessels and are usually arranged inorder from above downwards, vein, artery and nerve. When the nerve, artery and vein reach the fleshy part of the internal intercostalmuscle, they split the muscle into innermost intercostal and internal intercostal. In the region of the angles of the ribs, muscle fibersparallel to the fibers of the internal intercostal muscle are seen arising from one rib and inserting on the second rib above. These fibers formthe subcostal muscle.

Carefully clean the sympathetic trunk or chain keeping in mind that it has both anterior and posterior branches. Dissect the greatersplanchnic nerve the largest anterior branch which receives fibers from T5-9 (sometimes confused with the continuation of the trunk) andthe lesser splanchnic nerve with fibers from T10-11.The least splanchnic nerve from T12 cannot be seen at this time. The greater, lesserand least splanchnic nerves go through or behind the diaphragm to reach the abdominal cavity. The sympathetic trunk is connected to allthoracic nerves by communicating rami. Dissect the white communicating rami, the more distal one and the gray communicatingrami the more proximal communicating ramus.




Review: Autonomic structures and neurovasculature of the posterior thoracic wall

Fig. 31.5 Adapted from Gilroy et al. Atlas of Anatomy, second edition, Fig. 8.3 A.

div class="header">After completing this dissection module, you should be able to identify the following structures. For muscles you should knowattachments, innervations, and actions; for neurovasculature, the course and function; and for organs, the structure, relationships, and function.

EsophagusEsophageal plexusRight and left vagus nervesThoracic ductTracheaCarinaLeft main and primary bronchusRight main and primary bronchusThoracic aortaPosterior intercostal arteryEsophageal arteriesAzygos veinPosterior intercostal veinsArch of azygosAccessory hemiazygos veinHemiazygos veinSympathetic trunkSympathetic (paravertebral) gangliaGray rami communicantes




White rami communicantesSplanchnic nervesGreater splanchnic nerveIntercostal nerves

At this time it is appropriate to review the groups of lymph nodes in the thorax and their direction of drainage: three groups of lymphnodes lie on the diaphragm, anterior, middle and posterior phrenic nodes (Plate 202, 233; 1.40, 1.78); the sternal or internal thoracicnodes lie along the internal thoracic vessels in the upper three or four intercostal spaces (Plates 178; 1.7, 1.78); the intercostal nodes which liein the spaces near the heads of the ribs; the anterior mediastinal lymph nodes which lie in relation to the brachiocephalic veins, thus are in thesuperior mediastinum, not the anterior mediastinum; tracheal or paratracheal nodes which drain the trachea; pulmonary nodes are located insidethe lung in the regions of the bronchopulmonary segments. (The visceral pleura has lymph vessels but not lymph nodes); andthe bronchopulmonary nodes which are related to the lobar bronchi (Plates 202; 1.40, 1.78). The right lung lymph tends to stay on the right,that from the left upper lobe tends to stay on the left, but the lower left lobe may drain to either side. The tracheobronchial nodes drain into thebronchomediastinal lymph trunk. The lymphatic vessels draining the heart follow the coronary vessels. Those following the right coronary arterypass over the aortic arch to reach the anterior mediastinal nodes. Those following the left coronary artery pass deep to the aortic arch and enterthe tracheobronchial nodes. The lymphatics of the esophagus drain into inferior deep cervical nodes, paratracheal nodes, posterior mediastinalnodes or superior gastric nodes.

Be sure to identify all of the following in this unit:

trachea

cricoid cartilage

hilus of left and right lung

right lung

oblique fissure

horizontal fissure

upper lobe

middle lobe

lower lobe

left lung

upper lobe

lingula

lower lobe

oblique fissure

bronchopulmonary segments of right and

left lung

vagus nerve

right recurrent laryngeal nerve

left recurrent laryngeal nerve

esophagus

esophageal plexus

descending thoracic aorta

intercostal arteries, veins & nerves

azygos vein

hemiazygos vein

accessory hemiazygos vein

thoracic duct (in superior and inferior

mediastinum)

intercostal spaces

external intercostal muscle



internal intercostal muscle

posterior intercostal membrane

sympathetic trunk

greater splanchnic nerve

lesser splanchnic nerve

white communicating rami

gray communicating rami

lymph nodes (finding them depends

how large they are)



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Thieme Dissector - Lungs