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Schedule

Lab 4 - Lungs and Mediastinum

Suggested reading:
Gray's Ch. 3 (124-179)

Primary Lab Objectives and Goals:

  1. Expose the root of the neck and understand the relationship between structures in the thorax and the neck.

  2. Explore the pleural sacs, remove and examine the lungs.   Understand the position of the lungs relative to the mediastinum, the distinguishing features of right and left lungs, and the blood supply to the lungs (which follows bronchial branching and segments).

  3. Locate the sympathetic trunk.  Understand the relationship of the sympathetic trunk with ventral rami.

  4. Trace the course of the trachea, esophagus, inferior vena cava, and vagus nerves in the thorax.

  5. Examine the patterns of venous and lymphatic drainage in the thorax.   The key is to locate and understand the relationship between the azygos and hemiazygos veins and the thoracic duct.

 

Procedure:

1. THE NECK

  1. Saw through the middle of each clavicle between the insertions of the Trapezius and Sternocleidomastoid muscles.
  2. Make similar cuts through any costal cartilages remaining intact above the opening cut in the thorax.
  3. Reflect the manubrium cranially, together with the Sternocleidomastoid, the sternohyoid muscles, and the medial ends of the clavicles, until the great vessels are exposed below the root of the neck.
  4. Remove any remnants of the thymus.
  5. Strip the pericardium from the thoracic cavity.
  6. Identify the aortic arch and its three branches, the ligamentum arteriosum, pulmonary trunk, trachea, bronchi, superior vena cava, and brachiocephalic veins (PLAY MOVIE -courtesy of UMich)

Atlas Images:

 

 

2. PLEURAL SAC

  1. With your hands explore the recesses of the left and right pleural sacs or cavities.
  2. Note that the parietal pleura extends farther caudally than the lungs on both sides.
  3. Slip your hand down into the costodiaphragmatic recess located below the lungs and between the costal portion of the body wall and the diaphragm.

Atlas Images:

FUNCTIONAL ANATOMY:

The lungs are divided externally into lobes and internally into smaller broncho­pulmonary segments, the latter corresponding to the major branches of the bronchial tree inside the lung. The heart indents the left lung more deeply than the right; the left lung therefore is smaller, has only two lobes to the right's three. Each lung has 10 broncho­pulmonary segments, though in the left lung some of them fuse.

Atlas Images:

 

  1. Remove the lungs by transecting the pulmonary arteries and veins, the bronchi and accompanying nerves at the root (or hilum) of the lungs (PLAY MOVIE - courtesy of UMich)
  2. Note if visible the tracheobronchial lymph nodes.
 

 

  1. Examine the surface features of right and left lungs (PLAY MOVIE -courtesy of UMich)
  2. You will want to look at and probe the two fissures on the right lung (oblique and horizontal) and the single fissure on the left (oblique)
  3. Identify the lobes associated with those fissures.  On the left lung, additionally identify the little lingula in the inferior portion.
 

 

  1. Several thoracic organs make distinct impressions on the lung.  The heart makes a deep impression on the left lung, as does the arch of the aorta and descending aorta.  These features are less prominent on the right lung.
  2. Dissect the roots of the lungs, distinguishing pulmonary veins, arteries, and primary and secondary bronchi. This will involve some gentle work with finger and probe (PLAY MOVIE -courtesy of UMich) 
  3. Note, with the aid of the figure below, the normal arrangement of veins, arteries and bronchi.  These will vary a bit depending on how close to the surface of the lung you have cut.
  4. Cut into the lung and examine the spongy tissue.  Try to identify bronchi and vessels traveling through the lung tissue.

Atlas Images:

 

 

 3. SYMPATHETIC TRUNK

  1. On the right side, identify and trace the chain of paravertebral sympathetic ganglia in the thorax (PLAY MOVIE -courtesy of UMich)
  2. Follow the sympathetic trunk and the greater splanchnic nerve, which exits the sympathetic trunk between T5-9 vertebral levels, to the diaphragm.

Atlas Images:

FUNCTIONAL ANATOMY: 

The sympathetic nervous system has segmental ganglia located along the sides of the vertebral bodies. These paravetebral ganglia are strung together with connecting fibers. The connected chain of ganglia is the sympathetic trunk. Sympathetic motor fibers run medially from segments of the sympathetic trunk to supply the viscera. They conjoin to form cardiac nerves (in the upper thoracic region), which run down to the autonomic plexuses near the heart, and splanchnic ("visceral") nerves (in the lower thoracic region), which pierce the diaphragm and innervate the abdominal viscera. The cardiac nerve fibers are mainly postsynaptic (their cell bodies are in the paravertebral ganglia), but the splanchnic nerve fibers are presynaptic axons. The splanchnic nerves synapse on secondary neurons located in the prevertebral or preaortic ganglia of the abdomen.

Several major structures lie dorsal to the heart. In the superior mediastinum, the esophagus and trachea run downward in contact with the vertebral bodies, with the esophagus a bit to the left of the midline. The trachea ends behind the heart, and the aorta lies between the esophagus and vertebral column. The inferior vena cava, esophagus, and aorta all connect to structures in the abdomen, and therefore pierce the diaphragm. Their respective openings lie approximately in front of the 8th, 10th, and 12th thoracic vertebrae.

 

 

 

4. BACK TO THE MEDIASTINUM

  1. Trace the course of the esophagus and trachea through the mediastinum.
  2. Note their changing relationships to each other and to other mediastinal structures at various levels.
  3. Attempt to trace some of the vagal fibers passing into the cardiac nerves and plexuses. After passing the last aortic arch derivative on each side, the vagus nerves form an esophageal plexus which coalesce to form the anterior and posterior vagal trunks on the lower esophagus.
  4. Find the esophageal plexus and the vagal trunks on the anterior and posterior surfaces of the esophagus.
  5. Trace the vagal plexus on the esophagus down to the diaphragm (PLAY MOVIE -courtesy of UMich)

NOTE: You will probably find the vagal fibers condensing into anterior and posterior vagal trunks, derived mostly from left and right vagus nerves respectively. Owing to the rotation of the abdominal gut, the esophagus drifts to the left and twists as it approaches the esophageal hiatus, which brings most of the right and left vagus nerve fibers into positions respectively dorsal and ventral to the gut. See figure TG 4-45 above.

  1. Trace the inferior vena cava, esophagus, and aorta to the diaphragm.
  2. Verify that the aorta (along with the azygos vein and thoracic duct) passes through the opening between the right and left crura of the diaphragm (aortic opening).
  3. Verify that the esophagus, accompanied by the vagus nerves, passes through the esophageal opening in the fleshy portion of the diaphragm. The vena cava passes through the central tendon.

Atlas Images:

FUNCTIONAL ANATOMY:

Note that the phrenic nerves primarily supply motor and sensory innervation to the diaphragm. However, they also send small sensory branches to the pericardium and through the diaphragm to the parietal peritoneum that covers the diaphragm's abdominal surface.

 

 

5. VENOUS AND LYMPHATIC DRAINAGE OF THE THORAX

  1. Find the azygos vein where it empties into the superior vena cava (PLAY MOVIE -courtesy of UMich).
  2. Follow this vein down along the posterior wall of the thorax on the right side. 
  3. Note its relationship to the aorta on the left side of the posterior wall of the thorax.
  4. Locate midline branches of the aorta (bronchial and esophageal arteries) supplying the thoracic derivatives of the gut.
  5. Identify several of the segmental posterior intercostal veins on the right side emptying into the azygos vein (just to the right of the esophagus).
  6.  Look to the left of the trachea and the esophagus.
  7. Locate the left superior intercostal vein and the hemiazygos vein(s).

Atlas images:

FUNCTIONAL ANATOMY:

The azygos and hemiazygos veins collect blood from the posterior intercostal veins. The hemiazygos vein on the left crosses the midline to drain into the azygos vein on the right.  The azygos vein curves over the root of the lung to drain into the SVC.  The left superior intercostal vein, draining the second and third posterior intercostal veins, has a course ending in the left brachiocephalic vein.  All of the veins mentioned have a good deal of variability. Notice if your cadaver is different from the described pattern.

 

  1. Find the thoracic duct by looking for a thin-walled tubular structure lying between the azygos vein and the descending aorta (PLAY MOVIE -courtesy of UMich).

NOTE: The thoracic duct will seem hard to find, but if you probe carefully between the azygos and the descending aorta you will find it.

  1. Follow the thoracic duct up toward the brachicephalic vein.  It may be difficult at this stage to see its terminus. 

Atlas Images:

FUNCTIONAL ANATOMY:

The thoracic duct is a major lymphatic channel and it is important to be able to identify it.  Lymphatics generally accompany veins. The azygos vein is accompanied by the terminal lymph channel of the body, the thoracic duct. The lower part of the thoracic duct forms from lymphatics of the right side while the upper part forms from those of the left side. It passes up the thorax from right to left and empties into the brachiocephalic vein between the left internal jugular vein and subclavian vein. Its end point can vary; it can empty into the internal jugular or subclavian veins directly.  

 

CLINICAL CORRELATES

Foreign body aspiration

Aspiration of foreign bodies into the lungs happens most commonly in toddlers. In general, children under the age of 15 account for 75-85% of all foreign body aspirations. In adult, this usually occurs when there is some impairment – either drug/alcohol induced, medical or mental. Aspiration in adults occurs predominantly in the right side bronchus as opposed to the left side. This is due to the nearly vertical pathway of the right bronchus and the more acute pathway of the left bronchus. The right side is also slightly wider than the left side. An object that is aspirated into the lung can cause symptoms that persist over a period of time if it is not initially discovered. Symptoms include a recurrent cough, wheezing, bronchitis, and pneumonia. The foreign body can be removed using a rigid bronchoscope.

 

 

 

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