Thorax Lab 2B

Lab 2B: Lungs, Mediastinum, and Posterior Body Wall

Learning objectives

Dissect the outer surface of the heart to reveal and identify the coronary arteries.
Be able to distinguish between the right and left isolated lungs by their distinctive features. Identify the trachea and primary bronchi. Be able to list the structures contained within the hilum of the lung
Within a posterior intercostal space, identify the intercostal nerves (ventral rami of spinal nerves) and posterior intercostal aa. and vv.
Actually witness, with your own eyes, the sympathetic chain ganglia. This should aid your understanding of their relationship to the spinal nerves. You will also see one or two thoracic splanchnic nerves.
Actually witness, with your own eyes, the largest lymph vessel in the body—the thoracic duct (the only lymph vessel you are likely to see in gross dissection). Not all cadavers have a well preserved thoracic duct but there are generally a few good ones per room. If necessary, look at other cadavers to see the thoracic duct.

Short List of Structures - 2B

Orientation of the heart in the thorax
Intercostal nerves
Posterior intercostal arteries and veins
Coronary sinus
Left anterior descending artery
Sympathetic chain

Heart Dissection: Coronary Arteries

Before You Begin...

Part of your lab group should work on the heart and coronary vessels and the other should study the lungs and dissect the posterior mediastinum and posterior body wall. It is necessary to divide and conquer to be able to complete this long set of dissections in the time available.

Orientation

The heart and lungs have now been removed from the body and separated. Maintaining your understanding of the orientation of the heart as you spin it around is difficult without its relationship to the thorax for reference. The aorta has been cut close to the heart so you won’t see the whole aortic arch. Identify the aorta (thick walled) and the pulmonary trunk (=pulmonary artery, slightly thinner walled) and hold the heart in your hand as in the figure below. The anterior aspect of the heart should be facing you and your hand is cradling the posterior aspect. Your right is the patient’s (heart’s) left. Then try holding the heart in anatomical position up in front of your own thorax, so your right is the patient’s (heart’s) right.

Inspection of the removed heart

Identify the following features on the removed heart:

Superior and inferior vena cava- aligned along the right border of the heart.
Pulmonary trunk- bifurcates inferior to the aortic arch.
Ascending aorta- the right atrium may overlap the ascending aorta.
Pulmonary veins- four located posterior and inferior to the pulmonary trunk. The two right pulmonary vv. are positioned between the superior and inferior venae cavae.
Coronary sulcus- oblique groove that separates the atria from the ventricles, filled by coronary vessels and adipose tissue.
Anterior interventricular groove- filled by the left anterior descending (anterior interventricular) coronary a., great cardiac v., and adipose tissue.
Posterior interventricular groove- filled by the right posterior interventricular coronary a., middle cardiac v., and adipose tissue.
Apex of the heart- the inferolateral portion of the left ventricle. It lies posterior to the fourth or the fifth intercostal space about one hand width from the midsagittal line.
Base of the heart- opposite the apex, it includes the atria and the great vessels.

NOTE: The aorta was cut close to the heart, so you won’t see the aortic arch as it is shown in the figures to the right.

Anterior surface of the heart

Structure and function of the heart

Most of the substance of the heart is ventricle. The ventricles do the pumping work of the heart, so they are large and thick-walled. By comparison the atria are small and thin-walled. The apex of the heart is the muscular, rounded tip of the heart, and is part of the left ventricle. In anatomical position the apex points to the left and infero-laterally. The base of the heart is opposite the apex.

The most anterior of the great arteries of the heart is the pulmonary trunk. Just inferior to the pulmonary trunk is the wall of the right ventricle. This means that the anterior surface of the heart is mostly right ventricle. Also on the anterior surface of the heart to the left, you’ll find the great cardiac vein mostly embedded in fat. Deeper in the fat is the accompanying left anterior descending (LAD, or anterior interventricular) coronary artery. These vessels run superficial to the interventricular septum, which separates the left and right ventricles.

The ascending aorta is slightly posterior and to the right of the pulmonary trunk. The superior vena cava is to the right of the aorta. The position of the left ventricle can be verified by placing a finger into the aorta and following it down posterior to the pulmonary trunk and through the aortic valve. The position of the right atrium can be verified by placing a finger into the superior or inferior vena cava.

To find the left atrium, turn the heart so that you’re viewing its posterior surface and look for the pulmonary veins (typically 4, 2 upper and 2 lower). The auricles (“ears”) are the small, wrinkled flaps of tissue that project from both atria.

Anterior surface of the heart

HELPFUL HINTS

In order to actually see the coronary arteries, you must clean away the fat overlying them. Scissor-spreading is the technique to employ. This will take some time, be patient. Start scissor spreading at the base of the aorta where the coronary arteries to emerge from the aorta.

Right coronary artery and its branches

The right coronary artery arises from the ascending aorta directly superior to the right cusp of the aortic valve. The artery’s orifice is visible inside of the ascending aorta.

Follow the right coronary a. inferiorly between the right atrium and right ventricle in the coronary sulcus, using blunt dissection and scraping to remove the pericardial fat. Locate the following branches of the right coronary artery:

Right marginal a. - supplies the right surface of the right ventricle, it courses toward the apex along the inferior border of the heart.
Posterior interventricular a. - (posterior descending a.) descends in the posterior interventricular groove toward the apex.

Left coronary artery and its branches

The left coronary artery arises from the ascending aorta directly superior to the left cusp of the aortic valve. The artery’s orifice is visible inside of the ascending aorta.

The left coronary artery passes posterior to the pulmonary trunk and can be difficult to find at first. Make sure to loosen the pulmonary trunk and push it forward to expose the location of the left coronary artery.

Follow the left coronary artery to its bifurcation using blunt dissection and scraping to remove the pericardial fat. Locate the following branches of the left coronary artery:

Anterior interventricular branch - (left anterior descending a., LAD) courses in the anterior interventricular groove to the apex. It may anastamose with the posterior interventricular branch of the right coronary a. at the apex.
Circumflex branch - follows the coronary sulcus around the left border of the heart and gives off marginal aa. including the left (obtuse) marginal a. It may anastamose with the right coronary a. in the coronary sulcus.

PUTTING IT ALL TOGETHER

The main coronary vessels travel in the coronary sulcus or groove, which is a groove between the atria and ventricles. It doesn’t look like a groove because it’s filled with blood vessels and fat, but as you dissect into that fat to reveal the blood vessels you’ll see that it is a groove.

Cardiac veins and their branches

NOTE: The cardiac veins are named differently than the cardiac arteries.

Identify the following structures using blunt dissection or scraping to remove pericardial fat:

Coronary sinus - large dilated vein that travels in the coronary sulcus on the posterior surface of the heart. It receives venous blood from all of the cardiac vv. (except the anterior cardiac vv., which drain directly into the right atrium) and returns that blood to the right atrium.
Great cardiac vein - originates on the anterior surface of the heart near its apex, and ascends with the left anterior interventricular (descending) coronary artery.
Middle cardiac vein - originates on the posterior surface of the heart in the posterior interventricular groove and ascends with the posterior interventricular coronary a.
Small cardiac vein - originates on the anterior surface of the heart along the inferior margin of the right ventricle and ascends with the right (acute) marginal coronary a.

Clinical Correlation: Coronary Artery Bypass

Coronary artery bypass surgery, also known as coronary artery bypass graft (CABG, pronounced "cabbage") surgery, heart bypass or bypass surgery, is a surgical procedure to restore normal blood flow to heart tissue.

There are two main approaches. In one, the left internal thoracic artery (LITA, also called left internal mammary artery, LIMA) is diverted to the left anterior descending (anterior interventricular) branch of the left coronary artery distal to its obstruction to restore blood flow. In this method, the artery is "pedicled", which means it is not detached from the origin. In the other approach, a great saphenous vein is removed from a leg. One end is attached to the aorta or one of its major branches and the other end is attached to the obstructed coronary artery immediately distal to the obstruction to restore blood flow.

-- Wikipedia

Examining the External Features of the Lungs

Before You Begin...

For the remaining part of this lab, you will study the isolated lungs and then dissect and study the posterior mediastinum and posterior body wall.

Part of your lab group should work on the heart and coronary vessels and the other should study the lungs and dissect the posterior body wall. It is necessary to divide and conquer to be able to complete this long set of dissections in the time available.

External Lateral Features of the Lungs

Now that we have the lungs isolated, let’s identify some external landmarks that are visible on the lateral surfaces of the lungs.

Apex – the pointed “tip” of the lung. The apex of the lung is part of the superior (upper) lobe.
Base – the concave “bottom” of the lung. This is the part of the lung that sits on top of the diaphragm. The base of the lung is part of the inferior (lower) lobe of the lung.
Oblique fissure – divides the upper and lower lobes of the lungs, and the middle and lower lobes of the right lung
Horizontal fissure – divides the upper and middle lobes of the right lung.

Once you’ve had a chance to familiarize yourself with the lateral exterior of the lungs, flip them over to explore the medial surface. First, we will look at the left lung.

HELPFUL HINTS

Orient yourself when looking at the lungs in isolation. Note that the anterior border of the lung is thin and pointed, while the posterior border of the lung is rounded. Also note contact impressions from the ribs.

External Medial Features of the Left Lung

Note the apex and base of the lung, the superior and inferior lobes (separated by the oblique fissure), and the anterior and posterior borders. Find the following landmarks on the medial surface of the left lung:

Two distinct contact impressions on the medial surface of the left lung: the groove for the aorta (arch and descending aorta) and the cardiac impression.
The hilum of the lung is the region that connects the lung to the structures in the mediastinum:
- The bronchi have cartilage in their walls and are easy to distinguish from blood vessels.
- The pulmonary arteries are typically the most superior vessels of the hilum and are slightly thicker-walled.
- The pulmonary veins, are typically inferior and anterior to the bronchi and thinner walled.
- The bronchopulmonary lymph nodes are typically firm black nodules and can be quite large.

Realize that the most superficial tissue of the hilum is the pleura. At the hilum the visceral pleura of the lung is continuous with the parietal pleura of the mediastinum. The pulmonary ligament is an extension of the hilum that contains no airways or blood vessels. At the pulmonary ligament, the visceral and parietal pleura are also continuous.

The bronchial vessels, which supply the tissue of the lung, also enter the lung at the hilum.

Now that we’ve viewed the left lung, let’s move on to the right lung.

External Medial Features of the Right Lung

Note the apex and base, the superior, inferior and middle lobes (separated by the oblique and horizontal fissures), and the anterior and posterior borders. Find the following landmarks on the medial surface of the right lung:

The contact impressions of the right lung are typically not as regular and clear as those of the left, though you may see grooves for the superior vena cava (SVC) and the azygous vein.
The hilum of the right lung contains the same structures as the left:
- Bronchi
- Pulmonary arteries
- Pulmonary veins
- Bronchopulmonary lymph nodes
- Bronchial vessels

RALS - The position of the pulmonary artery relative to the airway (airway is easy to find because cartilage). In the Right lung, the artery is Anterior to the airway. In the Left lung, the artery is Superior to the airway.

Clinical Correlation: Irritation of the pleurae and referred pain

The visceral pleura is insensitive to pain because it receives autonomic innervation. Autonomic nerves reach the visceral pleura in company with the bronchial vessels.

The parietal pleura is sensitive to pain because it is richly supplied by somatic sensory neurons:

The costal parietal pleura is segmentally innervated by the intercostal nerves.
The mediastinal parietal pleura is innervated by the phrenic nerves.
The diaphragmatic parietal pleura is innervated at the periphery by the lower 6 intercostal nerves and in the center by the phrenic nerves.

Pain from the mediastinal and central diaphragmatic pleurae is often “referred” to the neck and shoulder because it is carried in the phrenic nerves, which arise from spinal segments C3-C5. These levels of the spinal cord more usually transmit the sensation of pain from the neck and shoulder, so the brain mistakes the pain sensations from the mediastinal and central diaphragmatic pleurae as originating in the neck and shoulder.

The Mediastinum and Posterior Thoracic Body Wall

Before You Begin...

The following section will lead you through the exploration of the posterior mediastinum. You will need to do a lot of cleaning of structures and look carefully to identify the structures of the posterior body wall.

The posterior mediastinum

Identify the borders of the posterior mediastinum:

Vertebral bodies T5-T12 (posterior border)
Posterior surface of the pericardial sac (anterior border)

Use scissors and blunt dissection to carefully reflect the remaining pericardium from the posterior mediastinum (leave the pericardium attached to the diaphragm). Identify the following structures:

Esophagus- muscular tube that is anterior and slightly to the right of the descending aorta.
Descending aorta- posterior and slightly to the left of the esophagus.

Use blunt dissection to clean up and follow the already exposed right and left vagus nerves superiorly and inferiorly.

Right vagus n.- find this nerve posterior to the root of the right lung.
Left vagus n.- find this nerve posterior to the root of the left lung or anterior to the aortic arch.

Try to identify the left recurrent laryngeal nerve, a branch of the left vagus n. that passes immediately to the left of the ligamentum arteriosum. It turns under the arch of the aorta and heads superiorly (recurs) back up in the neck to the larynx. Since you removed the heart and lungs you will not likely find the right recurrent laryngeal nerve.

The vagus nn. branch and become the esophageal plexus on the esophagus. If you clean the esophagus too aggressively, you will remove this plexus.

HELPFUL HINTS

The anterior and posterior vagal trunks will be difficult to find. Note that the nerve plexuses that surround tubular structures (e.g., the esophagus, abdominal aorta) are web-like and easily mistaken for connective tissue.

The azygos system of veins

The azygos, hemiazygos, and accessory hemiazygos veins form the venous drainage system for the posterior thoracic wall.

Peel the parietal pleura and endothoracic fascia away from the internal aspect of the rib cage. Identify the following structures:

Azygos v. - The azygous vein typically runs along the right side of the vertebral bodies, and receives blood from the right intercostal veins. At approximately the T4 level, it arches over the right main bronchus to drain into the superior vena cava.
The intercostal veins on the left side also drain into the azygous vein, typically through a hemiazygous or accessory hemiazygous vein.

You should expect variation within this system of veins. In some people, an accessory azygos vein parallels the main azygos vein on the right side. Other people have no hemiazygos system of veins. An uncommon, but clinically important variation is when the azygos system receives all of the blood from the inferior vena cava, except that from the liver. In these people, the azygos system drains nearly all the blood inferior to the diaphragm, except for the digestive tract.

Anterior view of the azygos system of veins

Clinical Correlation: Obstruction of the Vena Cavae

The azygos, hemiazygos, and accessory hemiazygos veins offer alternate means of venous drainage from the posterior thoracic and abdominal regions when obstruction of the inferior vena cava (IVC) occurs. When obstruction of the superior vena cava (SVC) occurs superior to the entrance of the azygos vein, blood can drain inferiorly into the veins of the abdominal wall and return to the right atrium through the IVC and azygos system of veins.

Azygos system of veins (anterior view)

Lymphatic system in the thorax

Using blunt dissection, identify the following lymphatic structures:

Thoracic duct- very fragile yet the largest lymphatic vessel in the body. This may be very difficult to find. Look between the esophagus and the descending aorta by pulling the esophagus to the right side. It passes deep to the esophagus and deep and medial to the descending aorta from the diaphragm to the neck. It terminates by emptying into the junction of the left internal jugular v. and left subclavian v. The thoracic duct is 3-4 mm in diameter and may look dull white.
Tracheobronchial lymph nodes- located below and on either side of the tracheal bifurcation. These nodes are often large and should be identifiabl if you dont see them on the posterior body wall they were likely removed with the heart and lungs.

Lymphatic system in the thorax

Nerves in the posterior mediastinum

Probe the intercostal spaces near the vertebral bodies on one side of the thorax to identify the following:

Thoracic sympathetic trunk - located bilaterally on the following:
- The heads of the ribs in the superior part of the thorax.
- The costovertebral joints in the midthorax.
- The vertebral bodies in the inferior part of the thorax.
Greater splanchnic nerves - originate from spinal levels T5-T9 and travel inferiorly towards the abdomen on the lateral surface of the vertebral bodies T6-T12.
Intercostal neurovascular bundles - veins, arteries, and nerves.

Nerves of the mediastinum (anterior view of the right side)

Thoracic sympathetic nerves (anterior view of the right side)

Nerves of the mediastinum (anterior view)

Nerves of the mediastinum (anterior view with the right side exposed)

Intercostal nerves and vessels

Follow a couple of the anterior intercostal aa. and vv. laterally from the sternum in a couple of intercostal spaces. Where the vessels disappear behind the innermost intercostal mm., use forceps to separate the innermost and internal intercostal mm. In each intercostal space, the intercostal vessels travel with an intercostal nerve. Collectively, the intercostal v., a., and n., in an intercostal space are known as an intercostal neurovascular bundle.

The main branches of the intercostal neurovascular bundles travel in the superior region of the intercostal space, running in the costal groove of the rib forming the upper boundary of the space. Smaller, collateral branches of the neurovascular bundle travel in the inferior part of the intercostal space.

The intercostal nn. are the anterior (ventral) rami of spinal nerves T1 to T11 (the anterior ramus of T12 is the subcostal nerve). The posterior (dorsal) rami of spinal nerves T1 to T11 (and T12) innervate the deep back (epaxial, from epimere) muscles and skin adjacent to the vertebral column.

Every thoracic spinal nerve gives off three cutaneous branches: a posterior branch from the posterior ramus of the spinal nerve, and a lateral and anterior branch from the anterior ramus of the spinal nerve. Cutaneous branches carry sensation from the skin, as well as sympathetic motor innervation (to sweat glands, arrector pili, and vascular smooth muscle).

You may have seen some of these cutaneous nerves as you removed the skin (they are very thin structures and look a lot like dental floss). They are usually accompanied by small arteries and veins, forming cutaneous neurovascular bundles. The vessels that contribute to the posterior and lateral cutaneous neurovascular bundles are branches of the posterior intercostal aa. and vv. The posterior intercostal arteries branch from the descending aorta.

Clinical Correlation: Thoracentesis

Sometimes it is necessary to insert a hypodermic needle through an intercostal space into the pleural cavity to obtain a sample of pleural fluid or to remove blood or pus (thoracentesis = thoracocentesis). The needle is inserted in the inferior part of the rib cage, because that's where the fluid will accumulate (gravity). To avoid damage to the intercostal nerves and vessels, the needle is inserted in the mid-point of the intercostal space, to avoid hitting the main intercostal nerve and artery (which are just inferior to a rib) and the collateral branches of the intercostal vessels and nerve, which run along the top of the rib. The midaxillary line is also used here to guide needle insertion.

The superior mediastinum

The contents of the superior mediastinum include the large arteries branching off the aortic arch:

Brachiocephalic a.
Left common carotid artery
Left subclavian artery

You should also see the large right and left brachiocephalic veins joining to form the superior vena cava.

Vessels of the superior mediastinum

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