Thorax Lab 1B

Lab 1B: Removal of Rib Cage, Pleura, Pericardium

Learning Objectives

Understand the flow of blood in the intercostal spaces.
Understand the anastomosis between the aorta and the internal thoracic arteries via the posterior and anterior intercostal arteries and understand the concept of arterial anastomoses.
Appreciate the bilateral symmetry of the left and right brachiocephalic veins relative to the asymmetry of the three first major branches of the aorta.
Understand the spatial relationships (i.e., the relative 3D positions) between the great vessels and the left and right vagus nerves.
Be able to distinguish the parietal and visceral layers of the pleura and be able to place your finger on each layer and name it.
Be able to distinguish the fibrous, serous parietal and serous visceral layers of the pericardium and be able to place your finger on each layer and name it.
Be able to visualize the parietal pleura as a continuous sac surrounding each lung, creating the left and right pleural cavities—analogous to the pericardial sac and pericardial cavity.

In this lab, you’ll use an electric bone saw to cut through the ribs and sternum, and remove the anterolateral thoracic wall to view the heart and lungs.

Short List of Structures - 1B

True ribs, false ribs, and floating ribs
Internal thoracic artery and vein
Phrenic nerves
External and internal intercostal muscles
Parietal and visceral pleura
Costodiaphragmatic recess

Before You Begin...

Be aware of a structure called the thoracic inlet/outlet (shown on the right) located at the top of the thorax (above the first rib) where key structures including the subclavian a. and v. pass through along with some roots of the brachial plexus before entering the upper extremity.

This will become relevant for your first step where you will be cutting the clavicles and the first ribs, so be mindful of this area and the important structures that pass through it.

Removal of anterior thoracic wall

In this lab, you’ll use an electric bone saw to cut through the ribs and sternum, and remove the anterolateral thoracic wall to view the heart and lungs.

Use your scalpel to cut through the overlying tissue on the ribs before you cut with a bone saw. Cut along the dotted lines in the picture below. On each side, make your cuts as far posterior as possible. This will give you better access to the lungs and thoracic cavity.
- In the area of the thoracoacromial artery (trunk), be especially careful as you cut through the rib cage. There are a lot of soft tissue structures deep to the clavicle that you don’t want to destroy.
- Check with a member of the teaching staff once you are ready to move on to the next step.
Once you are through the soft tissue, use a bone saw to cut through the ribs.
- Again, be careful while cutting along the superior lateral portion of the chest wall near the deltoid m., in the region where you were looking for the pectoral nerves and thoracoacromial artery (trunk). Don’t damage the surrounding nerves, arteries, and veins.
- Once you feel the bone give (in other words, once you feel like you’re through the bone) stop cutting with the bone saw! This will decrease accidental damage to the lungs.
When you have finished with the bone saw, work your way around the cut using a scalpel to carefully cut away any remaining soft tissue attachments. Slide your hands under the loosened anterolateral chest wall, being careful not to cut your fingers on the sharp edges of bone. Break any fascial connections between the internal surface of the chest wall and the thoracic viscera and carefully work the loosened piece up and away. This last step can be challenging - take your time and work carefully.

Cut along the dotted lines with a scalpel first and then with the bone saw.

Saw cuts completed on a cadaver.

Reflecting/Mobilizing the Manubrium

Reflecting the manubrium

The first step will be to reflect the manubrium to allow better access to the aortic arch and its branches.

Push any remaining soft tissue away from the manubrium to isolate it so that you don’t accidentally cut any structures deep to the manubrium. Do the same with the clavicle, pushing soft tissue structures (like the subclavian artery and vein) away from the clavicle so that those structures are not inadvertently cut. The subclavian vein, in particular, will be very close to the deep surface of the clavicle.

Now cut the isolated clavicles with nippers or a bone saw approximately at their mid-points as shown by the black dashed lines on the image to the right, leaving the medial ends of the clavicles attached to the manubrium. You will also need to cut through the first ribs, which are articulated with the manubrium. Feel for the first ribs and isolate them before cutting through them with nippers instead of the bone saw. If you have any questions along the way ask a member of the teaching staff to guide you through this procedure.

Reflect the manubrium and the attached parts of the clavicles and first ribs superiorly, carefully freeing the bone from underlying connective tissue.

Great vessels

Clean any remaining connective tissue and adipose off the surface of the aortic arch and find the three large arteries that branch from the arch:

The first branch is the brachiocephalic a., which runs superiorly a short distance before splitting to form the right common carotid a. and the right subclavian a. In the axillary region (armpit), the subclavian artery changes its name to the axillary artery, When the axillary artery enters the brachium (arm), its name changes to the brachial artery.
The left common carotid a.
The left subclavian a.

Note that there is no brachiocephalic a. on the left side. The left common carotid a. and the left subclavian a. branch directly from the aortic arch. Other than the lack of a brachiocephalic a. on the left side, the course of the left common carotid and the left subclavian are the same on the left side as on the right.

The superior vena cava is formed by the union of the left and right brachiocephalic vv. Note that there are two brachiocephalic veins (left and right), but only one brachiocephalic artery. The brachiocephalic a. is on the right side, but since there is only one, it’s not referred to as the right brachiocephalic a. (because there is no left brachiocephalic a.).

The brachiocephalic vv. are formed by the union of the subclavian v. and the internal jugular v. on each side. The subclavian v. travels with the subclavian a., changing its name to the axillary v. and then the brachial v. as does the subclavian artery. The internal jugular v. runs in the carotid sheath with the common and internal carotid arteries. The origin of the internal jugular v. is at the base of the skull, where it drains all the venous blood from inside the skull.

You may also be able to identify the ligamentum arteriosum, the fibrosed arterial connection between the pulmonary trunk and the aortic arch (a remnant of fetal circulation, will be covered in lecture). Bluntly dissect between the bifurcation of the pulmonary trunk and the aorta to find the ligamentum arteriosum.

Helpful Hint

Given the direction of the flow of blood (away from the heart in arteries, toward the heart in veins) it may be helpful to think of the arteries as splitting apart as they move away from the heart, and the veins as coming together as they toward the heart.

Relationship of the great vessels and the vagus nerves

The phrenic nerves pass through the thorax anterior to the root of the lungs, and thus are fairly easy to find. The vagus nerves, however, pass posterior to the root of the lungs, and so they are very difficult to see until the heart and lungs are removed. But you will be able to see the most superior part of the left vagus nerve prior to heart and lung removal, due to its association with the aortic arch.

The left vagus nerve courses anterior to the aortic arch immediately lateral to the ligamentum arteriosum, and then passes posterior to the left pulmonary a. and the hilum of the left lung. As it passes over the aortic arch it gives off a recurrent branch (left recurrent laryngeal nerve, seen in green in the figure to the right)) that passes posterior to the aortic arch and ascends through the neck to the larynx, where it innervates the muscles of phonation.

The right vagus nerve passes between the right subclavian (or brachiocephalic) a. and v., and then passes posterior to the hilum of the right lung. The right recurrent laryngeal nerve (yellow in the image to the right) branches from the right vagus nerve and passes posterior to the subclavian a., then ascends to the right side of the larynx.

Examining the Chest Wall

Features of the chest wall

Examine the internal surface of the removed anterolateral chest wall. Identify the sternum. On either side of the sternum, you should be able to find the internal thoracic vessels (an artery and a vein). The internal thoracic arteries originate from the subclavian artery, just posterior to the sternoclavicular joint.

The internal thoracic vessels give rise to the anterior part of the intercostal circulation. Look for anterior intercostal vessels (artery and vein) branching off the internal thoracic vessels in each intercostal space.

Identify the transversus thoracis m. Note that the muscle is attached to the sternal body and the costal cartilages of ribs 2-6.

Internal surface of the rib cage and sternum. The internal thoracic arteries are branches of the subclavian arteries. The anterior intercostal arteries are branches of the internal thoracic arteries.

Intercostal muscles

Next, dissect and identify the intercostal muscles in an intercostal space. Note that none of the 3 intercostal muscles (external, internal, innermost) run completely from the vertebral column to the sternum. The external intercostals extend nearly to the vertebral column posteriorly, but are only represented by a thin membrane anteriorly (toward the sternum). The internal intercostals extend all the way to the sternum but are only represented by a thin membrane posteriorly (toward the vertebral column). The innermost intercostals are mostly lateral, and do not extend as far anterior as the internal intercostals, nor as far posterior as the external intercostals.

External intercostal mm. This is the most superficial layer of intercostal mm. and will be visible on the external surface of the removed anterolateral chest wall. The muscle fibers course in an oblique fashion (superior/lateral to inferior/medial). The muscle belly (the fleshy part of the muscle) fills the intercostal spaces posteriorly and laterally. Anteriorly, just lateral to the sternum, the external intercostal m. is only represented by a membrane, which is thin, strong, nearly transparent connective tissue.
Internal intercostal mm. The muscle fibers course at a right angle and deep to the external intercostal mm. On the external surface of the removed anterolateral chest wall, you can see the internal intercostal m. through the membrane of the external intercostal mm. The muscle belly of the internal intercostal m. fills the intercostal space anteriorly and laterally, but not posteriorly.
Innermost intercostal mm., This is the deepest layer of intercostal mm. and is visible on the internal surface of the removed anterolateral chest wall. The fibers of this muscle follow the same course as the internal intercostal muscles and the two are difficult to distinguish. The muscle belly of the innermost intercostal mm. only fills the intercostal spaces laterally; anteriorly and posteriorly it is membranous. Thus, when you are looking at the internal surface of the removed anterolateral chest wall anteriorly, you are actually looking at the internal intercostal mm. through the membrane of the innermost intercostal mm. The best way to tell the two muscles apart is to follow the intercostal vessels and nerve laterally to where they disappear behind the innermost intercostal mm. (the intercostal vessels and nerve run between the internal and innermost intercostal muscles).

Anastomotic blood flow in the chest wall

Review the internal thoracic artery and anterior intercostal arteries (located in the anterior part of the chest wall that was removed). Blood flows from the internal thoracic (and musculophrenic) arteries to the anterior intercostal arteries. Blood also flows from the aorta to the posterior intercostal arteries. The anterior and posterior intercostal arteries join together on the lateral side of the thorax. The junction of two arteries is called an anastomosis.

Anastomoses permit redundancy of blood supply to important regions. In other words, there are two sources of blood in areas with anastomoses. Should one source be cut off, the other will continue to supply blood to the region. The concept of arterial anastomosis is clinically important, and the intercostal arteries are a classic example.

Redundant blood supplies can be exploited surgically. For example, the internal thoracic artery can be removed or diverted for coronary bypasses without causing ischemia (lack of blood flow) to the anterior chest wall because the intercostal anastomoses allow the aorta to supply blood to the anterior intercostal arteries via the posterior intercostal arteries (see the Coronary bypass clinical correlation in the Heart Dissection section).

Pleurae and Pericardium

Before You Begin...

A pleura is a serous membrane which folds back onto itself to form a two-layered membrane structure. The inner (visceral) pleura covers the lungs and adjoining structures (blood vessels, bronchi and nerves). The visceral pleura is distinct from the lung but is not easily removed from the surface of the lung. The parietal pleura, on the other hand, is associated with the body wall. When you removed the anterior thoracic wall, you almost certainly removed the costal parietal pleura with it. Thus, you can see the parietal pleura on the internal surface of the chest wall and visceral pleura on the lung’s surface.

The thin space between the two pleural layers is a potential space known as the pleural space or pleural cavity, and normally contains only a small amount of pleural fluid. In a cadaver, the pleural spaces are often filled with lots of fluid and coagulated material that will need to be removed from the thoracic cavity.

Parietal and Visceral Pleurae

The parietal pleura is most likely still attached to the inside of the body wall. Although it is a continuous sheet, it is divided into different regions, according to what part of the interior thorax it’s covering:

Costal parietal pleura - the part of the pleura that covers the inner surfaces of the ribs, both anterior and posterior to the lungs.
Mediastinal parietal pleura - the part of the pleura that covers the mediastinum (heart, trachea, esophagus, vertebral column).
Diaphragmatic parietal pleura - the part of the pleura that covers the diaphragm.
Cervical parietal pleura - the part of the pleura that covers the pleural cupola or cupula (the region of the thoracic cavity that is superior to the first rib).

You may be able to pull the thin layer of parietal pleura off the body wall, but you cannot remove the visceral pleura from the lungs without tearing the lung tissue.

Use your hands to explore the pleural cavities and break down any adhesions that may be present between the parietal and visceral pleurae. Be careful of the sharp ends of cut ribs.

Healthy lungs in young people are completely free to move around in the pleural cavities. In older or ill individuals, adhesions may form between the visceral and parietal pleurae, particularly from inflammation of the pleura, a condition called pleurisy or pleuritis.

Clinical Correlation: Collapsed Lung

The pleural cavity is a potential space between the visceral and parietal pleurae that contains a thin layer of fluid. If a sufficient amount of air or fluid enters the pleural cavity, the force holding the visceral to the parietal pleura (the lung to the thoracic wall) is broken, and the lung collapses because of its inherent elasticity (elastic recoil). When a lung collapses, the pleural cavity (normally a potential space) becomes a real space, and may contain air (pneumothorax) or blood (hemothorax).

Pleural Recesses

In some regions, the two layers of pleurae form pleural recesses. These are spaces where the pleura reflects so sharply that adjacent layers of parietal pleura come into contact with each other (e.g. during expiration). During inspiration, lung tissue fills this potential space.

Explore the costodiaphragmatic recess by sliding your fingers along the lateral aspect of the lung in an inferior direction. Your hand will enter a space between the costal parietal pleura and the diaphragmatic parietal pleura below the inferior margin of the lung; this is the costodiaphragmatic recess.

Clinical Correlation: Pleural effusion

The costodiaphragmatic recess is the most inferior part of the pleural cavity, so any pleural effusion will gather there and can be seen as a meniscus sign (red line in the x-ray below) in a standing chest x-ray. Plueral effusion refers to the buildup of fluid in the pleural space.

Costodiaphragmatic recess

Phrenic nerve and pericardiacophrenic vessels

The phrenic nn. that innervate the diaphragm arise from cervical spinal nerves C3-C5 and pass through the neck and into the thorax to reach the diaphragm. Find them now.

To identify the phrenic nn. and the pericardiacophrenic aa. and vv., take the following steps:

Pull the medial margin of both lungs laterally to reveal the pericardial sac.
Look along the anterolateral surface of the pericardial sac, just anterior to the root of the lung, to locate the neurovascular bundles containing the phrenic nn. and pericardiacophrenic vessels.
Dissect through the fibrous tissue and adipose to reveal and observe the neurovascular bundles on both sides of the pericardial sac.
Once you’ve found the phrenic nerves on the surface of the pericardium, follow them through the thorax and into the diaphragm.

Opening the Pericardial Sac

To open the pericardial sac, grab a fold of the pericardium with forceps. Cut the fold with scissors longitudinally from the superior vena cava to the apex of the heart to completely open the sac.

Fold back the parietal pericardium to identify the following:

Fibrous layer of pericardium - the outermost, tough layer.
Parietal serous pericardium - smooth layer lining the inside of the fibrous pericardium, cannot be separated from the fibrous pericardium.
Pericardial cavity (space)
Visceral serous pericardium (= epicardium) - lines the surface of the heart.

Open the pericardial sac widely to better investigate the heart. Do not dissect the posterior portion of the pericardial sac at this time!

Incision line in the pericardium

Zoomed-in view of the pericardium to show the layers

Clinical Correlation:

Position of the Heart Relative to the Sternum and Ribs for Chest Compressions and Auscultation

You should note the location of the heart in relation to the sternum and ribs. In cardiac compression for CPR the sternum is depressed 4 to 5 cm, forcing blood out of the heart and into the great vessels. Fortunately, the one-way valves of the heart prevent backward flow of the blood despite manual compressions. Also note the heart relative to the position of the ribs and sterum which is important for listening to heart sounds.

Clinical Correlation: Cardiac Tamponade

Cardiac tamponade is due to critically increased volume of fluid outside the heart, but inside the pericardial cavity, for example due to stab wounds or from perforation of a weakened area of the heart muscle after heart attack (hemopericardium). As pressure builds in the pericardial cavity, it becomes increasingly difficult for the heart to fill with blood during diastole.

Cardiac tamponade. Blood or other fluid fills the pericardial space and puts pressure on the heart.

Page updated

Report abuse