Thorax Lab 1b

Lab 1B: Removal of anterior thoracic wall, examination of heart and lungs

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.

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.
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.

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.
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.

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.

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).

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 n., a., and v. 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.

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).

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.

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 cupola, the part 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.

Anterior view of the thorax with the anterior rib cage removed.

Different regions of the parietal pleura

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).

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 recesses

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.

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!

Zoomed-in view of the pericardium to show the layers

Incision line in the pericardium

Anterior view of the sternum and its anatomical relationship to the heart

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

Clinical Correlation: Chest Compressions and Cardiac Tamponade

You should note the location of the heart in relation to the sternum. In cardiac compression, as in CPR, the sternum is depressed 4 to 5 cm, forcing blood out of the heart and into the great vessels.

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.

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