Session Three: Thoracic Wall, Pleura, & Pericardium

Reflect the anterior thoracic wall

• 1. Detach the serratus anterior m. from its proximal attachments to the ribs 1-8. Reflect the muscle superolaterally to expose the ribs and intercostal spaces.

Photo 1. Reflected serratus anterior m.

• 1. Remove remaining skin superficial to the clavicles, but DO NOT remove skin superior to the clavicles Take great care in this region to protect underlying neurovasculature.

  2. Cut the sternocleidomastoid mm. from its inferior attachments to the manubrium and clavicle. Detach the infrahyoid (strap) mm. from the posterior portion of the manubrium. If the thymus gland extends into this area, use blunt dissection to reflect this superiorly.

Photo 2. Detached sternocleidomastoid and infrahyoid mm.

• 1. Palpate the entire length of the clavicles. Remove any soft tissue on the anterior portions of the clavicles. Depress any contents deep to the clavicles to protect from damage.

Note: Make sure all soft tissue is removed from the clavicles before cutting. This will allow for a cleaner and more exact cut.

• 1. Cut through each clavicle with a small saw, just medial to the clavicular attachment of the trapezius muscle.

Photo 3. Clavicular cut

• 1. Free any clavicular attachments (subclavius mm. and costoclavicular ligaments) to the 1st ribs. This can be completed by blunt dissection and scissor cuts, if necessary. An important collection of neurovasculature and some muscle attachments are found just deep to the clavicle.

Photo 4. Subclavius m. and costoclavicular ligament

Photo 5. Protection of neurovasculature and muscle attachments.

• 1. Disarticulate the manubrium and the 1st ribs. This will involve chiseling or cutting (using a chisel or bone shears) through the synchondroses (cartilaginous joints) of the 1st ribs.

Photo 6. Disarticulation of the sychondroses of the 1st rib

• 1. Using scissors, cut the intercostal mm. in the intercostal space inferior to the 1st ribs, along the midaxillary line. Cut (using bone shears) the 2nd ribs along the midaxillary line. Continue cutting through intercostal mm. and ribs inferiorly to the 6th ribs.

Photo 7. Soft tissue and bone cuts

• 1. Lift the clavicles and manubrium anteriorly, and locate the internal thoracic aa. and vv. as they pass deep to the 1st ribs. Cut this vasculature at this level.

Photo 8. Internal thoracic vasculature cut

• 1. ‘Hinge’ (pull anteriorly) the cut portions of the anterior thorax to uncover deep structures.

Note: When pulling, this needs to be done forcibly to break ribs or tissues not completely cut in previous step. Care should be taken to not allow the sharp ends of cut bone to contact your skin.

Photo 9. Anterior thoracic wall hinged

Identify intercostal mm. and neurovasculature

Find these structures:

Check structures as you find them on the donor.

• • External intercostal mm.

  • External intercostal membrane

  • Internal intercostal mm.

  • Innermost intercostal mm.

  • Internal thoracic a. and v.

  • Intercostal neurovascular bundle

• 1. Return the anterior thoracic wall to anatomical position (i.e. ‘un-hinge’). Beginning at the cut mid-axillary line, identify the external intercostal mm. in each intercostal space.

Note: the muscle fibers run inferomedially from the rib above to the rib below. Think ‘putting your hands in your front pants pockets’ - the muscle fibers run the same direction.

Notice the transition from muscle fibers to external intercostal membranes as you move anteriorly and towards the sternum.

Photo 10. External intercostal mm. and membrane

• 1. In two intercostal spaces, carefully remove the external intercostal mm. and membranes to expose the internal intercostal mm.

Note: Start medially (close to the sternum), and remove the white and fibrous external intercostal membranes. These membranes are easily distinguishable from the deep internal intercostal mm. fibers.

Note: The internal intercostal m. fibers run perpendicular to the external intercostal m. fibers. The internal intercostal membrane is not visible in this dissection. It is posterior and close to the vertebrae.

Photo 11. Internal intercostal mm.

• 1. Hinge the anterior thoracic wall to get a clear view of the inner surface of the anterior thoracic wall. Uncover the innermost intercostal mm. by removing investing fascia. These are the deepest of the intercostal mm., and these muscle fibers run in the same direction as the internal intercostal mm. The intercostal neurovasculature runs between the internal intercostal and innermost intercostal mm.

The transversus thoracis mm. are also visible in this view. Remove any remaining investing fascia. These mm. are not located exclusively in the intercostal spaces. The proximal attachment for these mm. are the inferior sternum, and the fibers radiate superolaterally to ribs and costal cartilages.

Photo 12. Innermost intercostal and transversus thoracis mm.

• 1. Locate the internal thoracic aa. and vv. They were cut to allow the anterior thoracic wall to be reflected.

Photo 13. Internal thoracic a. and v.

• 1. Locate anterior intercostal branches of the internal thoracic aa. for the first 5-6 intercostal spaces (there are 2 branches per intercostal space). These branches anastomose with posterior intercostal aa. (from the superior intercostal aa. and the thoracic aorta) to supply blood to the thoracic wall. Follow one or two branches laterally to locate the intercostal neurovascular bundle. Innermost intercostal mm. must be removed to follow the neurovasculature laterally.

Note: Intercostal neurovascular bundles consist of a vein, artery, and nerve (vein nearest to the rib). The arteries are supplied anteriorly via anterior branches of the internal thoracic arteries (intercostal spaces 1-6) or the musculophrenic aa. (intercostal spaces 7-9), and posteriorly by posterior intercostal aa., which arise from either the superior intercostal aa. (intercostal spaces 1 & 2), or the thoracic aorta (intercostal spaces 3-11). The veins are similarly arranged, flowing anteriorly into the internal thoracic vv., and posteriorly into either the brachiocephalic vv. (intercostal spaces 1), superior intercostal vv. (typically for intercostal spaces 2-3, sometimes 4), and the azygos system (typically for intercostal spaces 4-11). All of the nerves are the ventral primary rami of thoracic spinal nn. 1-11.

Photo 14. Intercostal neurovasculature

There are two intercostal neurovascular bundles: larger superior bundles, and smaller inferior bundles. The superior bundles are easier to locate, as they lie in the costal groove of the inferior border of ribs.

Note: A useful mnemonic to remember the order of the superior neurovascular bundle is VAN: vein is superior, artery is intermediate, and nerve is inferior. The order is opposite for the inferior bundle: NAV, with the vein lying nearest to the rib below.

• 1. Follow each internal thoracic a. and v. inferiorly. Cut the transversus thoracis mm. from their attachments to the sternum, and reflect laterally. Expose the internal thoracic aa. and their points of bifurcation into the superior epigastric aa. and musculophrenic aa. around the sixth intercostal space.

Note: The superior epigastric a. is the direct continuation of the internal thoracic a. The musculophrenic a. is the lateral terminal branch.

Photo 15: Superior epigastric a. and musculophrenic a.

Explore each pleura and its recesses

Find these structures:

Check structures as you find them on the donor.

• • Parietal pleura

  • Visceral pleura

  • Costodiaphragmatic recess

  • Costomediastinal recess

• 1. Locate the parietal and visceral pleura.

Note: The pulmonary cavities are lined with parietal pleurae.  The parietal pleurae each has three parts: costal, diaphragmatic, and mediastinal. Cervical pleurae (extensions of the mediastinal and costal parts) form domes over the apices of each lung.  Visceral pleurae adhere to all surfaces of the lungs. The spaces between parietal and visceral pleurae are the pleural cavities.

Photo 16. Parietal and visceral pleura

• 1. Define and explore the costodiaphragmatic recesses and costomediastinal recesses.

There are several recesses/potential spaces in this area. Explore the costodiaphragmatic recesses with your hand. The costodiaphragmatic recesses are the potential spaces between the diaphragmatic and costal parietal pleurae.

The superior margins of the costodiaphragmatic recesses vary based on anatomical line: 8th rib in midclavicular line, 10th rib in the midaxillary line, and 12th rib at the paravertebral line.

Photo 17. Costodiaphragmatic recess

The costomediastinal recesses are potential spaces between the costal and mediastinal parietal pleura. The left costomediastinal recess is larger because of the location of the heart and the cardiac notch of the left lung. The costomediastinal may best be seen on a transverse section of a plastinated or prosected thorax.

Photo 18. Costomediastinal recess

Identify the pericardium, open the pericardium, and examine the great vessels and closely surrounding neurovasculature.

Find these structures:

Check structures as you find them on the donor.

• • Pericardium

    • Fibrous pericardium

    • Serous pericardium

      • Parietal layer

      • Pericardial cavity

      • Visceral layer (epicardium)

  • Phrenic n.

  • Pericardiacophrenic a. & v.

  • Superior vena cava

  • Inferior vena cava

  • Pulmonary trunk

  • Pulmonary aa.

  • Pulmonary vv.

  • Ascending aorta

  • Arch of aorta (aortic arch)

  • Brachiocephalic trunk

  • L. common carotid a.

  • L. subclavian a.

  • Ligamentum arteriosum

  • L. superior intercostal v.

  • L. recurrent laryngeal n.

• 1. Find the pericardium. Identify the fibrous pericardium.

Note: The pericardium is comprised of a fibrous pericardium (externally) and a serous pericardium (internally). The serous pericardium has two parts: the parietal layer, which adheres to the fibrous pericardium to form the pericardial sac, and the visceral layer (aka the epicardium), which adheres to the surface of the heart. The space between the parietal and visceral layers is the pericardial cavity.

Photo 19. Anterior pericardium

• 1. On each side of the pericardium, strip away the mediastinal pleura to reveal the phrenic n. and pericardiacophrenic a. & v.

Note: The phrenic n. and pericardiacophrenic vasculature are often adhered to the lateral portion of the pericardium, anchored under a layer of mediastinal pleura. If this is the case, use blunt dissection to carefully separate the vasculature from the fibrous pericardium.

Photo 20. Phrenic n. and pericardiacophrenic a. & v.

• 1. Open the pericardium by making a vertical midline incision (with scissors) through the fibrous pericardium from the diaphragm to the arch of the aorta.

Note: The parietal layer of the serous pericardium adheres to the fibrous pericardium, and also is cut in the step above.

Photo 21. Midline incision of pericardium

• 1. Make a second incision, from the apex of the heart to the right border along the diaphragmatic surface. Additionally, note where the fibrous pericardium covers the bases of the great vessels. To better see the great vessels, reflect the fibrous pericardium laterally in this location as well.

Photo 22. Inferior incision of the pericardium

• 1. Identify the great vessels: superior and inferior vena cavae, pulmonary trunk, and ascending aorta.

Photo 23. Superior vena cava, ascending aorta, and pulmonary trunk

• 1. Following the ascending aorta superiorly, locate the arch of aorta and its three branches: brachiocephalic trunk, L. common carotid a., and L. subclavian a.

Photo 24. Aortic arch, brachiocephalic trunk, L. common carotid, and L. subclavian a.

• 1. Locate the ligamentum arteriosum. This structure connects the inferior surface of the aortic arch to the origin of the L. pulmonary a. and is the remnant of the fetal ductus arteriosus.

Note: Lifting the aortic arch superiorly yields the clearest view of the ligamentum arteriosum.

Photo 25. Ligamentum arteriosum

While lifting the aortic arch, locate the L. superior intercostal vein, which typically lies anterior to the aortic arch, and typically drains into the L. brachiocephalic v.

Photo 26. L. superior intercostal v.

Locate the L. recurrent laryngeal n. as it branches from the L. vagus n. The L. recurrent laryngeal n. passes under the aortic arch and loops superiorly toward the neck.

Photo 27. L. vagus n. and L. recurrent laryngeal n.

Locate the oblique and transverse pericardial sinuses.

• 1. With the pericardium opened, locate the oblique pericardial sinus.

Note: Lift the apex of the heart, and slip several fingers behind the heart. This is the oblique pericardial sinus. It is bounded by pericardial reflections. Your fingers will be stopped from moving superiorly at the cul-de-sac formed by the pericardial reflection near the base of the heart.

Photo 28. Oblique pericardial sinus

• 1. Locate the transverse pericardial sinus. This can be located by inserting your index finger behind the pulmonary trunk and ascending aorta. Once this is completed, your finger will be in the transverse pericardial sinus.

Photo 29. Transverse pericardial sinus

Examine the gross features of the heart.

Find these structures:

Check structures as you find them on the donor.

• • Apex

  • R. border

  • L. border

  • Diaphragmatic surface

  • Coronary sulcus

  • Anterior interventricular sulcus

  • R. atrium

  • L. atrium

  • R. auricle

  • L. auricle

  • R. ventricle

  • L. ventricle

• 1. Observe the gross features of the heart: apex, R. border, L. border, diaphragmatic surface, coronary sulcus, anterior interventricular sulcus, R. & L. atria, R. & L. ventricles, and R. & L. auricles. DO NOT remove the heart from the pericardial cavity.

Photo 30: Apex, R. & L. border, & diaphragmatic surface

Photo 31. Coronary sulcus & anterior interventricular sulcus

Photo 32. R. auricle of atrium

Photo 33. R. & L. ventricles