Part 4: Opening the Thorax

To prepare for the internal thorax dissection in Chapter 4, we will now use the pruning shears/snips provided in the lab to open up the thoracic cavity. Follow the steps outlined below and reference the demonstration animals for further guidance if you need help.

Using the snips provided, make a sagittal incision at least 1cm away from the sternum on either side (make two cuts, one on either side, parallel with the sternum). Note that it is best to make these cuts lateral to the emergence of the cranial epigastric a. on either side of the sternum to preserve the internal thoracic aa. within the thorax.

• Begin these cuts caudally, alongside the xiphoid of the sternum, and extend them cranially through the first rib/thoracic inlet (cranial boundary of the thoracic cavity) on BOTH sides of the sternum (cutting parallel to the sternum on both sides). Be sure you are far enough lateral/away from the sternum that you avoid cutting the cranial epigastric artery (and internal thoracic a.)!

  • • Dissection Note: You may see the transversus thoracis m. which is a flat, fleshy muscle found inside the thorax on the medial surface of the costal cartilages (ribs 2 through 8) extending from the costochondral junctions to the sternum. (See DG Fig. 3-4, A & B) If needed, cut through the transversus thoracis m. along the cuts made parallel to the sternum (if you did not already cut through it when making your original cuts alongside the sternum).

Identify the diaphragm connecting to the ribcage inside the thorax and, using the snips provided, carefully make a vertical cut through the thoracic wall, and associated abdominal and intercostal muscles, (on both left and right sides) just CRANIAL to the diaphragm so that you do not enter the abdominal cavity.

Pull open the LEFT side of the rib cage, and using the snips provided in lab, begin INSIDE the ribcage and snip each individual rib about 2cm lateral to its vertebral articulation (this will hopefully avoid damage to the sympathetic trunk which will be dissected in a later lab). Cut only the ribs, from WITHIN THE THORAX, starting caudally and moving cranially to the first rib.

• Repeat this same procedure on the RIGHT side of the thorax.

When you are finished, each side of the ribcage should remain attached, but hinge open like a door.

• Comment: If you have questions on how to complete this procedure please reference the demonstration animals.