Part 3: Opening the Thorax

1. To prepare for the internal thorax dissection in Chapter 4, we will now use the Stryker saws and/or large pruning shears 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.
2. ALL specimens: As you open the thoracic cavity, identify the line of pleural reflection and the costodiaphragmatic recess through removal of intercostal mm. and soft tissues (proceed as directed).
   • The purpose of the following dissection is to demonstrate the line of pleural reflection (the line along which diaphragmatic pleura reflects off the diaphragm and onto the ribs as costal pleura). This clinically important landmark is the caudoventral extent of the pleural cavities.
     • The pleura reflect in a narrow angled space between the convex surface of the diaphragm and the thoracic body wall. This space is the costodiaphragmatic recess.
3. ALL specimens: Identify the line of pleural reflection on Figs. 3-9 (pony) or 3-10 (calf) and compare the image to your specimen.
4. ALL specimens: Remove the intercostal muscles (and associated soft tissue) from the intercostal spaces between each consecutive rib, except for the last three (pony) or two (calf) intercostal spaces. Begin this process caudally and proceed cranially.

• • Dissection Note: You should note that there are two types of intercostal mm., external and internal, but you do not need to differentiate them in your dissection.
  • In each intercostal space, begin your incisions dorsally (along the ventral edge of the longissimus m. system) and remove the intercostal muscles and soft tissue slowly, cutting cranially and caudally along each rib, noting the shiny surface of the pleural membrane on the inner surface as you reflect the soft tissues ventrally.
  • As you approach the ventral aspect of each intercostal space, probe with your finger inside the thorax/ribs to palpate the location of the reflection of pleura onto the diaphragm.
    • Dissection Note: Your finger will reach a ‘stop’ point where you cannot move further ventrally, this is the point of the pleural reflection (i.e., the line of pleural reflection).
  • Remove the intercostal muscles and other soft tissues down to the line of pleural reflection BUT NOT BELOW IT. 
    • Note that the line of pleural reflection curves dorsocaudally away from the costochondral junction of the 9th rib until it reaches the middle of the last rib where it then turns medially and ends dorsally in the last intercostal space.

Figure 3-9. Equine thorax, lateral view. The dotted line (1) is an outline of the heart; shown in black is where the heart is exposed by the cardiac notch; 2, basal border of lung; 3, line of pleural reflection.

Figure 3-10. Bovine thorax, lateral view. The dotted line (1) is an outline of the heart; 2, caudal extent of heart; 3, basal border of lung; 4, line of pleural reflection.

5. Use a Stryker saw or large pruning shears to remove the ribs (as directed below) to allow adequate visualization of the internal thorax in Chapter 4. Cut each rib as far dorsally as possible (just ventral to the longissimus m. system). Ventrally, cut each rib at/near (but not below) the line of pleural reflection.

• • ALL (PONY) specimens: Remove all ribs except 7 and 15-18 by making two cuts (one dorsal and one ventral) with a Stryker saw or large pruning shears.
    • Important Dissection Note: Be sure to remove the first rib, it will make visualization of other structures in Chapter 4 much easier!
  • ALL (CALF) specimens: Remove all ribs except 12-13 by making two cuts (one dorsal and one ventral) with a Stryker saw or large pruning shears.
    • Important Dissection Note: Be sure to remove the first rib, it will make visualization of other structures in Chapter 4 much easier!
  • ALL specimens:
    • Dorsally, cut each rib at the ventral edge of the longissimus muscle system.
      • • Important Dissection Note: In order to successfully complete the thorax dissection in Chapter 4, ribs should be removed as far dorsally as possible.
    • Ventrally, cut the ribs at (or just above) the line of pleural reflection.
      • • Dissection Note: Ventrally, many of the ribs can simply be broken off/cut at their costochondral junctions.