Part 1: Pleural Cavity/Space and Diaphragm

IMPORTANT NOTE: Be sure the thoracic cavity has been opened on your specimen before you begin this dissection!  (This step was completed at the end of Chapter 3; please reference Chapter 3, Part 4: Opening the Thorax as needed.)

1. Recall that between consecutive ribs there are intercostal spaces which, as mentioned in Chapter 3, contain the intercostal muscles (external and internal). On the internal surface of the thoracic wall, in the intercostal spaces between the ribs, identify a few of the intercostal nn. and intercostal aa. (dorsal and ventral) coursing along the caudal aspect of each rib. (See DG Figs. 3-4A, 3-5 and 3-6)

   Related Learning Objective

   • D4.1

    

   • Note that intercostal aa. can be regionally named as either dorsal or ventral. The ventral intercostal arteries arise from the internal thoracic artery (ventrally) while most of the dorsal intercostal arteries arise directly from the aorta (dorsally), with exceptions for the first few that come from a branch of the costocervical trunk. The dorsal and ventral intercostal aa. (and vv.) then anastomose with each other midway along the length of the ribs, creating a nice example of an arterial loop within the thorax.

     • Note the regional differences of the dorsal and ventral intercostal arteries on your specimen.

2. Within the thorax you will find pleurae, which are the serous membranes that cover the lungs and line the walls of the thoracic cavity. These pleurae form right and left ‘sacs’ that enclose the pleural cavities/spaces (R & L). Each ‘sac’ consists of visceral and parietal layers; the pleural cavity/space is found between the two layers of pleura (visceral and parietal) within the thoracic cavity. Identify the pleural cavities/spaces in your specimen. Within the pleural cavity identify the layers of pleura described. (See DG Figs. 3-8A, and 3-9A)

   Related Learning Objective

   • D4.2

    

   • Identify the pulmonary (visceral) pleura that is closely adhered to the surfaces of the lungs.

   • Identify the parietal pleura lining the thoracic wall. Parietal pleura consists of costal, diaphragmatic, and mediastinal pleurae.

     • • Costal pleura covers the inner surfaces of the ribs (and associated muscles).

       • Diaphragmatic pleura covers the cranial surface of the diaphragm that bulges into the thoracic cavity.

       • Mediastinal pleurae are the layers of pleura that cover the partition between the two pleural cavities (the mediastinum).

         • • Dissection Note: The mediastinum includes the two layers of mediastinal pleurae and the space between them.

           • Comment: There are two named parts of the mediastinal pleura, pericardial mediastinal pleura and plica venae cavae, but you do not need to identify them in your specimen. The pericardial mediastinal pleura is the pleura covering the heart and the plica vena cavae is a loose fold of pleura that surrounds the caudal vena cava.

   • Identify the region of the root of the lung which is composed of the pleura and the bronchi, vessels, and nerves entering each lung. At the root of the lung, the mediastinal parietal pleura is continuous with the pulmonary (visceral) pleura via a very small connecting pleural layer.

     • • Comment: Caudal to the hilus this connection forms a free border, known as the pulmonary ligament, between the caudal lobe of the lung (both left and right) and the mediastinum. (See DG Figs. 3-9A, and 3-10)

         Related Learning Objective

         • D4.3

          

3. Identify the mediastinum and note that it can be divided into a cranial part (cranial to the heart), a middle part (containing the heart), a dorsal part (dorsal to the heart), a ventral part (ventral to the heart), and a caudal part (caudal to the heart). As noted previously, the mediastinum includes the two layers of mediastinal pleurae and the space between them. Enclosed in the mediastinum you will find the following: thymus, thoracic lymph nodes, heart, aorta, trachea, esophagus, vagus nerves, as well as other nerves and vessels. You should know the contents of the mediastinum and be able to describe their general placement within the mediastinum.

   • Identify the thymus within the mediastinum (if possible).

     • • Dissection Note: The thymus is a bilobed structure that atrophies as the animal ages (i.e., young animals will have a large thymus, old animals will have little to none present). The thymus is located in the cranial mediastinum (cranial to the heart). (See DG Figs. 3-8 A, 3-14A, 3-16 and 3-20)

   • Identify the esophagus within the mediastinum and note that it is found in the dorsal mediastinum (dorsal to the heart). Note that you will uncover more of the esophagus as you progress through this dissection, so don’t spend too much time on it now. (See DG Figs. 3-9A, 3-11, 3-14A, 3-16, and 3-20)

     • • Dissection Note: Dissect carefully (blunt dissection with scissors) when looking for the esophagus in the dorsal mediastinum so that you do not cut vessels and nerves you still need to identify.

   • Comment: We will not dissect all of the thoracic lymph nodes, but you should be aware of their general placement within the mediastinum (as it relates to the Application portion of the course and also radiology); they are regionally named (sternal, cranial mediastinal, tracheobronchial). Sternal lymph nodes will be situated near the sternum in the ventral mediastinum. Cranial mediastinal lymph nodes will be in the cranial mediastinum (cranial to the heart) and tracheobronchial lymph nodes will be near the hilus of the lungs and the bifurcation of the trachea in the dorsal mediastinum (which we will look for later in this lab).

4. Inside the chest cavity, running alongside the deep surface of the sternum, on both left and right side, identify the internal thoracic artery. (Note that the transversus thoracis muscle may obscure your view of this artery and should be reflected if necessary.) (See DG Figs. 3-5, 3-14A, 3-16, 3-19 and 3-20)

   • Dissection Note: As previously mentioned in Chapter 3, the internal thoracic artery is continued as the cranial epigastric artery after it pierces through the body wall between the costal arch and the sternum.

5. In the caudal part of the thoracic cavity, identify the diaphragm which is the muscular partition between the thoracic and abdominal cavities. After identification of the diaphragm, identify and name its various parts: tendinous center, lumbar part (left crus & right crus), costal part, sternal part and cupula. (See DG Fig. 4-15)

   Related Learning Objective

   • D4.4

    

   • The tendinous center is a V-shaped tendinous sheet in the middle portion of the diaphragm.

   • The muscular parts of the diaphragm (attached to the tendinous center) are the lumbar, costal, and sternal parts; these are named according to their regions of attachment.

     • • The sternal part is narrow and is attached to the sternum just cranial to the xiphoid cartilage.

       • The costal part is broader than the sternal part and attaches along the deep face of the ribs.

       • The lumbar part attaches to lumbar vertebrae and forms the left crus and the right crus. Note that the right crus is typically larger than the left crus. The crura (plural) are somewhat triangular slips of muscle that surround structures passing through the diaphragm.

       • The cupula is the dome-shaped cranial extent of the diaphragm that bulges into the thorax.

6. Identify the three ‘passageways’ through the diaphragm, the aortic hiatus, esophageal hiatus, and caval foramen. (See DG Fig. 4-15)

   Related Learning Objective

   • D4.4

    

   • • The aortic hiatus is located dorsally in the central diaphragm, between the muscular crura. It allows for the passage of the aorta, azygos vein, and thoracic duct through the diaphragm.

     • The esophageal hiatus is centrally located in the diaphragm, between the muscular crura. It allows for the passage of the esophagus, esophageal vessels, and vagal nerve trunks through the diaphragm.

     • The caval foramen is usually found near the junction of the muscular part and the tendinous part, slightly to the right side of the diaphragm. It allows for the passage of the caudal vena cava through the diaphragm.