The Thorax
2.7 Pleura
Now we are ready to enter the thorax. Remove all of the muscles on the thoracic wall until you uncover the ribs. Care is necessary when remov- ing the intercostal muscles. By making vertical cuts along the cranial and caudal borders of the ribs (watch out for embalming fluid oozing out), being careful not to cut too deep, remove the muscles between all ribs.
Notice the intercostal vessels and nerves (vein, artery, and nerve) running along the caudal border of each rib. At the costochon- dral junctions these neurovascular bundles are present along both the cranial and caudal borders of the ribs. They are more visible inside the thorax deep to the transparent costal pleura.
You may choose one of two methods to expose thoracic viscera.
Method 1: Once you have cleaned the mus- cles of the thoracic wall, use a large nipper, pruning shears, or saw to cut each of the ribs at their vertebral and sternal attachments and along the costal arch but leave the first two ribs in place. These two ribs will be removed later when you are ready to visualize the vessels and nerves in the cranial thoracic region (Figure 2.8).
Method 2: This method is suitable for small and large cadavers laid on the table. It is
described in Appendix A. In this method, reflect the ribs dorsally and cut the diaphragm and abdominal wall ventrally along the dorsal edge of the rectus abdominis muscle. Reflect the ribs and abdominal muscles dorsally as a single flap (see Figure A.2 in Appendix A).
In both methods, use your probe and forceps to perform blunt dissection of intrathoracic structures.
Within the thorax, identify the transversus thoracis muscle fibers coursing on a trans- verse plane over the dorsal aspect of the ster- num. The internal thoracic artery and vein run deep to this muscle. Remember that these vessels are found on the other side of the body. Dorsally, identify the longus colli muscle as it courses from under the body of caudal cervical vertebrae passing through the thoracic inlet to continue along the ventral surface of the bodies of the first few thoracic vertebrae (Figure 2.5).
pleura, and connecting pleura). Identify the location of the thymus, caudal medi- astinal lymph nodes, and major bloods vessels, autonomic ganglia, and nerves (vagus and phrenic nerves, and sympa- thetic trunk and ganglia). Explain the meaning of the term “mediastinum” and its contents. Study the superficial and internal features of the heart.
Before you disturb or remove any structure, study the pleura, and lung lobes. The thin transparent serous membrane (mesothe- lium) that lines the interior of the thoracic cavity and its contents is the pleura.
Depending on where it is located or what it covers, the pleura is divided into parietal, visceral (pulmonary), or connecting pleura.
Again, note the vessels and nerves running at the caudal border of each rib. These are intercostal arteries, veins and nerves.
2.7.1 Parietal Pleura
The parietal pleura covers interior thoracic walls such as those of the ribs and diaphragm.
It is attached to the ribs and diaphragm by thin layer of connective tissue known as endothoracic fascia. Identify the following types of parietal pleura:
● Costal parietal pleura: covers the interior wall of the rib cage.
● Diaphragmatic parietal pleura: covers the thoracic surface of the diaphragm.
● Mediastinal parietal pleura: creates the thin partition (mediastinum) between the left and right pleural cavities.
2.7.2 Visceral Pleura
The visceral pleura is also known as the pul- monary pleura. It covers all surfaces of the lungs.
2.7.3 Connecting Pleura
An example of connecting pleura is the pul- monary ligament that connects the visceral pleura on the caudal lobe of the lung to the mediastinal pleura (Figure 2.9). Lift the cau- dal lobe of the lung cranially to observe the pulmonary ligament. In small animals, this ligament is transected in surgery when the caudal lobe of the lung is removed.
Left cranial lung lobe (cranial part)
Left cranial lung lobe (caudal part) Left caudal lung lobe
Cranial
Left cardiac notch
(a)
Principal bronchus
Esophageal impression Aortic impression Cardiac impression Pulmonary ligament
Ox
Right cranial lung lobe (cranial and caudal parts)
Right middle lung lobe Right caudal lung lobe Cranial
Tracheal bronchus
(b)
Right cardiac notch
Accessory lobe
Principal bronchus
Ox
Esophageal impression Aortic impression
Cardiac impression
Figure 2.9 Medial surfaces of the left (a) and right (b) bovine lungs.
Box 2.3
Surgical incision into the thorax through the intercostal muscles should be made midway through the intercostal space between the ribs. This is necessary to avoid damaging the intercostal vessels and nerves running at the caudal border of each rib.
Plica vena cava is another example of connecting pleura. It runs from the ventral surface of the caudal vena cava to the floor of caudal mediastinum and diaphragm.
The pleural sac on the right side extends through the thoracic inlet beyond the first rib. This cup‐shaped extension is known as the cupula pleura. The left pleural sac does not extend beyond the first rib.
2.7.4 Content of the Pleural Cavity
The pleural cavity has nothing inside but a small amount of serous fluid. The fluid helps in friction‐free movement of the lungs as they expand with air during inspiration.
2.7.5 Lung Lobes
Identify the lung lobes. The left lung has cranial and caudal lobes (Figure 2.9). A notch divides the cranial lobe into cranial and caudal parts. The left cardiac notch typically is located between the 3rd and 6th rib in the live animal.
The right lung has four lobes: cranial, middle, caudal, and accessory lobes.
The cranial lobe of the right lung is separately ventilated by a special tracheal bronchus that arises directly from the trachea. The trachea then terminates caudally by bifurcating into the right and left principal bronchi (Figure 2.9).
Remove the lungs by reflecting them dor- sally and cutting the vessels at the hilus or root of the lung. Stay close to the root of lung and be careful not to cut the phrenic nerve and branches of the vagus nerve that are close to the surface of the heart.
Study the mediastinum and remove the mediastinal pleura to study the heart, lymphocenters, vessels cranial to the heart, topography of the trachea and esophagus, thoracic duct, pathway of the vagus nerves, and sympathetic trunk (Figure 2.8).
2.7.6 Mediastinum
The mediastinum is the partition between the left and right pleural cavity. It consists of the mediastinal pleurae and organs contained between them including the thymus, heart, trachea, esophagus, and major blood vessels.
It is divided into cranial, middle, and caudal mediastinum. Examples of organs in the three parts of the mediastinum include the thymus in cranial mediastinum, heart in the middle mediastinum, and caudal portion of thoracic aorta and caudal portion of the esophagus in the caudal mediastinum.
Box 2.4
The cupula pleura is vulnerable to puncture by sharp objects leading to rush of free air into the pleural sac (condition is known as pneumothorax). This is especially serious in the horse should they run into a sharp objects such as a fence post.
Box 2.5
● Pneumonia is the most clinical problem affecting lungs in cattle.
● Endotracheal intubation is performed after induction of anesthesia. This procedure is best accomplished with the patient in sternal recumbency with the head and neck held in extension. Be cautious not to obstruct the tra- cheal bronchus when using an endotracheal tube with inflatable cuffs.
● Lung auscultation: on a triangular area deter- mined by the following anatomic boundaries:
Cranial: caudal border of the triceps brachii muscle
Dorsal: epaxial muscles
Caudal: the curved line for identification of the basal border of the lungs along the 6‐10‐11 ribs.
● Thoracentesis: a procedure for removal of access inflammatory fluid from the chest.
Cannulation can generally be performed at the lower 7th intercostal space (7 ICS).