CHAPTER 6: THORACIC CAVITY (S. 62-63)

1. RESPIRATORY MUSCLES AND THORACIC CAVITY WITH LUNGS

The deep shoulder girdle muscles and the vessels and nerves of the limb, with attention to their roots, are cut as close as possible to the thoracic wall, and the limb is removed. The diaphragmatic line of pleural reflection, where the costal pleura is reflected as the diaphragmatic pleura, is clinically important as the caudoventral boundary of the pleural cavity. In the dorsal end of the 11th intercostal space, a small opening is made through the intercostal muscles into the pleural cavity; then the caudoventral limits of the costodiaphragmatic recess (7) are probed and marked on the ribs as the intercostal muscles are removed. The line extends from the knee of the 7th or 8th rib, through the middle of the 11th, to the angle of the 13th rib at the lateral border of the muscles of the back. The basal border of the lung is also marked on the ribs. After study of the lung field, the ribs, with the exception of the 3rd, 6th, and 13th, are cut above the line of pleural reflection and removed, sparing the diaphragm and noting the slips of origin of the ext. oblique abdominal muscle.

a) The RESPIRATORY MUSCLES (see appendix on myology) belong partly to the muscles of the back and partly to those of the thorax. They function as expiratory muscles in the contraction of the thorax or as inspiratory muscles in the expansion of it. The obligate respiratory muscles are aided by the auxiliary respiratory muscles. The diaphragm is the primary respiratory muscle and the partition between the thoracic and abdominal cavities.

The line of diaphragmatic attachment rises steeply, running across the ribs from the knee of the 8th, across the 11th rib below its middle to the vertebral end of the 13th rib. In ruminants the two costal parts (3) of the diaphragm are clearly separated from the 13–15 cm wide sternal part (not illustrated) by clefts between muscle fibers. The lumbar part (2) resembles that of the horse in its relation to the aortic hiatus and esophageal hiatus, but sends muscle fiber bundles from the right and left crura, sometimes with fibrocartilaginous inlays, to the foramen venae cavae (5). This lies on the right in a relatively large tendinous center (4), which on inspiration is at the level of the 7th rib.

b) The THORACIC CAVITY is protected by the bony thoracic cage [thorax] and extends from the especially narrow cranial thoracic aperture [thoracic inlet] to the diaphragm. It contains the two pleural cavities of unequal size. The pleural sacs project into the thoracic inlet as the cupulae pleurae (15). The left one does not extend beyond the first rib. The right one projects 4–5 cm cranial to the first rib. The parietal pleura includes the costal pleura (6), diaphragmatic pleura (8), and the mediastinal pleura (16), where right and left pleural sacs adjoin and where they cover the pericardium as pericardial pleura (18).

The visceral pleura covers the lungs as the pulmonary pleura, which is connected to the mediastinal pleura by the short pulmonary ligament. This is present only in the caudal area. The mediastinal recess (9) is a diverticulum of the right pleural cavity containing the accessory lobe of the right lung. The costodiaphragmatic recess (7) is the potential space between the basal border of the lung and the diaphragmatic line of pleural reflection. The latter runs slightly craniodorsal to the line of diaphragmatic attachment, dipping ventrally at every intercostal space.

c) The MEDIASTINUM is thicker than in the horse. The heart occupies the middle mediastinum and divides the rest of the mediastinum into cranial (16), caudal, dorsal, and ventral parts. The mediastinum is composed of the two mediastinal pleural layers and the fibrous substantia propria between them. It encloses the usual organs and structures: the esophagus, trachea, blood and lymph vessels, lymph nodes, nerves, and the pericardium.

The cranial mediastinum is pushed against the left thoracic wall in the first and second intercostal spaces, ventral to the great vessels, by the cranial lobe of the right lung. The caudal mediastinum, containing the left phrenic nerve, is attached to the left side of the the diaphragm. Together with a fold on the right, the plica venae cavae (h), they enclose the mediastinal recess (9), containing the accessory lobe of the right lung. Perforations of the mediastinum, allowing communication between right and left pleural cavities, as described in the dog and horse, do not occur in the ox.