The extent and dimensions of the thoracic cavity are not apparent on inspection of the live animal.
The inclusion of the upper segments of the forelimbs within the skin of the trunk reveals the narrowness of the cranial part of the thorax but fails to indicate its shallowness or how much of the space enclosed by the ribs is occupied by the abdomen (Figure 27-1).
Certain features of the limb skeleton provide helpful guides to the location of deeper parts: the point of the shoulder projects a few centimeters in front of the lower part of the first rib, the caudal angle of the scapula lies over the vertebrae dorsal to the sixth rib, and the point of the elbow lies over the fifth intercostal space, just above the costochondral joints, and just a short way cranial to the vertex of the diaphragm (Figure 27-2, A-B).The thoracic wall of cattle, unlike that of sheep or goats, is mainly remarkable for the great breadth of the ribs, especially toward their lower ends, and the consequent narrowing of the intercostal spaces.
The ribs from the fifth to the thirteenth may generally be identified with ease, though possibly not palpated along their entire lengths. An increasing obliquity and a stronger bowing is revealed as the series is followed caudally, and there is an increasingly forward slope of their cartilages. The cartilages of the last five (asternal) ribs combine to form the costal arch that defines the cranial limit of the flank; those of the remaining (sternal) ribs join the sternum directly. The cranial part of the chest wall is rigid and contributes little to the respiratory movements; the wider, more caudal part makes a significantly larger contribution, but it is the activity of the diaphragm that predominates. Despite this, cattle survive diaphragmatic paralysis; however, they suffer greater distress than is usual in smaller species.
Surgical access to the thoracic cavity, though rarely indicated in cattle, is hampered by the narrowness of the intercostal spaces and may require resection of one or more ribs. The intercostal vessels follow both margins in the ventral parts of the spaces, which is a point relevant to pleurocentesis, which is best performed by puncture of the sixth or seventh space directly above the level of the costochondral joints.