» Conformation and Surface Anatomy
The height of the withers and the caudal prolongation of the rib cage make it difficult to obtain a reliable impression of the thoracic cavity from simple inspection of the exterior.
The narrow cranial part of the thorax is completely covered by the shoulder and arm. Some variation in the projection of the limb bones on the thoracic skeleton is due to the inconstant slope of the scapula. As a general guide, the caudal angle of this bone lies over the upper end of the seventh rib, while the supraglenoid tubercle projects in front of the first rib, a little above the manubrium of the sternum (Figs. 20.1 and 20.2). The humerus forms a lesser angle with the horizontal plane than in the smaller species, and this moves the elbow up within the skin of the trunk such that the summit of the olecranon is nearly level with the lowest part of the fifth rib or succeeding intercostal space. The triangle between the scapula and humerus is completely occupied by the massive triceps muscle, which severely restricts clinical access to the cranial part of the thorax.There are 18 pairs of ribs. Those caudal to the seventh rib and lying behind the triceps are individually identifiable on palpation even though they are covered in varying degree by certain muscles: cutaneus trunci, latissimus dorsi, serratus ventralis, and obliquus externus abdominis. The most caudal ribs may even provide visible landmarks. For example, the upper part of the last rib prominently marks the cranial limit of the flank. Palpation of the ribs reveals their changing orientation. The last two or three, which are relatively short, have a pronounced caudal inclination; the half-dozen or so (R9-R15) in front of these are longer and of equal length and curvature. The more cranial ribs are both shorter and less strongly curved. The first rib, the shortest of all, is almost vertical.
The increasing slope of the ribs as the series is followed caudally brings the last rib remarkably close to the coxal tuber (see Fig. 19.1).Between the forelimbs the thorax is covered by the powerful pectoral muscles that form paired swellings separated by a prominent groove along the line of the sternum (see Fig. 23.4). The cranial part of this bone, the manubrium, projects as a readily found landmark. The caudal xiphoid process is also palpable, although it is not quite so easily found as the manubrium. It is broad and flexible and is enclosed between the converging costal arches. External inspection does not convey the reduced depth of the cranial part of the thoracic cavity because of the upward sloping of the sternum toward the manubrium and the ventral slope of the cranial thoracic vertebrae. An exact appreciation of the position of the diaphragm is essential for the clinician. The vertex is level with the sixth intercostal space (or even the sixth rib) and thus comes to within a short distance of the point of the elbow in an animal standing square (Fig. 20.3).
There are naturally considerable breed and individual variations in conformation. Without considering these in detail, it may be said that a deep chest is generally favored. In saddle horses it is desirable that the ribs slope caudally without excessive lateral bowing because too pronounced a "barrel" makes for an uncomfortable seat.