CONFORMATION AND SURFACE ANATOMY
In the horse the difficulties in obtaining a reliable impression of the thoracic cavity from simple inspection of the exterior are increased by the height of the withers and the caudal prolongation of the rib cage.
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 (Figure 20-1). The humerus forms a lesser angle with the horizontal than in the smaller species, and this brings the elbow within the skin of the trunk. The precise position of the elbow joint is not immediately apparent but may be inferred from its relation to the olecranon whose summit (point of elbow) lies over 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 behind the triceps, that is, those from the seventh rib caudally, are individually identifiable on palpation even though they are covered in varying degree by certain muscles: cataneus trunci, latissimus dorsi, serratus ventralis, and obliquus externus abdominis. The most caudal ribs may even provide visible landmarks; this is most often true of the upper part of the last rib, which 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 (R9-15) 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 Figure 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 Figure 23-4). The cranial part of this bone, the manubrium, projects as a readily found landmark. The caudal xiphoid process is also palpable, though 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 fails to suggest the tilt of the sternum, which slopes upward toward the manubrium; this, in combination with the ventral slope of the cranial thoracic vertebrae, reduces the depth of the cranial part of the thoracic cavity.
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 (Figure 203). The inexperienced find it particularly hard to accept this crucial fact.
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.