But the proportionate measurements of the thorax are different. Indeed, we may recall that in man the thorax is flattened from before backwards, so that the distance between the sternum and the vertebral column is shorter than the distance from the rib of one side to the corresponding one of the opposite side (Fig. 5). In animals, on the contrary, it is flattened laterally. Its vertical diameter--measured from the sternum to the vertebral column--is greater than the transverse measurement (Fig. 6).
[Illustration: FIG. 5.--A TRANSVERSE SECTION OF THE THORAX OF A MAN PLACED VERTICALLY--THAT IS TO SAY, IN THE DIRECTION WHICH IT WOULD ASSUME IN A MAN PLACED IN THE ATTITUDE OF A QUADRUPED (A DIAGRAMMATIC FIGURE).
1, Dorsal vertebra; 2, sternal region; 3, costal region of one side; 3´, costal region of the other side.]
From this results a peculiar arrangement of the muscles that we are able to bring directly into prominence, which presents points of interest from the point of view of the contraction of the subcutaneous layer. Indeed, in man the region occupied by the pectorals is very broad; it is a wide surface turned directly forward. In quadrupeds, this region of the pectorals is narrowed. It is not spread out, as in the preceding instances; and the appearance it presents is explained by the fact that the thorax is compressed laterally. If we examine the thorax on one of its lateral surfaces, the muscles, on the contrary, are more extended. We see the contour of the vertebral column, and the median part of the abdomen; and, especially in the horse, between the great dorsal and the great oblique of the abdomen, we find a large space, in which the ribs, with the intercostals which join them, are uncovered; the muscles in question separate the one from the other, under the influence, it would seem, of the great dimensions of the lateral wall of the thorax.
[Illustration: FIG. 6.--A VERTICAL SECTION OF THE THORAX OF A QUADRUPED (DIAGRAMMATIC).
1, Fifth dorsal vertebra; 2, sternal region; 3, costal region of one side; 3´, costal region of the opposite side.]
=The Sternum.=--The sternum is, in quadrupeds, directed obliquely downwards and backwards; its form varies in different species. In the carnivora, it consists of eight bones, irregularly cylindrical in form, being slightly flattened from within outwards, and thickened at their extremities. They remain separate, and this contributes elasticity and flexibility to the thorax. The first nine costal cartilages articulate directly with the sternum. The first of these cartilages articulates with a nodule situated a little above the middle of the first bone of the sternum.