In the ox, the sternum is formed of two distinct bones, which are united by an articulation. One, the anterior, is short, and forms the first portion of the sternum; it is slightly flattened from side to side, and vertical in direction. The other, the posterior, is longer, and is formed by the fusion of several small bones; it is placed horizontally, and is flattened from above downwards. At the level of articulation of these two portions, and because of their different directions, the bone is bent. This bend occurs at the point of articulation of the second costal cartilage. On the superior border of the anterior segment the cartilage of the first rib is articulated. The xiphoid appendix, which is cartilaginous, is attached to the extremity of a long process of the last bone of the sternum. The shape of the anterior extremity of the sternum is influenced by the presence or absence of clavicles. We have seen that in some quadrupeds the clavicles are wanting. In the first case, this extremity is large, and approaches in shape to the corresponding part of the human sternum, which is so clearly designed to give a point of support to the anterior bone of the shoulder. In the second, on the contrary, this extremity is narrow. The sternum in birds is very different from that in mammalia, which we have been studying. It varies greatly in extent and shape, under the influence of certain conditions. To understand the cause of these variations it is necessary to remember that in man (as, indeed, in other animals; but the example of man, for that which follows, will be more striking, on account of the mobility of his upper limbs) the sternum gives origin to the pectoral muscles, and that these muscles are inserted into other parts of the thoracic limbs, designed by their contraction to draw the arms downwards, forwards, and inwards--that is, when these are in a state of abduction and in a horizontal direction, they draw them towards the anterior surface of the thorax and downwards. Now, this movement is similar to that made by birds during flight. It is necessary to add that, in the latter case, the more the displacement of the upper limbs has of force and extent, the more the pectoral muscles are developed. For these reasons, birds, in which, during flight, the movements of the thoracic limbs--the wings--are necessarily energetic, present a great development of the pectoral muscles; having consequently, because an extent of surface for the origin of the muscles commensurate with their development is necessary, a very large and peculiarly shaped sternum (Figs. 18, 6; and 21, 6). Indeed, not only is the sternum large, but, further, in order to form a deeper surface, proportionately adapted to the muscles which arise from and cover it, its anterior surface presents, in the median line, a prominent crest known as the _keel_. This prominence forms two lateral fossæ. We cite as examples, the sternum of the eagle, the vulture, the falcon, and the hawk.