OSTEOLOGY AND ARTHROLOGY 15 



half. The six bones which form the sternum are connected 

 by cartilage. The keel-shaped piece, situated in front of the 

 sternum, is also cartilaginous. This process, but slightly 

 marked posteriorly, becomes more and more prominent in 

 front, and terminates at its anterior extremity by a prolonga- 

 tion, slightly curved backwards, which projects for some 

 centimetres beyond the cavity in which the first costal 

 cartilage is received. This process is known as the tracheal 

 process, or rostral cartilage. The posterior extremity of the 

 sternum, flattened from above downwards, ends in a car- 

 tilaginous plate ; concave superiorly, and convex inferiorly : 

 this is the abdominal prolongation, or xiphoid appendix. 



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 articu- 

 lation 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 mam- 

 malia, 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 



