COMPOSITION OF STERNUM 



brates, the sternum of the Mammalia consists of a series of pieces, 

 as many as eight or nine or even sixteen in Choloepus, of which 

 the first is called the manubrium sterni, and the last the ensifonn 

 cartilage, xiphisternum, or xiphoid process. The latter often 

 remains largely cartilaginous throughout life ; in fact this is 

 generally but not universally the case with that part of the 

 breastbone. The most extraordinary modification of the xiphoid 

 process is seen in the African species of the genus Manis, where 

 it diverges into two long cartilages, which run 

 back to the pelvis and then, curving round, run 

 forwards and fuse together in the middle line 

 anteriorly. These processes serve for the attach- 

 ment of certain tongue-muscles. They were looked 

 upon by Professor Parker as the equivalents of 

 the " abdominal ribs " of reptiles elsewhere non- 

 existent among mammals. This view is not, 

 however, usually held. The manubrium sterni 

 is often keeled in the middle line below ; this 

 is so with the Bats, which thus approach the 

 birds, and probably for the same reason, i.e. the 

 need of an enlarged origin for the pectoral muscle, 

 which is concerned in the movements of flight. 

 In many forms this part of the sternum is much 

 FIG. 1 9. sternum broader than the pieces which follow ; this is so 



of the Pig (Sus . . T _ _. 



scrofa). x . ms, with the Viscacha. In the rig the precise reverse 

 Mesostemum; j s seen the manubrium being narrower than the 



ps, presternum ; 



o, xiphisternum. rest of the sternal bonelets. It will be noticed, 

 (From Flower s h owev er. that in this and similar cases there are 



Osteology.) ' 



no clavicles. Eibs are attached between the 

 successive pieces of the sternum. When the sternum is 

 reduced, as it is in the Cetacea and in the Sirenia, it is the 

 intermediate part of the series of bones which becomes abbrevi- 

 ated or vanishes. The Sperm Whale has only a manubrium 

 sterni and a following piece belonging to the mesosternum. 

 It is fair to say that the xiphoid process and the rest of 

 the sternum have disappeared, since among the Toothed Whales 

 a progressive shortening of the sternum can be seen. In 

 the Whalebone Whales the sternum is still further reduced ; 

 the manubrium is alone left, and to it are attached but 

 a single pair of ribs. In Balaena, however, a rudimentary 



