RIBS AND STERNUM 



aie persistently free for a very long period, and in some eases 

 never become ankylosed with their vertebrae. But it should be 

 noted that in this group there is no approximation to the state 

 of affairs which exists in many lower Vertebrates, where there is 

 a gradual transition l)etween the ribs of the cervical and those of 

 the dorsal region of the vertebral colunni ; for that of the seventh 

 ribs in Monotremes is smaller tlian those which precede it. 



The Sternum. — All the Mannnalia so far as is known possess 

 a sternum. Tliis is the bone, 

 or series of bones (sternebrae), 

 which lies upon the ventral 

 surface of the chest, and to 

 which the ril:)S are attached 

 below. The development of 

 the sternum has been shown 

 to take place from the fusion of 

 the ribs below into two lateral 

 bands, one on each side ; the 

 approximation of these ])ands 

 forms the single and unpaired 

 sternum of most mammals. 

 Very consideral)le traces, how- 

 ever, of the paired state of the 

 sternal bones often exist ; thus 

 iu the Sperm Whale the first 

 piece of the sternum is divided 

 into two by a longitudinal 

 division, and the second piece 

 is longitudinally grooved. The 

 development of the sternum 

 out of the fused ends of ribs is shown in 

 plete condition in some specis of Manis than in many other 

 mammals. Thus in M. tricuspis the last ribs of those which are 

 attached to the sternum are completely fused together into a 

 single piece on each side.^ As a general rule tlie last ribs 

 which come into relation with the sternum do so only in an 

 imperfect way, being simply firmly attached at their sides to, 

 but not fused with, the last ribs which are definitely articulated 

 with the sternum. Contrary to what is found in lower Verte- 

 1 Khlrr's Zool. Mlscdlcn. i. 1894. 



Fig. 18. — Sternum ami sternal rilis of the 

 Common Mole {TaJpa ennqxien), with 

 the clavicles (d) and humeri (//) ; J/, 

 nianulirium sterni. Nat. size. (From 

 Flower's (Mfohnjii. ") 



a more com- 



