RIBS AND STERNUM 



are persistently free for ;i very Ioul;' period, and in some eases 

 never become aukylosed with their vertebrae. But it shoukl be 

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

 of affairs which exists in many lower ^"ertebrates, where there is 

 a gradual transition between the ribs of the cervical and those of 

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

 ribs in Monotremes is smaller than those which precede it. 



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

 a sternum. This is the bone, 

 or series of bones (sternebrae), 

 which lies upon the ventral 

 sm'face of the chest, and to 

 which the ribs 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 bands 

 forms the single and unpaired 

 sternum of most mammals. 

 Very considerable traces, how- 

 ever, of the paired state of the 

 sternal bones often exist ; thus 

 in 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 a more com- 

 plete condition in some specis of Man.is 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 the 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 Ehler's Zool. lliscdlen. i. 1894. 



Fig. 18. — Sternum and sternal ribs of the 

 Common Mole {TaliJa, eirropaea), with 

 the clavicles (cl) and humeri (//) ; ilf, 

 manubrium stemi. Nat. size. (From 

 Flower's Osteuhgy. ) 



