6o THE HUMAN STERNUM 



absent or defective, and the sternum, at the same time, is modified in a 

 different direction. The presternum is more rudimentary, and the meso- 

 sternum may be throughout life in a quasi-segmented condition.* 



In the case of the biped, fusion of the sternal elements is a necessity, 

 in order to obtain the proper use of the fore limb. In the quadruped, 

 segmentation and pliancy of the sternum are equally a necessity to ensure 

 ease and readiness in lateral movement of the trunk. In both the respiratory 

 necessities are the same, and are equally provided for by the bony and 

 muscular mechanism. 



The mammalian sternum is essentially characterized by the possession 

 of presternum, mesosternum (simple or compound), and metasternum. 

 The only element constantly present, however, is the presternum, the other 

 elements being in different cases either absent or undifferentiated. Lastly, 

 the most variable features of the sternum are produced by the variations in 

 the connexions of the ribs (Tables XXIII, XXIV). 



* In the ox the mcsosternal * segments ' are united by cartilaginous laminae ; and one finds, in old examples, a 

 gradual fusion of the segments by osseous union, beginning in the hinder part of the mesosternum. In one case in 

 my collection, the mesosternum is in one piece, and has the metasternum fused with it. 



