2 THE HUMAN STERNUM 



show that the notion of segmentation may be stretched too far. And even 

 in regard to the trunk itself, where segmentation is most obvious, each 

 particular segment has its specific differences. Segmentation, indeed, may 

 be looked upon as an architectural device utilized for what it is worth, both 

 in invertebrates and in vertebrates, and modified and discarded where it 

 would be hurtful. Modifications of segmentation occur in the formation 

 of the sacrum, in the cervical vertebrae of cetacea ; and there is no real 

 segmentation in the urostyle of the frog, or in the mammalian basis cranii. 



Segmentation, as Herbert Spencer points out, is essentially con- 

 nected with progressive movement. The elongated worm-like organism 

 for purposes of progression becomes segmented, on account possibly of 

 alternate stress and strain produced by^ the attempt at a wriggling move- 

 ment. Where stability and fixation are required segmental features are 

 modified or absent. 



In a mammal, the thorax presents the most complete and characteristic 

 example of segmentation, by the association together of the vertebrae, 

 ribs, and sternum. In other regions the costal elements are rudimentary 

 or absent, and it is almost certain that no element comparable to the sternum 

 exists elsewhere in the mid-ventral line of the trunk. 



In such an animal as the ant-eater {TVLyrmecophaga jubata, PI. IX, 

 Fig. 63), the sternum consists of presternum, mesosternum, and meta- 

 sternum. The mesosternum is composed of eight separate median pieces, 

 which regularly alternate with costal attachments. 



In most other quadrupedal mammals the general structure of the 

 bone is the same, but {e.g., jackal, PL IX, Fig. 64) there is a failure in the 

 segmental alternation of mesosternal and costal elements ; and the costal 

 cartilages of the last two sternal ribs unite at the same point with the 

 junction of mesosternum and metasternum. 



Again, in reptiles, birds, and certain mammals (notably bipeds) the 

 same three elements are present — presternum, mesosternum, meta- 

 sternum — but the mesosternum here consists only of a single bone, or 

 fewer pieces, without any proper segmental relation to the ribs, but afford- 

 ing by its lateral margins attachment for a certain number of costal 

 cartilages. 



