32 THE HUMAN STERNUM 



but variable, approximation of the lower costal cartilages at their junction 

 \Nith the sternum ; this approximation causes the traction of these costal 

 cart la^cs to be exerted simultaneously in the growinfi; sternum, ami so 

 prevents the formation of an osseous centre, which might make its ajipearance 

 it the sternal attachments ot these cartilages were further removed from one 

 another. 



It the terms ' sternebrae ' and "■ sternal segments ' possess any real 

 miirjihological significance, some more adequate explanation is required of 

 the alisence of these sternebrae between the sixth ami seventh, and between 

 the fifth and sixth costal attachments, than the bare unci meaningless 

 statement that ' the)' have been lost in the process of evolutitjn.' 



l*'rom this process oi ossification one is led to a behef that morpho- 

 l(iL:icall\- the sternum is a simple cartilaginous structure (possibly bilateral 

 in origin), median in position, associated with shoulder-girdle ami ribs, and 

 subserving the purposes (in different degrees in different animals) ot shoulder- 

 girdde nv>\ements and of resp r.ition. Its ossification is con.'tucte;.! I)y a slow 

 an>] jilastic metliod, by means of which its form can be adapted to the needs 

 ol the animal, and which leads to the ultimate production id different types 

 ol sternum - on the one hand, the clavicular or hi.nnan type; and -)n the 

 ( idler hand, the purely thoracic or respiratory type, as in the carnivora or 

 unrrulata. 



o 



This view appears further to explain those cases m ^vhich on one or 

 both sides the distal endi ol a rib overlaps the sternum, and reathes the middle 

 line ; the sternum itself persisting or being absent at the same time ; it 

 acc('iints for the non -occurrence of segmentation in the cartilaginous tneso- 

 sternum ; and lor the variabilit)' in the matter ot fusion or sc|Mratioii oi the 

 cartilaginous mcsosternum in relation to the presternum ami nietasternuin. 



In dealing with the observations ot previous observers, it woukl be a 

 matter of great difficulty to summarize the extraorilinarily discrepant 

 accounts that are cjiven ot the (ossification ot the sternum. My task is 

 li!/litened by reason of the appearance of an important memoir by 

 Makkowski,'*"* in which these statements are given with some fulness. 

 This author also deals exhaustively with some parts of the cpiestion luider 

 consideration. As, however, his statistics are compiled from an examination 



