THE HUMAN STERNUM 63 



positive observations the weight of evidence is all on the side of the primary- 

 association of the sternum with the shoulder-girdle, and its secondary- 

 connexion with the ribs. And 1 venture, further, to submit that my view 

 is borne out and supported by- the evidence of comparative anatomy. 



In favour of the view put forward are the following arguments : — 



1. The early development of the sternum shows it closely related 

 to the shoulder-girdle and independent of the ribs. 



2. The ossification of the several parts, whether median or lateral, 

 is not truly segmental, and is not dependent upon costal connexions, 

 except in the mesosternum. 



3. The so-called 'segments' of the mesosternum are not truly 

 metameric. As a rule they are not the same in number as the ribs which 

 articulate with the mesosternum, but fewer. They are not costal but 

 inter-costal. And the formation of segments is due to the deposition of 

 bony masses, whose separation from one another — like their origin — is 

 associated with the mechanical effects produced by the costal attachments 

 to the borders of the sternum. 



4. Individual variations are common in the arrangement of the ribs 

 in relation to the sternum. But they occur without producing any funda- 

 mental differences in the morphology of the bone itself. 



5. A study of the comparative anatomy of the sternum demon- 

 strates that the bone may exist without any connexion with the ribs ; and 

 that the ribs may be highly developed and of great functional importance 

 in cases of absence of the sternum (and shoulder-girdle). Moreover, 

 modifications in the form of the sternum are more dependent upon modifi- 

 cations in the shoulder-girdle than upon differences in its rib-connexions. 



In opposition to the views submitted, the following are the chief 

 arguments : — 



I. The accounts given by previous observers — notably Ruge — of 

 the early development of the sternum. In previous pages the memoir of 

 this distinguished observer has been referred to, and it need only be re-stated 

 here that, admirable as Ruge's observations are, and valuable as a demonstration 

 of fact, there is really nothing in the observations made that prevents one 



