330 



Paterson (5) again has found that the sternum first makes its appear- 

 ance as a single mesoblastic structure in which are developed two 

 cartilaginous bars which are formed independently of the ribs. With 

 these bars the ribs meet and fuse and become secondarily marked off. 

 Paterson also regards the metasternum as a distinct element and in 

 this he agrees with a view expressed by Götte (11) and Cleland 

 and Mackay (12). 



A complete estimation of the value to be attached to each of these 

 theories cannot be entered into in this paper but their application to 

 the facts above noted in connection with the sternal articulations of 

 the 7th and 8th ribs may be studied. 



If with Bruch (9) and Paterson (5) we regard the mesosternum 

 and metasternum as median structures formed independently of the 

 ribs then the advance of the 7th and 8th costal cartilages to the front 

 of the metasternum must be regarded either as indicating a tendency 

 to an increase in the length of these ribs or as a malformation pro- 

 duced by some constricting force applied to the trunk — as in Hyrtl's 

 theory that it was brought about by tight lacing. The first of these 

 inferences is disproved by the history of degeneration of the ribs 

 throughout the order Primates and the second by the frequent occur- 

 rence of the condition in both male and female adults and in the foetus 

 as shown by Paterson's (5) series. 



According to Ruge (10) 9 ribs enter into the formation of the 

 human sternum but the connection of the 8th and 9th with the sternum 

 fails at a later stage — a sternal portion however remaining to form 

 the metasternum. Ruge also in the course of his paper points out 

 that in the process of coalescence of the two sternal bars the fusion 

 takes place first dorsally and gradually proceeds ventrally. 



If this view of the development of the sternum be assumed then 

 a rib undergoing degeneration would first tend to dissociate itself from 

 taking part in the formation of the sternum. The ventral aspect being 

 the last to unite would be the first to fail but as degeneration pro- 

 ceeded the dorsal portion would also fail — the rib end becoming 

 thus entirely separated from the sternum. The separated rib cartilages 

 would be at first continuous with one another in front but later the 

 cartilaginous junction would give place to a fibrous one. 



These changes, it will be seen, correspond exactly with the con- 

 ditions found in the case of the 7th and 8th ribs. 



1) The articulation of the rib end with the lateral facets on the 

 meso- and metasternum. 



2) The articulation by a dorsal projection with the lateral facets 



