62 THE HUMAN STERNUM 



GENERAL CONCLUSIONS 



The conclusions at which one has arrived from the foregoing considera- 

 tions may be summarized here. 



Surveying the vsrhole field, the sternum is to be regarded as evolved, 

 developed, and constructed rather in relation to the shoulder-girdle than to 

 the thorax. It primarily subserves the functions of the fore-limb, and is 

 secondarily utilized for purposes of respiration. It therefore receives the 

 attachment of a variable number of ribs. 



In those animals in which the fore-limbs are limited in functions and 

 serve solely for quadrupedal movement, the limitation is associated with 

 (i) a failure in development of the shoulder-girdle, and (2) a coincident 

 modification of the sternum, which in such cases is connected only with the 

 ribs, and assumes a quasi-segmental character. 



If the quadrupedal mammal is to be regarded as the type, then it 

 might be fairly assumed that its sternum is also typical. But can this be 

 asserted ? The mass of evidence seems to point to the conclusion that the 

 fore-limb is primarily developed as a prehensile organ ; that its more 

 primitive type is to be looked for in animals with a strongly developed 

 shoulder-girdle : and that the fore-limb of quadrupedal mammals is a 

 modification from this primitive type, associated with defects in the forma- 

 tion of the shoulder-girdle and modifications in the form of the sternum. 



In other words, segmentation of the sternum is not a primitive con- 

 dition. It is a secondary event, in two senses — both ontogenetically and 

 phylogenetically. It is an architectural device adopted to suit the particular 

 needs of a modified type, utilized for a particular end in one case and 

 abandoned in another ; just as in the formation of the dorsal axial 

 skeleton, segmentation is made use of in the formation of the vertebral 

 column, but is discarded in the construction of the basis cranii, and is 

 modified in the formation of the sacrum. 



In advancing a view which is opposed to orthodox belief, one has to 

 face the essential foundations of current opinion. In this light, I venture 

 to express the view that there is no solid evidence of the ontogenetic or 

 phylogenetic origin of the sternum from the ribs. In my opinion and by 



