THE HUMAN STERNUM 3 



Lastly, by a study of comparative anatomy of lower forms, we are 

 faced with the presence, in certain reptiles and amphibia, of a sternum which 

 is not connected with ribs at all, but is associated with the shoulder-girdle 

 only ; and in fishes, with one exception, an entire absence of the sternum, 

 although there is a complex development of both shoulder-girdle and ribs. 



The commonly accepted view of the morphology of the sternum is 

 that it is made up of ' sternebrae,' derived from the costal cartilages ; and 

 that each ' sternebra,' with a pair of ribs and a vertebra, constitute a typical 

 and complete body segment. 



The more one examines the subject (and even admitting the attrac- 

 tiveness and plausibility of this view) the more one feels that it is founded 

 upon an insufficient basis, and presents such unsatisfactory features, that one 

 is forced to enquire for some other hypotheses for the formation of the bone 

 which will more adequately account for its special characteristics, and for 

 the differences which it presents throughout the vertebrate series. 



Apart from any hypotheses or theoretical considerations, the sternum 

 is a bone which occurs in air-breathing animals possessing four limbs ; and 

 its chief modifications are dependent upon differences in the mode of 

 progression. It is the central bone of the thorax, comparable in that way 

 to the basis cranii, an adjunct of the shoulder girdle, of prime importance 

 especially in bipeds, and an essential factor in the construction of the osseous 

 respiratory chamber. At the same time it forms a platform or a shield for 

 the heart, and serves important mechanical purposes. Muscles radiate from 

 it for the neck, fore limbs, thorax, and abdomen.* 



The main thesis which I have to present is that there is insufficient 

 evidence of the origin of the sternum from the rib elements, and indeed 

 that the association of sternum with costal cartilages is a secondary one, and 

 that the primary condition of the sternum is one in which it is in association 

 with the shoulder-girdle. One does not claim originality for this view. It 

 has been already foreshadowed by T. J. Parker, and Professor Cleland 

 apparently does not accept the view held by most anatomists. 



* One may draw particular attention to the correlation of the infra-hyoid muscles and the diaphragm, 

 and the fact of their associated innervation, with the intervention of the sternum ; and also to the presence of the 

 internal mammary artery, and its prolongation downwards to the abdomen. 



