THE HUMAN STERNUM 



I. DEVELOPMENT AND OSSIFICATION OF THE 



STERNUM 



A. General Considerations. The Materials and the Process 



One of the fundamental difficulties in the study of the growth of all 

 mesoblastic tissues arises from the gradual alteration of the primitive 

 embryonic cellular tissue into more complex tissues — connective tissue, 

 cartilage, and bone. At first there is a simple uniform tissue, in which 

 and out of which, in definite areas, conglomerations of cells occur so as to 

 produce a clearly demarcated mass in the situations in which, for example, 

 vertebral column, ribs, sternum, and shoulder-girdle are to arise. Out of 

 this mass of cells are produced connective tissues, cartilages, and bones. 

 The usual mode of proceeding is the conversion of the cellular mass into 

 cellular and afterwards hyaline cartilage, and in that, either by a perichondral 

 or an endochondral mode of ossification, the cartilaginous model is converted 

 into bone. 



Besides the histological changes, there are also fundamental chemical 

 differences to be remembered in the conversion of this embryonic tissue 

 into its more stable and permanent form ; a gelatinogenic change in the 

 formation of fibrous tissue, a chondrinogenic change in the formation of 

 cartilage, and an osteogenic change in the formation of bone. 



In exceptional cases {e.g., lower jaw, vault of skull, and clavicle in part), 

 the chondral stage is omitted and a process of periosteal or membrane bone 

 formation takes place before any cartilaginous an/age can appear. 



It is important in connexion with the development of the sternum 

 particularly to be conversant with the pre-chondral stage in its formation, 

 as by a recognition of this it can be demonstrated that the shoulder-girdle 

 Is associated with it, that the girdle becomes cartilaginous, while the sternal 

 anlage is still in the cellular condition, and that the ribs are independent of 

 the sternum at first, and in mammals become joined to it as cartilaginous 

 bars, when the sternum itself is a cellular layer. And, further, after 

 the bone has been modelled out into its definite form in cartilage, the 



