74 



THE STERNUM. 



[chap. 



often persistent through life in the form of fissures or 

 fenestras in the middle line of the sternum. Each segment 

 ossifies from a single nucleus, or from two nuclei placed 

 one on each side of the middle line, and which usually 

 become blended together in the course of growth. Some- 

 times epiphyses are added to the ends of the segments. 

 The terminal portion of the xiphisternum generally remains 

 cartilaginous through life. 



Special Characters of the Sternum in the various Orders. \ 

 Order Primates.- — In Man (see Fig. 31, p. 73) the presternum 

 is broad and flat, hollowed in the middle line in front, and 

 expanded laterally to give large surfaces for the attachment 

 of the clavicles and the first pair of 

 ribs. The mesostemum is elongated, 

 but is also comparatively broad and 

 flattened. It consists of four distinct 

 segments. The xiphisternum is a 

 more or less elongated posterior ap- 

 pendage, varying somewhat in form 

 and size in different individuals. 



The ossification of the human 

 sternum is endosteal, or commencing 

 within the substance of the primitive 

 hyaline cartilage. The presternum 

 Fig 32.— sternum of young ossifies from one, or sometimes two, 

 mesosterl centres, which may be placed side by 

 side, or one in front of the other. 

 Each of the segments of the mesostemum has a distinct 

 centre, though these may be double in their earliest con- 

 dition, and sometimes remain so for a long period. 



The segments of the mesostemum usually unite together 

 SO as to form one continuous bony piece, to which the pre- J 

 sternum often remains throughout life connected only by 



xs 



Orang [Stoma satyrus). 

 presternum ; ms 

 num ; -cs xiphisternum. 



