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 Ime, 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 Steniiun in the various Orders. 

 Order Primates.' — In Man (see Fig. 31, p. 73) \ht p?rsteniu?n 

 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 mcsosterniim is elongated, 

 but is also comparatively broad and 

 flattened. It consists of four distinct 

 segments. The xiphistermiin is a 

 more or less elongated posterior ap- 

 pendage, varying somewhat in form 

 and size in diflerent individuals. 



The ossification of the human 

 sternum is endosteal., or commencing 

 within the substance of the primitive 

 hyaline cartilage. The presternum 

 ossifies from one, or sometimes two, 

 centres, which may be placed side by 

 side, or one in front of the other. 

 Each of the segments of the mesosternum 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 mesosternum usually unite together 

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

 sternum often remains throughout life connected only by 



of young 



Fig 32. — Sternu 



Orang(6Vw/<i satyrus). J>s 

 presternum ; n/s mesoster- 

 num ; a:s xiphisternum. 



