92 THE STERNUM. [CHAP. 



life in the form of fissures or fenestrse 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. Sometimes 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,) 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 mesosternum 

 is elongated, but is also comparatively broad and flattened. 

 It consists of four distinct segments. The xiphisternum is a 

 more or less elongated posterior appendage, varying some- 

 what 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 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 condition, 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 

 fibrous tissue, although it is not unfrequently ankylosed in 

 old age. 



The xiphisternum ossifies irregularly and imperfectly. 



The Gorilla, Chimpanzee, Orang, and Gibbons, resemble 

 Man, and differ from the other monkeys in the breadth 



