100 



THE STERNUM. 



[CHAP. 



segments, and consequently no ribs, other than the first 

 pair, are attached to it. 



The presternum is ossified from one, or perhaps a pair of 

 symmetrical nuclei. In the Right Whales {Baltzna, Fig. 38) 

 it is heart-shaped, or longitudinally oval; in the Fin Whales 

 (Balcenoptera, Figs. 39, 40, and 41) it is transversely oval 

 or trilobate, with a backward projecting xiphoid process. 1 

 In young animals ossification of the cartilaginous sternum 

 advances forwards on each side of the middle line, so that 



FIG. 39. Sternum of Common Rorqual or Fin 

 Whale {Balcenoptera musculus), jV 



FIG. 40. Sternum of Pike Whale 

 (Balcenoptera rostra to), T V 



the ossified portion at one period appears deeply notched in 

 front ; as the bone meets across the middle line anteriorly, 

 this notch usually becomes converted into a hole (see 

 Fig. 39), which finally closes with complete maturity. 



1 In the cartilaginous sternum of a young BaLznoptera sibbaldii 

 Professor Turner found the xiphisternum to be quite distinct from the 

 presternum, and connected with it by fibrous tissue. (Journal of 

 Anatomy, May 1870.) In most Whales the sternum shows no such 

 evidence of segmentation. 



