OSTEOLOGY 129 



margin of the bone is entire, without any processes at all. 

 In passerine birds generally, and in some others also, there 

 is but a single pair of these processes ; while, finally, by 

 excessive growth of the parts concerned the processes have 

 joined and converted the notches into foramina. The con- 

 verse course of events has been suggested i.e. that deficient 

 ossification leads to the fenestrated condition, whence to 

 the posterior notches is an easy step. Development, how- 

 ever, shows that the former view is the more correct. The 

 diversities in the form of the sternum undoubtedly must 



ca 



mr 



FIG. 74. STERNUM OF EMU (AFTEK MIVART). ^ NATURAL SIZE. 



crt, anterior lateral process ; c, grooves for eoracoids ; /, elevation in centre ; ?.r, posterior 

 end ; w, lateral view showing articulation of rilis. 



have some relation to the muscles which are inserted on to 

 and take their rise from the margins of the bone. Thus, as 

 already mentioned, the flat sternum of the Ratitse is associated 

 with the slight development of the pectorales muscles and 

 the consequent loss of capacity for flight. It has been 

 ingeniously suggested that the relative development of the 

 posterior lateral processes of the sternum has possibly an 

 analogous explanation. The muscles that are attached 

 thereto are mainly the pectorals and the abdominals. Now r 

 the pull of these two is in an opposite direction. The 

 tendency of the action of the pectorals would be to 



Iv 



