SKELETON. 



641 



the vertebral portion with the sternum. Each of the first 

 nine ribs has a double head — the capitulum articulating 

 with the centrum of the corresponding vertebra, and partly 

 with that of the one in front, the tubercle articulating with 

 the transverse process of the corresponding vertebra. The 

 posterior ribs have no tubercles, and the capitular articula- 

 tions are restricted to the corresponding vertebrae. 



The sternum is a narrow jointed plate, with a large keeled 

 praesternum or manubrium, then five segments composing 



Fr. 



rmx. 



Fig. 277. — Side view of rabbit's skull. 



Pmx., Premaxilla ; Na , nasal ; Fr., frontal ; Pa., parietal; Sq., 

 squamosal ; S.O., supraoccipital ; Per., periotic ; T., tympanic 

 (the reference line points to the bony external auditory meatus, 

 beneath it lies the inflated bulla) ; P.O., paroccipital process. 



the mesosternum, then a posterior xiphisternum ending in 

 cartilage. 



The skull consists, as in all the higher Vertebrates, of two 

 sets of bones, — cartilage bones preformed in the cartilage of 

 the original gristly brain-box and its associated arches, and 

 membrane bones developing in the investing membrane and 

 not preformed in cartilage. (The names of the membrane 

 bones are printed in italics.) 



We have already noticed the chief characteristics of the 

 mammalian skull, such as the usual persistence of sutures, 

 the two condyles, the bony palate, the fusion of the periotic 

 bones, the articulation of the mandible with the squamosal, 



41 



