194 



VERTEBRATES. 



VI. The Skull. The study of the vertebrate skull is 

 well begun with the skull of the frog, which is wide and 

 open, and in which all the bones can be seen from the 

 outside. 



Observe its triangular outline as seen from above, and 

 the large orbital and nasal apertures. Observe the pos- 

 terior opening into the cranium (foramen magnum), and 

 the pair of smooth, articular surfaces (occipital condyles) 



pmx 



SKULL OF BULLFROG (Rana catesbiana), dorsal view: pmx, premaxilla; 

 v, vomer; mx, maxillary; n, nasal; g, girdle bone or sphenethmoid ; 

 pf, parietof rontal ; ptg, pterygoid; po, prootic; sq, squamosal; qj, quad- 

 ratojugal; o, occipital region surrounding the foramen magnum. 



close beside. These meet corresponding surfaces on the 

 anterior end of the first vertebra. 



The bones composing the skull are as follows : 



1. The bone which forms the back of the brain case, 

 and immediately surrounds the foramen magnum, is the 

 occipital. 



2. The stout pillar of bone, which extends laterally at 

 the base of the skull at right angles to the median line, 

 is the pro otic, so called because it lies in front of and covers 

 the capsule of the inner ear. 



