Metazoa Rana. 



301 



l63 _ DoRSAL VIEW OF THE FROG>S SKULL> 



(Milnes Marshall.) 



Nat 



The several vertebrae are bound together by connective 

 tissue, but in such a maner as to permit of a limited amount 

 of movement. 



The skull is too complicated a structure to allow of de- 

 tailed treatment here, and therefore only the essential 

 features will be referred to. It is to be noted that the neural 

 canal communicates with the cavity of the skull by the 

 foramen magnum, a large aperture in the posterior wall 

 of the skull. The skull is hinged to the first vertebra by 

 two condyles, or 



SmOOth processes be- 



tween which a peg- 

 like projection of the 

 first, or atlas, vertebra 

 fits. 



The skull itself 

 may be said to con- 

 sist of three parts, a 

 cylindrical box, the 

 cranium, or skull 

 proper, to which are 

 attached two pairs of 

 sense-organs, the ol- 

 factory and auditory 



Capsules I a frame- 



WOrk attached tO the 

 .j _ . T i 



sides of the cylinder 

 and forming the upper jaw, the maxilla ; and a lower jaw, 

 or mandible, articulating on either side with the posterior 

 part of the upper framework. The cranium itself is com- 

 posed partly of cartilage, partly of bone. Posteriorly, i.e. 

 in the occipital region, it consists of a floor and two side 

 walls of bone (the basi- and two ex-occipitals) and a roof of 

 cartilage, the ring thus formed bounding the foramen mag- 

 num. The floor of the middle portion of the box is formed 

 mainly of cartilage strengthened by a dagger-shaped bone, 



-, premaxilla ; Na., nasal ; S.e., sphenethmoid ; 

 Sr., parieto-frontal ; Pr.O., prootic ; E.G., ex- 

 occipital ;Q. 7., quadrato-jugal ; Sg,'', squamosal ; 

 Pt'i pterygcid ; Pa., palatine. 



