INTRODUCTORY. THE CRANIUM. 19 



already mentioned, in many animals the eye likewise is 

 included in a cartilaginous capsule. 



(1) THE CRANIUM PROPER Or BRAIN CASE. 



The cranium varies much in form and structure. In lower 

 vertebrates, such as Sharks and Lampreys, it remains entirely 

 cartilaginous and membranous, retaining throughout life much 

 of the character of the embryonic rudiment of the cranium of 

 higher forms. The dogfish's cranium, described on pp. 73 to 

 76, is a good instance of a cranium of this type. But in the 

 majority of vertebrates the cartilage becomes more or less 

 replaced by cartilage bone, while membrane bones are also 

 largely developed and supplant the cartilage. 



The cranium of most vertebrates includes a very large 

 number of bones whose arrangement varies much, but one 

 can distinguish a definite basicranial axis formed of the 

 basi-occipital, basisphenoid, and presphenoid bones, which is 

 a continuation forwards of the axis of the vertebral column. 

 From the basicranial axis a wide arch arises, composed of a 

 number of bones, which form the sides and roof of the brain- 

 case. These bones are arranged in such a manner that if both 

 cartilage and membrane bones are included they can be divided 

 into three rings or segments. The hinder one of these seg- 

 ments is the occipital, the middle the parietal, and the anterior 

 one the frontal. 



The occipital segment is formed of four cartilage, bones, the 

 basi-occipital below, two exoccipitals at the sides, and the 

 supra-occipital above. The parietal segment is formed of 

 the basisphenoid below, two alisphenoids at the sides and 

 two membrane bones, the parietals above, and the frontal 

 segment in like manner consists of the presphenoid below, 

 the two orbitosphenoids at the sides, and two membrane 

 bones, the frontals, above. The parietals and frontals, being 

 membrane bones, are not comparable to the supra-occipital, 

 in the way that the presphenoid and basisphenoid are to the 

 basi-occipital. 



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