THE SKELETAL SYSTEM 



223 



phylogenesis, cartilage is gradually destroyed and later replaced by 

 bone. 



Since reptiles have only a single occipital condyle to articulate the 

 skull with the atlas vertebra, while mammals have two, the descent of 

 mammals from reptiles has on this account been questioned. Also for 

 this reason, the attempt has been made to prove that mammals have 

 evolved directly from amphibia, which also have two occipital condyles. 

 But some reptiles have a tripartite condyle, and mammals may have 



iPARASPHENOID 



PARASPHENO 



'PREMAXILLARY 



PALATINE 



MAXILLA- - 

 ISPHENOID 



OCCIPITAL 



■STAPES 



pfemaxillary/ 



int.nares' 



DIMETROOON. 



PALATI NE' 



PTERYGOID^-'' 

 QUADRATE + 

 QUADRATOJUGAL 



CYNOGNATHUS. 



j;emporal aOCCipital 



PRESPI-CNOIO 

 PALATINE 

 PREMAXILLARY 



IPITAL PALATINE 



-BASIOCCIPITAL 



INTERNAL NARES 



^INTERNAL NARES p-jL 



C. DOG. Varietal-' 



D MAN 



Fig. 183. — Stages in the recession of the internal nares (choanae) are represented in 

 the Tertiary reptiles, Dimetrodon and Cynognathus. By the growth of a horizontal 

 septum from the maxillary and palatine bones the narial passages become separated 

 from the mouth cavity in higher reptiles and mammals. Thus the internal nares or 

 choanae finally open, not into the mouth cavity directly as in amphibia, but into the 

 pharynx. (Redrawn from Romer's " Man and the Vertebrates," University of Chicago 

 Press.) 



derived their two condyles from this tripartite reptilian condyle by the 

 disappearance of its median basioccipital element. 



One of the most conspicuous evolutionary developments of the skull 

 has been the relative enlargement of the membrane bones and the reduc- 

 tion of the cartilage elements. The only cartilage bones which persist 

 are those which support the brain, those which cover the brain being 

 exclusively dermal. This change is correlated with the increased size 

 of the brain. See Fig. 178. 



Except in proportion, there is little difference between the skull of man 

 and that of other mammals. The enlargement of the brain and the 

 correlated enlargement of the roofing bones of the skull carries the olfactory 

 lobes, the foramen magnum and the otic capsules to the floor of the 



