164 



CHORDATE ANATOMY 



The difficulty of explanation of the emergence of bony skeletons from 

 cartilaginous beginnings is greatly increased by the complexity of the 

 processes by which in ontogenesis, and therefore presumably in phylo- 

 genesis, 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 amphibians, which also have two occipital condyles. 

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



Fig. 156 — Diagram of the bones of the mammalian skull. Cartilage bones dotted, 

 membrane bones lined. With slight changes the skull of mammals may be derived 

 directly from that of reptiles (Fig. 155). 2-12, nerve exits. (Altered from Flower, 

 from Kingsley's "Comparative Anatomy of Vertebrates.") 



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 

 excliisively dermal. This change is correlated with the increased size 

 of the brain. See Fig. 152. 



Except in proportion, there is Uttle 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 

 cranium. Among other changes is an increase in the facial angle from 

 an acute to a right angle. The facial angle is the angle between a line 

 from the frontal bone to the maxilla and one from the basioccipital to the 



