80 COMPARATIVE ANATOMY. 



the chapter on the respiratory system.) A complete bony palate 

 like that of Crocodiles is never present, so that the Bird's skull 

 here manifests a decidedly lower stage of development than that 

 of the higher Reptiles. The quadrate is nearly always movable 

 upon the skull, as is also the whole maxillopalatine apparatus, the 

 palatopterygoid bar sliding on the rostrum of the basisphenoid, 

 and so allowing the beak to be raised or lowered to a greater 

 or less extent. This mobility of the upper jaw is most marked 

 in Parrots, in which the frontonasal joint forms a regular hinge. 

 Teeth are no longer developed in existing Birds, their place being 

 taken functionally by horny sheaths covering the bones of the 

 jaw, and forming a beak. As in Reptiles, a fenestra ovalis and 

 fenestra rotunda are present, as well as a tympanic cavity opening 

 into the mouth. 



The visceral skeleton is greatly reduced, though the first 

 branchial arch not only persists, but may, as in the Woodpecker, 

 grow out into a pair of very long jointed rods extending far 

 over the skull. 



(For other details, compare Fig. 65, A, B, C.) 



E. Mammals. 



In Mammals there is a much closer connection between the 

 cranial and visceral regions of the skull than is the case in the 

 Vertebrates already described (comp. Figs. 66A and B). In the 

 fully-developed skull both maxillary and palatopterygoid regions 

 are united to the cranium, though even in Man a facial and a 

 cranial region can be distinguished. The higher we pass in the 

 Mammalian series, the more does the former come to lie below 

 the latter ; so that, in the highest types, their mutual relations to 

 one another can no longer be so well expressed as being before 

 and behind, as by under and above. Thus in Man the facial 

 skeleton is proportionately small when contrasted with the great 

 cranial portion, and the reduction of the angle between the basi- 

 cranial and vertebral axes is carried still further than in Birds. 



The base of the skull is preformed in cartilage, as in Reptiles 

 and Birds : the parasphenoid has disappeared almost entirely, the 

 anterior part of the basis cranii being formed by the ossification 

 of the basal cartilage, which may be distinct, as a presphenoid 

 (Marsupials, Rodents, and some Insectivores), or result from a 

 union of the basal parts of the two orbito-sphenoids : a basi- 

 sphenoid and basioccipital are always present. Most of the bones 

 of the roof of the skull are developed directly in the subcutaneous 

 fibrous membrane. 



The cranial cavity is closed in anteriorly by the cribriform 

 plate of the ethmoid, which is perforated by the olfactory nerves. 



