in.] THE CRANIAL SKELETON. ni 



The internal pterygoid processes of man represent very im- 

 portant and constant parts of the cranium, which exist in all 

 classes down to and including bony Fishes. 



The share which they take in man in bordering and bound- 

 ing the posterior nares is by no means their normal office in 

 Vertebrates generally. In Birds and in all lower Vertebrates 

 (except the Crocodiles) they have no connexion with these 

 apertures, but are quite posterior to them, serving to connect 

 the hinder part of the palatine with the point of suspension ot 

 the lower jaw, and, except in Fishes, have also a connexion 

 with the sphenoidal region of the basis cranii. 



Sometimes, as in Serpents, these bones may be very 

 movable. 



In certain forms, t.e. in the Crocodiles, Ant-eaters (see 

 Fig. 117), and in the Echidna, these bones may develop 

 horizontal plates, which form the posterior part of the bony 

 palate. So that, in these animals, the posterior nares are 

 bounded by the pterygoids, both laterally and inferiorly. 

 The pterygoid may be swollen and bullate, as in the Mole 

 and in some Sloths. 



The internal (but not the external) pterygoid processes 

 really belong, not to the cranial bones proper, but to those of 

 the face. 



In Lizards, a peculiar dismemberment of the pterygoid, 



Frc. 105. Diagrammatic Vertical, Fro. 106. Diagrammatic Vertical, 



Transverse Section of the Skull of Transverse Section of the Skull of 



a Lizard, showing the columellcz a Chelonian, showing the meeting on 



ascending from the pterygoid bones each side of upward processes of the 



to the parietals. pterygoids with downward processes 



/, parietal ; //, pterygoid bone ; pt ', of the parietals. 



columella, or dismemberment of p, parietal ; pt, pterygoid ; s, basi- 



pterygoid ; s, basi-sphenoid, diverg- sphenoid, 

 ing processes of which join the two 

 pterygoids. 



called the columella, may ascend and join the parietal. In 

 Chelonians a corresponding process of the pterygoid may 

 ascend and meet a corresponding downwardly directed pro- 

 cess of the parietal. 



24. The ETHMOID is another complex aggregation of ele- 

 ments which are distinct in lower animals. In man's class, 



