in.] THE CRANIAL SKELETON. 131 



Essentially palatal structures, namely the pterygoid bones, 

 in forms below the mammalian class, generally connect the 

 palatine bones with the quadrate bone which suspends the 

 lower jaw. The bony palate may be enriched by the addition 

 of a bone not known in Mammals (the os transvermm], as is 

 the case in Reptiles ; or of two extra bones (the ecto-pterygoid 

 and the ento-pterygoid) ; besides a third peculiar ossification 

 (the meta-pterygoid), as in bony Fishes. 



(2) The middle, or guttural part, is a noteworthy one, both 

 on account of the physiological importance of the structures 

 which modify its composition, and also on account of the 

 value which its various modifications possess for the purposes 

 of zoological classification. 



Its condition in man is exceptional, both as regards the 

 large proportion it bears to the anterior part of the basis 

 cranii, and also as regards its small elongation antero-pos- 

 teriorly as compared with its breadth. 



The variations presented as regards the mastoid and sty- 

 loid processes, the carotid foramina, alisphenoidal canals (for 

 the external carotid arteries), &c., have been already noticed 

 in speaking of the separate cranial bones, and need not here 

 be repeated. 



Sometimes, as in the Crocodile and Great Ant-eater, this 

 guttural part of the basis cranii is nearly concealed by the 

 immense extension of the palate. At other times, as in the 

 Horse, it is much exposed by the smallness of the backward 

 prolongation of the bony palate. 



The pterygoid fossae are found only in man's class, and by 

 no means always in it. Generally, as in the Dog, the pterygoid 

 bones are present, but the descending external pterygoid 

 process is absent ; so that the pterygoid fossae want seemingly 

 their outer walls. 



The pterygoids may be swollen out into bullate expansions, 

 as in the Mole and Sloths. 



Very often, especially in the smaller Mammals, the part 

 answering more or less to the petrous part of the temporal 

 and to the meatus auditorius externus may be, as it were, 

 blown up into vesicular air-containing prominences, termed 

 auditory bulla?. 



The Eustachian tubes may unite to open into the mouth 

 by a median foramen, as in Birds, or may be utterly absent, 

 as in Fishes. 



(3) The posterior part of the inferior region of man's skull 

 is very exceptionally developed, owing to the advanced situa- 



K 2 



