in.] THE CRANIAL SKELETON, 101 



21. The development attained by the FRONTAL bone in the 

 human subject is very greatly above the average of man's 

 class, and bears relation to the vast development of his brain 

 Sometimes, on the contrary, as in the Cetacea, the orbital 

 part may be enormously developed outwards on each side. 



The primitive double condition in which, as we have seen 

 this bone exists in man, very often persists in adult life in 

 other Mammals. It may do so in Apes, though sometimes in 

 that order (e.g. Pithecid] not only is it united, but it even 

 develops a median ridge, continuing fonvards the sagittal one. 



In many Reptiles and Fishes, however, this bone is single, 

 as in the Gecko and the Cod. 



Each half of the frontal bone may meet below as well as 

 above, so as to form a complete bony ring, as in the Python. 



The external angular process of the 

 frontal may rarely (as e.g. in the Horse) 

 join not the malar, but the zygomatic 

 process of the temporal bone. 



In the majority of the members of 

 man's class this process joins neither FIG. QS.-DIAGRA.M REPRE- 



Of those bones, but merely forms a SENTING A TRANSVERSE 



freely projecting post-orbital process. SSS^cSS^S 



Sometimes it is completely absent, f frontalj m its fdlow 



as is the case in several forms, e.g. O f the other side both 



the TanrCCS. above and below in the 



On the other hand, in the Sauro- l^f^STSt 

 ps.cla, a distinct bone (either tempo- cuia crami which persist 

 rally as in Birds, or permanently as in "SttckSatt 

 Reptiles) extends downwards from the ba<,e of the skull, 

 postero-external part of the frontal. 



This either ends freely, as in most Birds; or it contributes to 

 form a sort of upper zygoma, as in some Parrots and Lizards ; 

 or, as in Python, passes downwards to abut against the single 

 zygomatic arch. It is called the post-frontal. 

 ' A bone exists in the skull of osseous Fishes which has 

 often been called the " post-frontal," but which has no rela- 

 tion to that bone of Reptiles, being really an ossification of the 

 ear capsule, and therefore the representative of part of the 

 petrous bone of man. 



The superciliary ridges, which in some races of men are 

 much marked, may attain a far greater size, being at their 

 maximum in one of the highest Apes, viz. the Gorilla. 



The two sides of the frontal may unite behind the cribri- 

 form plate, as is the case in the commonest Monkeys. 



