in.] THE CRANIAL SKELETON. 127 



33. Those CONNEXIONS OF THE BONES of the skull one 

 with another which take place in man are not all constant 

 in Vertebrates generally, as we have already seen. Certain 

 unions, however, are absolutely constant and invariable, as 

 that of the basi-occipital and basi-sphenoid the pre-sphenoid 

 and basi-sphenoid the parietal and occipital the maxillary 

 and jugal, when the last-mentioned bone is present. 



Several exceptional connexions have been already given. 



In the skull of many Vertebrates vacuities often exist where 

 bone is present in man tracts osseous in him being repre- 

 sented by membrane only. An example of such defect is seen 

 in that region of the skull of Lizards (e.g. Iguana) which cor- 

 responds with the wings of the sphenoid of the human skull. 



The transitory fontanelle 1 of man is permanent in some 

 animals, as e.g. certain Sharks, where the cranium is to a 

 large extent roofed by membrane instead of by bone or 

 cartilage. 



The excessively dentated condition of certain sutures in 

 man is exceptional, nevertheless not unparalleled. We may 

 sometimes (e.g. in the Gorilla) find sutures even more 

 complex. 



The persistence of the sutures in man is less than in 

 many animals, and also greater than in many. Thus we may 

 find, as in Fishes and Reptiles, a variety of bones distinct 

 which in him are united. We may, on the other hand, as in 

 Birds, find a number of bones united which in him are 

 distinct. 



One suture, however, which, as has been mentioned, almost 

 constantly persists in other forms than man, is not repre- 

 sented in him (except on the palate), even at birth. This is 

 the one between the maxilla and the pre-maxilla. Again, 

 the intimate union which exists in him between the basi- 

 sphenoid and the pre-sphenoid is very far from persisting 

 even in his own class. 



On the other hand, that distinctness which always obtains 

 in him between the petrous part of the temporal bone and 

 the occipital portion of the skull, is very early lost in the 

 Sauropsida. 



The skull may present a much less compact mass than in 

 man s class. 



Thus in Birds the elongated facial bones which go to form 



1 The fontanelle is a space on the crown of the head, which, in the new-born 

 infant, is only closed by fibrous structures, the parietals and frontals not having 

 yet joined. 



