150 HISTORY OF THE HUMAN BODY 



to endless controversy, such as always occurs with regard to 

 places where the records are incomplete, and this vital period 

 in the history of the skull would have lost much of its reality. 



As to the necessity which caused the appearance of these 

 endochondral ossifications in the primordial skull, there has 

 been pointed out a curious relationship between them and the 

 principal cranial nerves, namely, that the ossifications de- 

 velop about their places of exit from the brain cavity as though 

 to protect them. Thus we have the olfactory nerve surrounded 

 by the ethmoid, the optic nerve perforating the orbito-sphe- 

 noid, and similar relations existing between the trigeminus 

 and the alisphenoid, the facial nerve and the prootic, and the 

 ninth and tenth and the exoccipital. These are certainly the 

 topographical conditions, but whether a causal relation really 

 exists between them is not known. 



In completing the history of the skull, it remains to no- 

 tice the amphibian stage, best exhibited by urodeles, and the 

 amniote stage, typically represented by reptiles and mammals. 

 In the first of these the dermal bones are no longer external at 

 any stage of their development and have become definitely 

 incorporated with the skull as physiological parts of the in- 

 ternal skeleton. Aside from this the characteristically piscine 

 elements, like the rostrals, the orbitals and those associated 

 with the operculum, have become lost, and the bones assume 

 more the number and relationships of the higher terrestrial 

 forms (Fig. 19, B, and Fig. 39). 



In the Amniota one of the fundamental changes is the loss 

 of the parabasal as the main element of the cranial floor, and 

 its functional replacement by a series of median cartilage 

 bones, the basi-occipital, basi-sphenoid and prce-sphenoid. 

 The parabasal may be entirely lost, but in the light of recent; 

 investigation it seems probable that it is continued as the| 

 median vomer, which is thus not the same as the paired vc 

 mers, which form a portion of the roof of the mouth of fish( 

 and amphibians, and which, if this view is the correct on< 

 probably disappear in amniotes. 



Another characteristic is the secondary fusion of elements 3 



