416 



of the walls of the skull, but have the same relation to the anterior 

 and upper expanded edge of the prolonged lamina perpendicularis 

 or body of the ethmoid, as the vomer or vomers have to its lower 

 edge. 



If the conclusions which I have laid before you are correct, the 

 following propositions are true of all the bony skulls of Vertebrata. 



1. Their axis contains at most five distinct bones, which are, 

 from before backwards, the basioccipital, the basisphenoid, the pre- 

 sphenoid, the ethmoid, and the vomer ; but any of these bones, 

 except the basisphenoid, may be represented by cartilage, and they 

 may anchylose to an indefinite extent ; so that the number distin- 

 guishable as separate bones in any skull cannot be predicated. The 

 craniofacial axis invariably presents the same regions, but the 

 histological character of these regions may vary. 



2. Their roof contains at most, leaving Wormian bones out of con- 

 sideration, five bones (supraoccipital, parietals and frontals), or seven, 

 if we include the epiotic bones in the roof. The number falls below 

 this in particular cases, for the same reason as that given for the 

 apparent variations in composition of the axis. 



3. Their inferolateral wall contains at most six pair of bones 

 (exoccipitals, mastoids, petrosals, alisphenoids, orbitosphenoids, pre- 

 frontals), whose apparent number, however, is affected by the same 

 causes. 



4. The axial bones have definite relations to the brain and nerves. 

 The basioccipital lies behind the pituitary body, the basisphenoid 

 beneath it, the presphenoid in front of it. In fact the pituitary 

 body may be regarded as marking the organic centre, as it were, of 

 the skull its relations to the axial cranial bones being the same, as 

 far as I am aware, in all Vertebrata. 



The olfactory nerves pass on either side of the ethmoid, which 

 bounds the cranial cavity in front, the greater part of its substance 

 and that of the vomer being outside the cranial cavity. 



5. The lateral bones have definite relations to the brain, nerves, 

 and organs of sense. The exoccipital lies behind the exit of the par 

 vagum ; the mastoid lies in front of it ; the petrosal lies behind the 

 exit of the third division of the trigeminal ; the alisphenoid lies in 

 front of it ; though either bone may, to a certain slight extent, en- 



