Mr. T. H. Huxley on the Theonj of the Vertebrate Skull 431 



of the identification of the preoperculum with the tympanic bone ; 

 but there are many arguments on the other side, and at present I 

 do not see my way to the formation of a definite conclusion on this 

 subject. 



If the conclusions which I have brought forward 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 histo- 

 logical 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 pairs 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- 

 croach on the province of the other. The optic nerve passes out 

 more or less in front of the alisphenoid, and behind, or through, the 

 orbitosphenoid. 



The organ of hearing is always bounded in front by the petrosal 

 bone, which limits the anterior moiety of the fenestra ovalis. 



The organs of smell always lie on each side of the ethmovomerine 

 part of the axis. 



The greater part, or the whole, of the petrosal lies behind the 

 centre of the mesencephalon. 



6. The attachment of the mandibular arch to the skull is never 

 situated further forward than the posterior boundary of the exit of 

 the trigeminal ; consequently it cannot belong to any segment of the 

 skull in front of the petrosal. 



