106 ELEMENTARY ANATOMY. [LESS. 



The three elements (pro-otic, epiotic, and opisthotic) which, 

 as we have seen, coalesce to form the petro-mastoid part 01 

 the temporal bone of man, may be differently proportioned 

 and conditioned from what they are in him. 



The Pro-otic is the largest and most important element ol 

 the three in Vertebrates below Mammals. 



It may meet with its fellow of the opposite side below, and 

 thus form part of the inside floor of the skull, as is the case 

 in many Fishes, e.g. the Pike. 



The Opisthotic constantly anchyloses with the lateral part 

 of the occipital before it unites with the pro-otic in all Verte- 

 brates below Mammals. In many forms, e.g. the Cod, it 

 never unites with the pro-otic at all. 



The Epiotic constantly anchyloses, in all Vertebrates below 

 Mammals, with the squamous portion of the occipital bone 

 before it unites with the pro-otic and opisthotic elements. 



In Fishes and in Chelonians it preserves its distinctness 

 from the occipital, though in Chelonians it coalesces with the 

 opisthotic. 



These three bony barriers protecting the internal ear may 

 be conveniently spoken of as the periotic mass. 



This mass may have added to it a part which, in some 

 Mammals, appears as a lamelliform expansion, as in the Mole 

 and still more in the Echidna, where it constitutes a con- 

 siderable part of the cranial wall between the occipital, the 

 squamosal, and the parietal bones. 



In Fishes, e.g. the Cod and Pike, it appears as a bone pro- 

 jecting at the postero-external angle of the roof of the skull. 

 It is called the pterotic? It may also, in Fishes, have added 

 to it a large and distinct ossification, the sphenotic? which 

 has been called by mistake the post-frontal, and which takes 

 part in suspending the lower jaw. 



The tympanic element of the temporal bone of man may, 

 even in forms so little removed as are the American Apes, be 

 reduced to the ring-like condition which it has in the human 

 infant. On the other hand, in many Mammals, e.g. in the 

 Dog and especially Macroscelides, it forms a large inflated 

 structure termed a bulla. In oviparous Vertebrates it is not 

 represented except by the frame of the tympanic membrane. 



The styloid element may be more conveniently treated of 

 under the head of hyoidean structures. It is sufficient to say 



1 n-rf piif , a wing. 



2 So called because adjoining the alisphenoid,- by its discoverer, Mr. W. K. 

 Parker, F.R.S., who also discovered and named the " pterotic." 



