24 THE ANATOMY OF VERTEBRATED ANIMALS. 



The cartilaginous cases of the organs of hearing, or the 

 periotic capsules, are, as has been said, incoi^porated with the 

 skull between the ex-occipitals and the alisphenoids — or, in 

 other words, between the occipital and the parietal segments 

 of the skull. Each of them may have three principal ossifi- 

 cations of its own. The one in front is the pro-otic ; the one 

 behind and below, the opisthotic ; and the one which lies above, 

 and externally, the epiotic. The last is in especial relation 

 with the posterior vertical semicircular canal ; the first with 

 the anterior vertical semicircular canal, between which, and 

 the exit of the third division of the fifth nerve, it lies. These 

 three ossifications may coalesce into one, as when they con- 

 stitute the pefrosa? and masiotcZ parts of the temporal bone 

 of human anatomy ; or the epiotic, or the opisthotic, or 

 both, may coalesce with the adjacent supra-occipital and 

 ex-occipitals, leaving the pro-otic distinct. The pro-otic is, 

 in fact, one of the most constant bones of the skull in the 

 lower Vertebrata, though it is commonly mistaken, on the 

 one hand for the alisphenoid, and on the other for the 

 entire petro-mastoid. Sometimes a fourth, pterotic ossifi- 

 cation, is added to the three already mentioned. It lies on 

 the upper and outer part of the ear-capsule between the 

 pro-otic and the epiotic (see the figure of the cartilaginous 

 craniiim of the Pike, infra). 



In some Vertebrata the base of the skull exhibits a long 

 and distinct splint-like membrane bone* — the par asphenoid, 

 which underlies it from the basi-occipital to the pre- 



♦ Bones may be formed in two Lizards, the bony matter is simply 

 ways. They may be preceded by ossified cartilage, or cartilage bone, 

 cartilage, and the ossific deposit The parietal or frontal bones, on 

 in the place of the future bone the other hand, are always devoid 

 may at first be deposited in the of cartilaginous rudiments, or, in 

 matrix of that cartilage ; or the other words, are membrane bones. 

 ossific deposit may take place, In the higher Vertebrata the 

 from the first, in indifferent, or cartilage bones rarely, if ever, re- 

 rudimentary connective, tissue. main as such; but the primitive 

 In this case the bone is not pre- ossified cartilage becomes, in great 

 figured by cartilage. In the skulls measure, absorbed and replaced 

 of Elasmobranch fishes, and in by membrane bone, derived from 

 the sternum and epicoracoid of the perichondrium. 



