26 THE ANATOMY OF YERTEBRATED ANIMALS. 



called ^jarietal ^iV,^ frontal. Thus the walls of the cranial cav- 

 ity in the typical ossified skull are di\^sible into three segments 

 — I. Occipital, II. Parietal, III. Frontal — the parts of which 

 are arranged with reference to one another, the sensory organs 

 and the exits of the first, second, fifth, and tenth pairs of 

 cranial nerves (i., n., Y., and x.), in the manner shown in the 

 diagram * on the preceding page. 



The cartilaginous cases of the organs of hearing, or the 

 periotlc capsules^ are, as has been said, incorporated 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 probtic / the one 

 behind and below, the opisthotic / and the one which lies 

 above, and externally, the epiotic. The last is in especial re- 

 lation 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. 

 Tliese three ossifications may coalesce into one, as when they 

 constitute \kiQ 2')etrosal and inastoid 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-occipi- 

 tals, leaving the prootic distinct. The prootic is, in fact, one 

 of the most constant bones of the skull in the lower Vertebra- 

 ta, though it is commonly mistaken, on the one hand for the 

 alisphenoid, and on the other for the entire petro-mastoid. 

 Sometimes a fourth, ^^^ero^/c ossification, is added to the three 

 already mentioned. It lies on the upper and outer part of the 

 ear-capsule between the prootic and the epiotic (see the fig- 

 ure of the cartilaginous cranium of the Pike, i7ifrd). 7> /Ji. 



In some Vertebrata the base of the skull exliibits a long 

 and distinct splint-like mxcmbrane bone f — the parasphenoid^ 



* The names of the purely memhrane bones in this diagram are in large 

 capitals, as PARIETAL ; while those of the bones which are preformed in 

 cartilage are in smaller type, as Basisphek-qid. 



t Bones may he formed in two ways. They may he preceded by cartilage, 

 and the ossific deposit in the place of the future bone may at first be deposited 

 in the matrix of that cartilage, or the ossific deposit may take place, from tiie 

 first, in indifferent, or rudimentary connective, tissue. In this case the bone 

 b not prefigured by cartilage. In the skulls of Elasmohranch fishes, and in 

 the sternum and epicoracoid of Lizards, the bony matter is simply ossified car- 

 tilage, or cartilage bone. The parietal or frontal bones, on the other hand, 

 are always devoid of cartilaginous rudiments, or, in other words, are memhrane 

 hones. 



In the higher Verteh'ata the cartilage bones rarely, if ever, remain as such; 

 but the p-imitive ossified cartilage becomes, in great measure, absorbed ana 

 '■fcplaced by membrane bone, derived from the perichondrium. 



