26 THE ANATOMY OF VERTEBRATE!) ANIMAI,S. 



called parietal and frontal. Thus the walls of the cranial cgv 

 iiy in the typical ossified skull are divisible into three segment." 

 —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., ii., v., and x.), in the manner shown in the 

 diagram * on the preceding page. 



The cartilaginous cases of the organs of hearing, or the 

 periotic capsules, are, as has been said, incorporated with the 

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

 other words, between the opcipital 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 prootic ; the one 

 behind and below, the opisthotio • 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. 

 These three ossifications may coalesce into one, as when they 

 constitute the petrosal and mastoid parts of the temporal bone 

 of human anatomy ; or the epiotic, or the opisthotio, or both, 

 may coalesce with the adjacent supra-occipital and ex-occipi- 

 tals, leaving the prootio distinct. The proQtic is, in fact, one 

 of the most constant bones of the skull in the lower Vet'tebra- 

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

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

 Sometimes a iourth, pter otic ossification, is added to the three 

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

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

 ure of the cartilaginous cranium of the Pike, infrd). 



In some Vertehrata the base of the skull exhibits a long 

 and distinct splint-like membrane bone f — the parasphenoid, 



* The nam-es of the purely membrane bones in this diagram are in large 

 capitals, as PAEIETAL ; whila tiiosu of the bones which are preformed in 

 cartilage are in smaller type, as Basisphenoid. 



+ Bones may bo formed in two ways. They may be preceded by cartilage, 

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

 iu the matrix of that cartilage, or the ossiflc deposit may take place, from the 

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

 is not prefigured by cartilage. In the skulls of Elasmobraneh 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 memimne 

 bones. 



In the higher Vertehrata the cartilage bones rarely, if ever, remain as such : 

 but the p"iniitive ossified cartilage bcoomea, in great measure absorbed and 

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



