. The Cambridge natural history. Zoology. 44 STRUCTURE OF TEETH of the nature of teeth is founded. I ^ ir^ M. ITi. It has been pointed out that the scales of the Elas- mobranch fishes consist of a cap of enamel upon a Ijase of dentine, the former Ijeing derived from the epi- dermis and modelled upon a papilla of the dermis whose cells secrete the dentine. TJie fact that similar structures arise within the mouth { the teeth) is explicable when it is remembered that the mouth itself is a late in- vagination from the out- side of the body, and that therefore the retention by its tissues of th
. The Cambridge natural history. Zoology. 44 STRUCTURE OF TEETH of the nature of teeth is founded. I ^ ir^ M. ITi. It has been pointed out that the scales of the Elas- mobranch fishes consist of a cap of enamel upon a Ijase of dentine, the former Ijeing derived from the epi- dermis and modelled upon a papilla of the dermis whose cells secrete the dentine. TJie fact that similar structures arise within the mouth { the teeth) is explicable when it is remembered that the mouth itself is a late in- vagination from the out- side of the body, and that therefore the retention by its tissues of the capacity to produce such structures is not remarkable. The relations of the three constituents of the tooth in its simplest form is shown in the accom- panying diagram, where the intimate structure of dentine, and cement (or crusta petrosa Fig. 33.âDiagrammatic sections of various forms of teeth. /, Incisor or tusk of Elephant, ivitli pulp cavity persistently open at base ; the enamel //, Hnman incisor during development, with root imperfectly formed, and pulp cavity widely open at ; III, completely formed aS it is Sometimes Called) Human incisor, with pulp cavity opening by . j. ⢠i- . i mi i a contracted aperture at base of root; IV, IS not indicated. 1 he latter Human molar with broad crown and two J^aS the closest resemblance roots; V, molar of the Ox, with the enamel . covering the crown deeply folded, and the tO bonC. The dentine IS depressions filled up with cement; the sur- traversed by fine canals lace IS worn by use, otherwise the enamel . â ' coating would be continuous at the top of the wllich run parallel to Cach ridges. In all the figures the enamel is black, â4-V,p,, and amstni-nncp liovo the pulp white ; the dentine represented by °^^^^ ^^^ auastomose lieie horizontal lines, and the cement by dots, and there. The enamel is (After Flower and Lydekker.) o i p i lormed of long prismatic fibres, and is excessively hard in structure, containing
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectzoology, bookyear1895
