. The Cambridge natural history. Zoology. DENTITION 249 dermal spines form a continuous series with those wliich invest the jaws aud eventually become teeth (Fig. 151). It is only later, when lips become apparent, that the continuity of the teeth and dermal spines is interrupted, and the two structures assume their distinc- tive characters. The tissues of which the teeth of Fishes are composed are (1) den- tine, which is a non- vascular, calcified tissue, traversed by numerous radiating, branched, dentinal tubuli, into which ex- tend protoplasmic pro- longations from the cells (scleroblasts) b
. The Cambridge natural history. Zoology. DENTITION 249 dermal spines form a continuous series with those wliich invest the jaws aud eventually become teeth (Fig. 151). It is only later, when lips become apparent, that the continuity of the teeth and dermal spines is interrupted, and the two structures assume their distinc- tive characters. The tissues of which the teeth of Fishes are composed are (1) den- tine, which is a non- vascular, calcified tissue, traversed by numerous radiating, branched, dentinal tubuli, into which ex- tend protoplasmic pro- longations from the cells (scleroblasts) by which the dentine is secreted. Dentine forms the greater part of the body of a tooth. (2) vasodentine and (3) osteodentine are modifications of ordi- nary dentine, former containing blood - vessels ramify- ing in its substance but no dentinal tubules, and the latter more closely resembling bone. (4) enamel, an exceptionally dense, non - vascular, non- tubular tissue, which may or may not exhibit traces of the prismatic structure so characteristic of this tissue in the higher Vertebrates, forms the outer investment of the teeth. As regards their fixation, the more primitive forms of teeth, such as those of Elasmobranchs, are simply embedded in the gums, and are only connected with the jaws by fibrous tissue; but in some of the older fossil Sharks the fixation of the teeth is effected by the mutual articulation of the basal plates of the teeth with one another. The Chondrostean Polyodon, so shark-. f V. g Fig. 151.—Transverse section through the lower jaw of an embryo Scyllium, to show the gradual transition from dermal spines (d, d, d) on the outer surface of the jaw to teeth (t, t, f) on the oral surface, c. Car- tilage of the lower jaw. (From Gegenbaur.). Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectzoology, bookyear1895
