This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1908 Excerpt: ...rays, which give rise distally to a series of polygonal plates. It will be noticed that while the skeleton of the crayfish is a series of articulated tubes, with the muscles inside them, that of the dogfish and of the frog is a series of articulated rods with the muscles outside. The joints, formed by two rods applied ...
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This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1908 Excerpt: ...rays, which give rise distally to a series of polygonal plates. It will be noticed that while the skeleton of the crayfish is a series of articulated tubes, with the muscles inside them, that of the dogfish and of the frog is a series of articulated rods with the muscles outside. The joints, formed by two rods applied at their ends and bound together by ligament, are not all confined to movement in one plane, like the hinge-joints of the crayfish, but may be capable of more or less rotatory movement. Digestive organs.--The mouth, as we have seen, is a transverse aperture bounded by the upper and lower jaws. Fig. 116.--Diagram of the development of a tooth. Bg, Bg. mesoderm; DS. dentine; KM. epithelium of mouth; Ma. epithelium of enamel-organ; O. odontoblasts;.S A'e dental lamina; ZK. dental papilla. (From Wiedersheim's Comparative Anatomy.) In the mucous membrane covering the jaws are imbedded large numbers of teeth--conical, calcified bodies, with enamelled tips, arranged in transverse rows. They are to be looked upon as special developments of the placoid scales or dermal teeth (p. 433) enlarged for the pur Chap, x ARTERIES 451 pose of seizing prey, and are continually renewed on the inner sides of the jaws as they are worn away on the outer sides. The teeth, in Vertebrates generally, are developed in the following manner. The ectodermal epithelium of the mouth (stomodaum, p. 204)--or in the case of the dermal teeth of the dogfish that covering the body generally--grows inwards to form a ridge or dental lamina (Fig. 116, SK) which projects into the underlying mesodermal connective-tissue and becomes enlarged distally to form a bell-shaped enamel-organ, into the base of which a mesodermal dental papilla (ZK) extends: the superficial part of this papilla fo...
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