The Dental cosmos . full-grown animal, are the gill arches, which origi-nally are common to all vertebrate animals, but which at a later period are transformed into the most differentorgans. Everyone knows the gill-arches of fish, those arched boneswhich lie behind one another to thenumber of three or four, on each sideof the neck, and which support thegills, the respiratory organs of thefish. Now, these gill-arches origi-nally exist in embryo exactly the samein man, in dogs, in fowls, in tortoises,as well as in other vertebrate animals.(See Fig. 1.) It is only in fishes thatthese remain in th
The Dental cosmos . full-grown animal, are the gill arches, which origi-nally are common to all vertebrate animals, but which at a later period are transformed into the most differentorgans. Everyone knows the gill-arches of fish, those arched boneswhich lie behind one another to thenumber of three or four, on each sideof the neck, and which support thegills, the respiratory organs of thefish. Now, these gill-arches origi-nally exist in embryo exactly the samein man, in dogs, in fowls, in tortoises,as well as in other vertebrate animals.(See Fig. 1.) It is only in fishes thatthese remain in their original formand develop into respiratory the other vertebrate animals theyare partly employed in the formationof the face (especially the jaw appa-ratus) and partly in the formation of the organ of hearing. An examination of the human embryo in the third or fourth weekof its existence shows it to be altogether different from the fully de-veloped man, and that it exactly corresponds to the undeveloped. Embryo ok Man at Eighth Week. THE ORIGIN AND EVOLUTION OF THE FACE. 76l embryo form presented by the ape, dog, rabbit, or other mammalsat the same stage of their ontogeny. At this stage it is a bean-shapedbody of very simple structure, with a tail behind and two pairs of pad-dles, resembling the fins of a fish. Nearly the whole of the front halfof the body consists of a shapeless bud without a face, on the sidesof which are several gill-fissures and gill-arches as in the embryo of man, as in all other vertebrates, the very remarkableand important structures which are called the gill-arches and gill-openings appear at a very early period, on each side of the are among the characteristic and never-failing organs of verte-brates. . The number of these gill-arches and of the gill-openings between amounts in the higher vertebrates to four or fiveon each side ; but the lower forms have a yet larger number. Origi-nally they are used for respiration. In
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectdentist, bookyear1890
