. Animal physiology. Physiology, Comparative. BONES OF FISHES : TEETH. 67 cartilage, but its conversion into true bony structure never taking place. In a few, not even a firm cartilage is produced; and all the trace of a skeleton is a cylinder formed of hex- agonal cells, resembling those of the pith of plants, which takes the place that is generally occupied by the " boclies " of the vertebrae (§ 71). Such a cylinder, "vvhich is termed the chorda dorsalis, precedes the formation of the vertebral column in other vertebratecl animals (§757). In the curious Arnphioxzis (ZooL. § 64
. Animal physiology. Physiology, Comparative. BONES OF FISHES : TEETH. 67 cartilage, but its conversion into true bony structure never taking place. In a few, not even a firm cartilage is produced; and all the trace of a skeleton is a cylinder formed of hex- agonal cells, resembling those of the pith of plants, which takes the place that is generally occupied by the " boclies " of the vertebrae (§ 71). Such a cylinder, "vvhich is termed the chorda dorsalis, precedes the formation of the vertebral column in other vertebratecl animals (§757). In the curious Arnphioxzis (ZooL. § 642), even this is wanting; and the only rudiment of the bony skeleton is to be found in the fibrous sheath that surrounds the nervous centres, and sends off prolongations between the successive transverse bands of muscles, which are attached to these, as they are in other fishes to the ribs and the spines of the vertebrge. 54. In connexion with the structure of Bone, it will be convenient to describe that of Teeth, although the general description of the form and development of these organs will be more appropriately given in connexion with the account of their instrumental uses (§§ 181—183). The principal part of the substance of all teetli is made up of a solid tissue, which has been appropriately called Dentine. Of tliis sub- stance, one variety, which is peculiarly close in texture, and susceptible of a high polish, is familiarly known as 'diiori/. The more perfect forms of dentine, such as present them- selves in Man and the Mammalia generally, consist of a-hard transparent substance formed by the union of animal matter and calcareous salts (chiefly phos- phate of lime), in the proportion of about 28 of the former to 72 of the latter; the mineral matter thus bearing a somewhat larger ratio to the organic, than it' bone. This dentinal substance is traversed by minute tubuli of about 1-10,000th of an inch in diameter, Avliich appear as dark Lines, generally ver
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