BONES OP 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 “ bodies ” of 
the vertebrae (§ 71). Such a cylinder, which is termed the 
chorda dorsalis , precedes the formation of the vertebral 
column in other vertebrated animals (§ 757). In the curious 
Amphioxus (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 vertebrae. 
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 teeth is made up of a solid tissue, 
which has been appropriately called Dentine. Of this sub¬ 
stance, one variety, which is peculiarly close in texture, and 
susceptible of a high polish, is familiarly known as %vory. 
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" does dn 
bone. This dentinal substance is 
traversed by minute tubuli of 
about 1-10,000th of an inch in 
diameter, which appear as dark 
fines, generally very close to¬ 
gether : these pass in a radiating Portion of Dentine (highly magni- 
^ n jV , -i fied), showing its tubular structure. 
manner from the central cavity h 5 
of the tooth, diverging from each other as they approach 
its exterior; but when seen in only a small part of their 
f 2 
Fig. 18. 
