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 “ bodies” of 
the vertebree (§ 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 (Zoot. § 642), even this is wanting; and the 
only rudiment of the bony skeleton is to be found im 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 vertebre. 
54. In connexion with the structure of Bone, it will be 
convenient to describe that of Zeeth, 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 2vory. 
The more perfect forms of dentine, such as present them- 
Ives in Man and the Mammalia generally, consist of a-hard 
nt 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 ; themineral matter thus 
‘bearing a somewhat larger ratio | 
to the organic, than it’ does -in 
me. This dentinal substance is 
traversed by minute tubuli of 
ut 1-10,000th of an inch in 
liameter, which appear as dark & 
es, generally very close to- Fig. 18. 
: iating P EXTINE (hi 
wea from fhe contral cavite, Sc) sboring ts tabuat sacane. 
the tooth, diverging from each other as they approach 
exterior ; but when seen in only a small part of their 
F 2 
