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 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 "ivory* 

 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- _ T ^_ mr __ TTTTT ^_^ n _ T ^. r ^ rimr _ TTr . i 

 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 an 

 bone. This dentinal substance is 

 traversed by minute tubuli of 

 about l-10,000th of an inch in 

 diameter, which appear as dark 

 lines, generally very close to- Fig. is. 



gether ; these pass in a radiating PORTION OF DENTINE (highly magni- 



manner from the central cavity fied), showing Us tubular structure. 

 of the tooth, diverging from each other as they approach 

 its exterior; but when seen in only a small part of their 



F 2 



