198 ANIMAL PHYSIOLOGY. [LESSON 56. 



the tissues will be invariably as follows : crusta petrosa in tbe centre, 

 bounded on each side by ivory, which always adjoins enamel ; crusta 

 petrosa and enamel never come together (except on the outer sur- 

 face) ; they are always separated by the ivory. 



845. These facts are well illustrated by a figure of the grinder 

 of a Sheep (Fig. 321) ; here the enamel (a, a) is constantly seen in 

 connection with the ivory (5, 5), and inside of this the crusta petrosa 

 (c). The elegant wavy form of the dentinal tubuli renders this a 

 very charming object for the microscope. The crusta petrosa (more 

 properly called cementum in these teeth, as it performs the office of 

 a cement to the other tissues) forms, in many instances, little more 

 than a line in the centre of the ivory, yet quite sufficient for its soft- 

 ness to be felt, and the required hollow to be formed. 



LESSON LYI. 



THE STEUCTUEE OF THE TEETH, CONTINUED. 



846. In the Rodentia, the form of the teeth is very interesting : 

 the enamel in the large, curved incisors, is always found upon the 

 opposing surface (front) alone. These teeth, in every member of the 

 class, are constantly being pushed, as it were, from behind, forwards ; 

 in other words, continually growing; and to gnaw, from which they 

 acquire their name, is in them an imperative necessity, to keep these 

 teeth in check, and reduce them within usable limits. Were Rats, 

 Mice, Rabbits, and their kindred, not to keep their ever-growing 

 teeth well cut down, by gnawing hard substances, by which the teeth 

 wear considerably, they would soon be unable to use them on legiti- 

 mate food, and die of starvation. 



847. If a Rat, or Rabbit, chance to break one incisor tooth, the 

 * opposite tooth fails to oppose it ; the consequence is, it continues to 



grow, and forms a perfect ring. The broken tooth also grows, and 

 having attained its original length, and finding no opposition, also 

 forms a ring. 



848. To obtain such illustrations for Museum purposes, to show 

 the ever-growing nature of such teeth, one of them has frequently 

 been designedly cut with a fine saw, or wire-nippers, to procure 

 specimens of the curled, or even double-curled teeth ; such examples 

 are common in European Museums. 



