670 



MAMMALIA. 



course, intercligitate with the ivory-forming pulps that arise from 

 the bottom of the sockets, and thus the hard materials formed by 

 them take the same arrangement. After these structures have been 

 completed, one or other of the sets of pulps, most probably the 

 enamel pulps, changing their action, fill up all the intervening 

 spaces with the crusta petrosa. 



(748.) As during the growth of a quadruped the size of the 

 jaws is continually increasing, a necessity exists for changing the 

 teeth once or oftener during the life of the animal, in order to 

 adapt these organs to the altered conditions required : hence the 

 necessity for shedding the teeth of young animals, and replacing 

 them with others of larger dimensions or more numerous than the 

 first set. 



This is effected in two different ways, each of which demands 

 our separate notice. 



In most quadrupeds, as, for example, in the Carmvora, the 

 Quadrumana, and the greater number of herbivorous genera, 

 the succession of the teeth is provided for precisely in the same 

 way as in our own persons, namely, by the formation of a new 

 tooth below each of the deciduous ones (fig- 313, d, d) ; so that, 

 when the latter falls out in consequence of the absorption of its 

 fangs, the former is ready to take its place. The germ of the second 

 tooth is at first found imbedded in the jaw-bone, in the immediate 

 vicinity of the roots of the one which it is destined to replace ; 

 and, as its growth advances, the old and used tooth is gradually 

 removed to make way for the new comer. The steps of this pro- 

 cess are exactly similar to those by which the milk-teeth of a child 

 are changed, and the details connected with it are familiar to every 

 anatomist. 



But in the Elephant, and some other genera of PACHYDER- 

 MATA, the succession of the teeth is effected in a different man- 

 ner ; the place of the first formed being supplied by others that 

 advance from behind as the former become used. Animals exhi- 

 biting this mcde of dentition have the grinding surfaces of their 

 molar teeth placed obliquely ;* so that, if they were to issue 

 altogether from the gum, the anterior portion would be much more 

 prominent than the posterior, notwithstanding that the opposed 

 teeth act upon each other in a horizontal plane. The consequence 

 of this arrangement is, that the anterior portion of these teeth is 

 ground down to the roots, and worn away sooner than the poste- 



* Cuv. Lemons d'Anat. Comp. torn. iii. p. 122. 



