268 ANATOMY OF VERTEBRATES. 



socket, fig. 256, which results from the development of two or 

 more fangs, is peculiar to animals of the class Mammalia. 



In the formation of a single fang, the activity of the reproduc- 

 tive process becomes enfeebled at the circumference, and is pro- 

 gressively contracted within narrower limits in relation to a single 

 centre, until it ceases at the completion of the apex of the fang, 

 which, though for a long time perforated for the admission of the 

 vessels and nerves to the interior of the tooth, is, in many cases, 

 finally closed by the ossification of the remaining part of the 

 capsule. 



When a tooth is destined to be implanted by two or more 

 fangs, the reproduction of the pulp is restricted to two or more 

 parts of the base of the coronal portion of the pulp, around the 

 centre of which parts the sphere of its reproductive activity is 

 progressively contracted. The intervening parts of the base of 

 the coronal pulp adhere to the capsule, which is simultaneously 

 calcified with them, covering those parts of the base of the crown 

 of the tooth with a layer of cement. The ossification of the sur- 

 rounding jaw, being governed by the changes in the soft but 

 highly organised dental matrix, fills up the spaces unoccupied by 

 the contracted and divided pulp, and affords, by its periosteum, a 

 surface for the adhesion of the cement or ossified capsule covering 

 the completed part of the tooth. 



The matrix of certain teeth does not give rise, during any 

 period of their formation, to the germ of a second tooth, destined 

 to succeed the first. This, therefore, when completed and worn 

 down, is not replaced ; all the true Cetacea are limited to this 

 simple provision of teeth. In the Armadillos, Megatherioids, and 

 Sloths, the want of germinative power, as it may be called, in the 

 matrix, is compensated by its persistence, and the consequent un- 

 interrupted growth of the teeth. In most other Mammals, the 

 matrix of certain of the first developed teeth gives origin to the 

 germ of a second tooth, which displaces its predecessor and parent. 

 All those teeth which are so displaced are called temporary, de- 

 ciduous, or milk teeth, fig. 293, d i, d 1-4. The mode and direc- 

 tion in which they are displaced and succeeded, namely from below 

 upward in the lower jaw, in both jaws vertically, are the same 

 as in the crocodile ; but the process is never repeated more than 

 once in the present class. A considerable proportion of the dental 

 series is thus changed ; the second, or permanent teeth, ib. 

 i i-p } 2-4, having a size and form as suitable to the jaws of the 

 adult as the displaced temporary teeth were adapted to those of 

 the young animal. Those permanent teeth, ib. m \-m 3, which 



