Fi?. 156. 



348 TBE DIGESTIVE APPABATUS IN MAMMALIA. 



a kind of cup hollowed on the summit of the latter appendages (o). These 

 are covered by the membrane of cylindrical cells mentioned above (d). 



With regard to the enamel organ, its internal face also presents a layer 

 of cylindrical cells. 



it is in the interval between these two papillary systems, that the dental 

 substance is deposited as in a mould, consequent on a process of secretion 

 and transformation, the mechanism and progress of which are somewhat 

 complicated. The dentine is produced by the metamorphosis of the super- 

 ficial cells of the dental germ. These cells send out ramifying prolongations 

 which constitute the tubuli of the dentine, and those of the middle layer 

 secrete an intertubular amorphous substance in which the earthy salts are 

 deposited from without inwards. The enamel is deposited on the dentine, 

 and results from the transformation of the cylindrical 

 cells of the germ into enamel prisms. The cement is, 

 in its turn, deposited either on the enamel or the 

 dentine after their formation, and is produced, like 

 the bony tissue, by the internal face of this (perio- 

 dontal) sac, which has become alveolar periosteum. 

 This development will be alluded to in the chapter on 

 the foetus. 



When formed by the process above indicated, the 

 tooth pierces its follicle and appears in the interior of 

 the mouth, after having traversed the table of the 

 maxillary bones, if there is any, and the gingival 

 membrane. (When the calcification of the different 

 tissues of the tooth is sufficiently advanced to enable 

 THEORETICAL SECTION OF it to boar the pressure to which it will be afterwards 

 subjected, its eruption takes place, the tooth making 

 its way through the gum. The gum is absorbed by 

 the pressure of the crown of the tooth against it, 

 which is itself pressed up by the increasing size of 

 Papilla of the 'external the fang. Concurrent with this, the septa between 

 cavity (pit) of the tooth, the dental sacs, at first fibrous in structure, soon ossify, 

 mermembranf-'DT!- '*°'^ constitute the alveoli; these firmly embrace the 

 thelial layer of the den- necks of the teeth, and afford them a solid basis of 

 tine membrane ; E, Cy- Support — Gray.) Though it has SO far become esta- 

 lindiical cells of the blished in its functions, the process of growth in the 

 Den"Tne"T''Tname^' ^^°^^ ^^^ °°' ^^^ ceased. The pulp lodged in the 

 The secretion of the ce- internal dental cavity, and charged with the formation 

 ment is not supposed to of the ivory Or dentine, continues its functions : inces- 

 have commenced. santly depositing new layers on those which were 



originally secreted. The dental cavity gradually 

 diminishes in extent ; the papilla becomes atrophied, and finishes by disap- 

 pearing altogether at a period of life more or less advanced, according to 

 the kind of teeth and species of the animal. 



In considering the entire dental apparatus, with regard to its develop- 

 ment, very interesting differences are remarked in the progress and period 

 of evolution ; differences which have been made available for ascertaining 

 the age of animals, but the details of which would be out of place here. 

 It may only be noted, that all animals have two successive dentitions : the 

 first, composed of a certain number of teeth designated the caducous (tem^ 

 porary, deciduous, or milk-teeth — caduques, decaying or frail), because they 

 are soon shed and give place to others which are stronger and more solid 



THE DENTAL SAO OF A 

 PEUMANEN'T INCISOR IN 

 THE HORSE. 



A, Proper membrane of the 

 sac; B, Dental pulp; C, 



