THE MOUTE. 



415 



Fig. 228. 



organization. (For details, see 'Embrjologj— Development of the Digestive 

 Apparatus.) 



The dental follicle is constituted by an external enveloping membrane of a 

 cellulo-vascular nature (Fig. 228, a). It shows at its bottom the simple or 

 compound papilla, which at a later period is termed the dental pulp (b) ; this 

 organ, destined for the secretion of the dentine, then fills nearly the whole of 

 the follicle. In its upper part is observed the enamel organ, or germ {enamel 

 membrane), formed by a prolongation of the gingival epithelium, and connected 

 with the latter by a small mass of cells named the gubernaculum dentis. Most 

 frequently there is, opposite the bottom of the follicle, one or more papillae 

 which, in some cases, adhere by their whole length to one of the lateral walls 

 of the follicular sac, and the free extremities of which 

 cross those of the dentine papillse, or are buried in a 

 kind of cup on the summit of the latter appendages (c). 

 These are covered by the membrane of cyUndrical cells 

 mentioned above (d). 



With regard to the enamel organ, its internal face 

 also presents a layer of cyhndrical 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 trans- 

 formation, the mechanism and progress of which are 

 somewhat comphcated. The dentine is produced by the 

 metamorphosis of the superficial cells of the dental germ. 

 These cells send out ramifying prolongations which con- 

 stitute 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 (periodontal) sac, which has be- 

 come alveolar periosteum. 



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 

 it to bear 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 the fang. Concurrent with this, the septa between 

 the dental sacs, at first fibrous in structure, soon ossify, and constitute the 

 alveoli ; these firmly embrace the necks of the teeth, and afford them a solid 

 basis of support.) Though it has so far become estabhshed in its functions, 

 the process of growth in the tooth has not yet ceased. The pulp lodged in the 

 internal dental cavity, and charged with the formation of the ivory or dentine, 

 continues its functions — incessantly depositing new layers on those which were 



THEORETICAL SECTION OP 

 THE DENTAL SAC OF A 

 PERMANENT INCISOR IN 

 THE HORSE. 



A, Proper membrane of the 

 sac; B, dental pulp; C, 

 papilla of the external 

 cavity (pit) of the tooth, 

 a dependency of the ena- 

 mel membrane ; D, epi- 

 thelial layer of the den- 

 tme membrane ; E, cy- 

 lindrical cells of the 

 enamel membrane ; F, 

 dentine ; G, enamel. 

 The secretion of the ce- 

 ment is not supposed to 

 have commenced. 



