244 



THE DIGESTIVE ORGANS. 



the papillae become lengthened, and after passing through further 

 changes, finally develop into the enamel prisms of the teeth. At 

 the periphery of the dentinal papillae, there is differentiated a layer 

 of columnar cells, the odontoblasts, which have a connective-tissue 

 origin, and later form the dentin. During these processes a 

 connective -tissue mantle, the dental sac, rich in cellular and fibrous 

 elements, is formed around each tooth anlage. 



The earliest appearance of the enamel is in the form of a cuticu- 

 lar membrane, developed from the ends of the enamel cells resting 

 on the dentinal papilla, this cuticular membrane appearing in the form 

 of a thin layer covering the top of the dentinal papilla. Sometime 

 later, short striated processes Tomes' processes appear on the 



Odontoblasts. 



Terminal nerve- 

 fiber. 



Odontoblasts. 



- - Terminal nerve- 

 fiber. 



Fig. 185. Nerve termination in the pulp of a rabbit's molar, stained in methylene- 

 blue (intra vi(am) : a, Odontoblasts seen in side view ; b, a number of odontoblasts seen 

 in end view, showing a terminal branch of a nerve-fiber situated between the odonto- 

 blasts and the dentin (Huber, "Dental Cosmos," October, 1898). 



lower end of each of the enamel cells (the end toward the dentinal 

 papilla). These are imbedded in a cement-substance, forming a 

 continuous layer. The Tomes' processes are regarded as the be- 

 ginnings of the enamel prisms. Calcification begins in the middle 

 of these processes ; they thicken at the expense of the cement- 

 substance surrounding them, which later also calcifies. The enamel 

 as a whole thickens by the elongation of the Tomes' processes of 

 the enamel cells and by their subsequent calcification. The process 

 ends finally in the death and partial absorption of the enamel cells 

 and the remaining elements of the enamel organs ; these structures 

 persist for a short time after the eruption of the tooth as a cuticular 

 sheath. 



The dentin is developed by the odontoblasts by a process 

 analogous to that observed in the formation of bone by the osteo- 

 blasts. These epithelioid cells secrete at their outer surfaces a 

 homogeneous substance which fuses to form a continuous layer, 

 the membrana pr&formativa. The further development of the dentin 

 is as follows : Its ground-substance is deposited at the cost of the 

 lateral portions of the odontoblasts (under the membrana praeforma- 



