FORMATION OF DENTINE. 81 



little uncalcified organic matter usually remains upon 

 the outer surface of the enamel. The markings seen 

 in a transverse section of enamel receive a simple 

 explanation upon this vieAv of the development of the 

 tissue. The outer uncalcified portion of the rods, 

 when acted upon by acetic acid, swells up ; and the 

 appearance as of a membrane covering the enamel is 

 produced. Some have been led to the conclusion that 

 an actual membrane (basement membrane) or mem- 

 brana preformativa, actually existed in this situation. 

 Consult Mr. Tomes's " Dental Surgery," p. 268. 



121. Formation Of Dentine. — The dentine of the tooth 

 begins to form before the enamel. But very soon after 

 the formation of enamel has commenced the two opera- 

 tions go on together until a short time before the tooth 

 emerges from the gum. The production of enamel 

 is then completed, while that of dentine continues 

 more slowly as age advances, but the development 

 of this tissue does not cease in some cases before a 

 considerable age is reached. In certain instances — 

 as, for example, in the case of the canines of some of 

 the lower animals and in the incisors of the rodents — 

 the formation of both structures continues throusrh 

 life, so that in the teeth of the adult the development 

 of the enamel and dentine may be studied as well as 

 during the very early period of life in other cases. ^ 

 The oval masses of bioplasm taking part in the 

 formation of the dentine are larger than those of the 

 enamel, and the formed material produced by them 

 appears as a continuous matrix rather than as dis- 

 tinct and separate columns. Moreover, instead of 

 each mass forming a separate oval body, a thin line 

 of tissue is drawn oiit as the dentine bioplasm 

 moves inwards. These lines of soft tissue correspond 

 to what are generally termed the dentinal tubes, and 

 may be forcibly withdrawn where the process of 

 calcification is nearly completed. In thin sections 

 the corresponding "tubes" from which the processes 



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