CHAP. XXIII.] FIRST AND SECOND DENTITION. 179 



part in the development and subsequent nutrition of the organs. 

 The nerves of the teeth are derived from the second and third divi- 

 sions of the fifth pair. 



Of the First and Second Dentitions. As teeth are required before 

 the jaw-bones have attained their full growth, and yet are organs in- 

 capable of enlarging pari passu with those bones, the young child is 

 provided with a temporary set, commonly known as the milk-teeth, 

 adapted to the size and form of its alveolar arches, and to the nature 

 of the food consumed in early life. This set consists of twenty 

 teeth four incisors, two canines, and four molars in each jaw. They 

 are formed in the manner already described : the papillae of the an- 

 terior molars appearing first between the sixth and seventh week of 

 foetal existence, according to Mr. Goodsir followed by those of the 

 canines, incisors, and posterior molars, about the eighth, ninth, and 

 tenth weeks respectively. About the fourth month all these are in 

 their saccular stage, the mouths of the follicles having closed ; and 

 there then appear behind the opercula, or lids of the follicles, small 

 crescentic depressions of the mucous membrane, soon becoming closed 

 cavities, called by the last-named author " cavities of reserve, to fur- 

 nish delicate mucous membrane for the future formation of the pulps 

 and sacs of the ten anterior permanent teeth" in each jaw. For two 

 or three weeks longer the primitive dental groove behind the pos- 

 terior milk molar is furnishing the papilla of the first permanent 

 true molar ; and as this becomes gradually in its turn enclosed in 

 a sac, a cavity of mucous membrane is said, by Mr. Goodsir, to be 

 left uiiobliterated between its sac and the surface of the gum, which 

 is the " cavity of reserve " from which the development, first, of 

 the second true molar, and, secondly, of the third or wisdom tooth, 

 is afterwards to proceed. 



The temporary teeth usually make their way through the gum 

 as follows, those of the lower jaw taking precedence : the four cen- 

 tral incisors about the seventh month after birth; the four lateral 

 incisors from the seventh to the tenth ; the anterior molars from 

 the twelfth to the fourteenth ; the canines from the fourteenth to 

 the twentieth ; and the posterior molars from the eighteenth to the 

 thirty-sixth month. This whole period is called that of the first 

 dentition, and is of great importance to the child, from the various 

 sympathetic morbid states which universal experience attributes to 

 the process of " cutting the teeth" ; but it would be beside our 

 purpose to dilate in this place on so interesting and prolific a theme. 

 It may suffice to say, that, in our opinion, the practice of 

 lancing the gum over an advancing tooth is often unnecessarily and 



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