66 DEVELOPMENT OF TEETH. 



cementura. Mr. Nasmyth has shewn that this, like the other struc- 

 tures composing a tooth, is formed of cells having a reticular arrange- 

 ment. 



Development. The development of the teeth in the human subject 

 has been successfully investigated by Mr. Goodsir, to whose interest- 

 ing researches I am indebted for the following narrative :* 



The inquiries of Mr. Goodsir commenced as early as the sixth week 

 after conception, in an embryo, which measured seven lines and a half 

 in length and weighed fifteen grains. At this early period each upper 

 jaw presents two semicircular folds around its circumference ; the 

 most external is the true lip ; the internal, the rudiment of the palate ; 

 and between these is a deep groove, lined by the common mucous 

 membrane of the mouth. A little later, a ridge is developed from the 

 floor of this groove in a direction from behind forwards, this is the 

 rudiment of the external alveolus ; and the arrangement of the appear 

 ances from without inwards at this period is the following: Most 

 externally and forming the boundary of the mouth, is the lip ; next 

 we find a deep groove, which separates the lip from the future jaw ; 

 then comes the external alveolar ridge; fourthly, another groove, in 

 which the germs of the teeth are developed, the primitive dental 

 groove ; fifthly, a rudiment of the internal alveolar ridge; and, 

 sixthly, the rudiment of the future palate bounding the whole in- 

 ternally. At the seventh week the germ of the first deciduous molar 

 of the upper jaw has made its appearance, in the form of a " simple, 

 free, granular papilla " of the mucous membrane, projecting from the 

 floor of the primitive dental groove ; at the eighth week, the papilla 

 of the canine tooth is developed ; at the ninth week, the papillae of the 

 four incisors (the middle preceding the lateral) appear ; and at the 

 tenth week the papilla of the second molar is seen behind the anterior 

 molar in the primitive dental groove. So that at this early period, 

 viz. the tenth week, the papillae or germs of the whole ten deciduous 

 teeth of the upper jaw are quite distinct. Those of the lower jaw are 

 a little more tardy ; the papilla of the first molar is merely a slight 

 bulging at the seventh week, and the tenth papilla is not apparent 

 until the eleventh week. 



From about the eighth week the primitive dental groove becomes 

 contracted before and behind the first deciduous molar, and laminae of 

 the mucous membrane are developed around the other papillae, which 

 increase in growth and enclose the papillae in follicles with open 

 mouths. At the tenth week the follicle of the first molar is com- 

 pleted, then that of the canine ; during the eleventh and twelfth 

 weeks the follicles of the incisors succeed, and at the thirteenth week 

 the follicle of the posterior deciduous molar. 



During the thirteenth week the papillae undergo an alteration of 

 form, and assume the shape of the teeth they are intended to repre- 



" On the Origin and Development of the Pulps and Sacs of the Human 

 Teeth," by John Goodsir, jun. in the Edinburgh Medical and Surgical Journal, 

 January 183Q. 



