DEVELOPMENT OF TEETH. 95 



osseous substance analogous to the cementum. Mr. Nasmyth has shown 

 that this, like the other structures composing a tooth, is formed of cells 

 having a reticular arrangement. 



Development. The development of the teeth in the human subject has 

 been successfully investigated by Mr. Goodsir, to whose interesting re- 

 searches 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 be- 

 tween 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 appearances 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 de- 

 veloped, the primitive dental groove ; fifthly, a rudiment of the internal 

 alveolar ridge; and, sixthly, the rudiment of the future palate bounding 

 the whole internally. At the seventh week the germ of the first decidu- 

 ous 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 pa- 

 pilla of the canine tooth is developed ; at the ninth week, the papillaB 

 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 ante- 

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

 viz. the tenth week, the papillse 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 ; 

 seventh week, 

 week. 



From about the eighth week the primitive dental groove becomes con- 

 tracted before and behind the first deciduous molar, and laminaB of the 

 mucous membrane are developed around the other papilla, which increase 

 in growth and enclose the papillae in follicles with open mouths. At the 

 tenth week the follicle of the first molar is completed, 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 papillaB undergo, an alteration of form, 

 and assume the shape of the teeth they are intended to represent. And 

 at the same time small membranous processes are developed from &e 

 mouths of the follicles ; these processes are intended to serve the purpose 

 of opercula to the follicles, and they correspond in shape with the form 

 of the crowns of the appertaining teeth. To the follicles of the incisoi 

 teeth there are two opercula ; to the canine, three ; and to the molars a 

 number relative to the number of their tubercles, either four or five. Dur 



* " On the Origin and Development of the Pulps and Sacs of the Human Teeth," \ y 

 John Goodsir, jun., in the Edinburgh Medical and Surgical Journal, January, a 839 



L.1 J"" CliV> ^UttC \J.li3LllJLV^(,. JL HUOC Ul LiiC 1U W Cl I Cl W CU C O. LLlllC 



; the papilla of the first molar is merely a slight bulging at the 

 iek, and the tenth papilla is not apparent until the eleventh 



