DEVELOPEMENT OF TEETH. 65 



vested by a thin layer of membrane, which is continued over the 

 root, and is reflected through the .opening in the apex of the fang 

 into the cavitas pulpae, which it lines throughout. This membrane 

 is considered by Mr. Nasmyth to be the " persistent dental capsule" 



The cortical substance, or cementum, forms a thin coating over 

 the root of the tooth, from the termination of the enamel to the 

 opening of the apex of the fang. In structure it consists of true 

 bone, characterized by the existence of numerous calcigerous 

 cells and tubuli. The cementum increases in thickness with the 

 advance of age, and gives rise to those exostosed appearances 

 occasionally seen in the teeth of very old persons, or in those who 

 have taken much mercury. In old age the cavitas pulpae is often 

 found 1 filled up and obliterated by osseous substance analogous to 

 the cementum. 



Developement. The developement.of the teeth in the human sub- 

 ject has been most successfully investigated by our countryman, 

 Mr. Goodsir, to whose interesting 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 jaw presents two semicircular folds around its circum- 

 ference ; 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 appearances from without inwards at this 

 period is the following : Most externally, and forming the boun- 

 dary 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 rudi- 

 ment 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 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 papilla? 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, 

 the tenth week, the papilla? or germs of the whole of the ten deci- 

 duous 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 



* " On the Origin and Developement of the Pulps and Sacs of the Human Teeth," 

 by John Goodsir, jun., in the Edinburgh Medical and Surgical Journal, January 1839. 



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