50 ON THE ORIGIN AND DEVELOPMENT OF THE 



been completed, tlie external lip begins to increase, and the 

 internal to diminish. This increase of the external lip goes 

 on after the closure of the secondary groove, until, at the fifth 

 month, it becomes very prominent, and is divided into an in- 

 cisive, a canine, and molar portion, each of which has a general 

 similarity in shape to the acting portions of the correspond- 

 ing divisions of the future tooth-ranges. As long as it remains 

 in this condition it is employed by the infant as a masti- 

 cating organ. During this period the internal lip has alto- 

 gether disappeared, except a small portion posteriorly ; but a 

 short time before the milk-teeth appear, it again increases, and 

 the raphe of the dental groove, instead of being hid behind the 

 base of the external lip, is situated on the ridge of the dental 

 arch, which now, as at first, is composed of two equally-deve- 

 loped portions. The raphe forms a little border in the 

 situation just mentioned, and is familiar to the eye of the 

 surgeon, who, by its disappearance at any particular point, 

 can satisfy himself of the proximity of the milk-tooth under it. 



Careful observation of the wliole process of Dentition in man 

 leads to the following conclusions: — 



Milk Teeth. — 1. The milk-teeth are formed on both sides of 

 either jaw, in three divisions, a molar, a canine, and an incisive, 

 in each of which dentition proceeds in an independent manner. 



2. The dentition of the whole arch proceeds from behind 

 forwards — the molar division commencing before the canine, 

 and the latter before the incisive. 



3. The dentition of each of the divisions proceeds in a 

 contrary direction, the anterior molar appearing before the 

 posterior, the central incisive before the lateral. 



4. Two of the subordinate phenomena of dentition also 

 obey this inverse law, the follicles closing by commencing at 

 the median line, and proceeding backwards, and the dental 

 groove disappearing in the same direction. 



