34 ON THE ORIGIN AND DEVELOPMENT OF THE 



of the gum. This cavity is a reserve of delicate mucous mem- 

 brane to afford materials for the formation of the second per- 

 manent molar, and of the third permanent molar or wisdom- 

 tooth. 



A little before this period tooth-substance begins to be de- 

 posited on the tubercles and apices of the pulps, which have 

 acquired round the point of deposition a raised border and a 

 zone-like vascularity ; and, synchronous with this deposition, 

 absorption takes place on the inner surface of the granular 

 matter immediately in contact with it. No vessel can be de- 

 tected running to the point of absorption, but ultimately the 

 granular matter becomes so thin as to allow the subjacent 

 vascularity to appear. The absorption goes on increasing as 

 the tooth-substance is deposited, and when the latter reaches 

 the base of the pulp the former disappears, and the interior of 

 the dental sac assumes the villous vascular appearance of a 

 mucous membrane. This change is nearly completed about 

 the seventh or eighth month. 



Up to this period little change has taken place in the ten 

 anterior, or in the two posterior or great cavities of reserve. 

 The ten anterior have been gradually receding from the sur- 

 face of the gum, so as to be posterior, instead of inferior, to the 

 milk-sacs. The two or four anterior began about the fifth 

 month to dilate at their distal extremities, across which a fold 

 appears (which is the germ of the future pulp) lying in the 

 direction of the cutting edge of the future tooth ; and at the 

 proximal or acute extremities of the cavities two other folds, 

 an anterior and a posterior, appear.* These round off the un- 



* These two folds are strictly analogous to the opercula of the milk-tooth 

 sacs. They never attain, however, the same high development as those of the 

 latter, remaining in a rudimentary state, apparently in consequence of the 

 almost saccular condition of the cavities of reserve. The existence of these 

 laminae in a rudimentary state proves that in the formation of the permanent 

 teeth there is a strict adherence to the law of follicular development even 

 when, in man at least, there is no apparent necessity for it. 



