102 TOMES, ON THE DENTAL TISSUES. 



ment of the tooth, having resumed its full formative powers, 

 and produced new, or secondary dentine^ the action having 

 been excited either by the wearing away of the tooth or 

 by tlie presence of caries. If the irritation be continued 

 until it extends down the fang as far as its extremitv, and 

 signs of inflammation show themselves, the aperture of the 

 fang will become enlarged by absorption, and after awhile the 

 enlargement is continued to a considerable distance up the 

 root of the tooth. The canal may be again contracted by the 

 formation of dentine, or by the development of ccmentum ; and 

 I have seen one or two instances in which the greater part of 

 the pulp cavity in permanent teeth has been lined with 

 cementum. This condition of tissues is very common in teeth 

 that have been long the subject of caries, but I believe it is 

 not confined to carious teeth. I have several specimen of 

 temporary teeth, in which the lower part of the root has 

 suffered from absorjition, and then has become the seat of 

 deposition of cementum, leaving only a small canal in the 

 centre. High up the root small patches of dentine have been 

 removed, some of which only have been made good with 

 cementum, while the contiguous parts have retained their 

 usual condition. 



It will be seen that the foregoing facts bear upon the 

 opinions advanced by JNIr. De jNIorgan and myself, in the 

 paper on the structure and development of bone, before 

 cited : that we have indications in teeth, as in bone, of alter- 

 nations, of removal, and deposition of tissue. In the young 

 subject, the development of bone tissue is in excess of absorp- 

 tion, allowing the bones to increase in size ; that in middle 

 life the two powers, under ordinary circumstances, balance 

 each other, and the bones preserve their adult dimensions ; 

 while in old age the absorbent action appears to prepon- 

 derate. Conditions pretty nearlv parallel occur in the dental 

 tissues after the temporary tooth has been fully formed ; por- 

 tions of cementum are removed, and with it, in some cases, a 

 little dentine; the lost parts are replaced by cementum, and the 

 tooth is again perfect. When the time approaches for shed- 

 ding the teeth, the two actions alternate ; but the absorption 

 being in excess of the development, the tissues disappear, 

 aud the tooth is shed. After the formation of the permanent 

 teeth we have occasional alternatives of the two actions ; but 

 they are balanced, and neither increase or diminution of size 

 is observed. But as age comes on, it often happens that 

 absorption is in excess, the fangs diminished in size, the teeth 

 become loose, and fall out. 



