Reactions to Standard Stimuli and Ageing 57 



pattern of the dermal papillae is very variable and changes 

 profoundly in places less than 1 mm. apart, this is a far less 

 suitable index of response than the behaviour of the cornified 

 layer. 



It is tempting to speculate whether the power to shed intra- 

 cellular water may be the underlying property, and this pro- 

 perty be increased by age. So far the only cases which have 

 shown anomalous changes in the thickness of the cornified 

 layer have been females. As no records of the menstrual 

 history of female patients have been taken, the possibility of 

 endocrine influence upon epithelial responses must be borne 

 in mind. 



In two of the four researches mentioned above we have 

 tried — and are still, in fact, trying — the reaction of rat and 

 human gums to the extra stimulus of being brushed with 

 tooth-brushes. Unfortunately, in the case of the rats, there is 

 no significant variation in the response from the time of 

 puberty onwards, and it is not easy to devise a fully com- 

 parable stimulus for use in rats during the neonatal-prepubertal 

 stages ; the gum area is small enough even in the older animals, 

 and there are other problems. In the human subjects a trouble 

 not experienced in the rat series is the difficulty of getting 

 comparable numbers of biopsy specimens in all age groups, 

 and above I have referred to other difficulties. I think we shall 

 eventually get an answer in respect of human ageing which 

 will be more declarative than that from the rats, and we have 

 already had a minor indication of some factors that may be 

 concerned in that answer. The final comment that I will make 

 on these gum experiments is that measurements dependent 

 upon fairly complex histological procedures are not entirely 

 satisfactory and are certainly laborious and time-consuming. 

 One comforting point about the human research is that the 

 brushing has already demonstrated therapeutic powers so that, 

 in addition to any contribution the work may make to the 

 study of ageing, it may also make one to the furtherance of 

 dental hygiene and thereby to a longer and healthier lifespan 

 for vast numbers of people. 



