Reactions to Standard Stimuli and Ageing 55 



determine the effects of radioiodine, injected into the mother, 

 upon the embryos and foetuses at different stages of develop- 

 ment. This will be of interest in the light of Van Middles- 

 worth's (1956) report on the surprisingly high values of radio- 

 iodine found at necropsy in the thyroid glands of cattle, sheep 

 and human beings on account of fall-out from nuclear weapons. 

 The long-term effects of this persistent source of internal 

 radiation need to be studied in relation to development and 

 longevity of the exposed lower animals and of Man. 



The sensitivity of the rat thyroid to internal irradiation has 

 been studied and an average dosage of 10 [jlc/100 g. body 

 weight has been found sufficient to destroy thyroid activity 

 in adult rats. The effects of this dosage, and of much larger 

 and of much smaller ones, are being tested on the long-term 

 thyroid activity. In a first series of 30 animals, following 

 doses of 25 jj.c, no diminution of function has been found as 

 measured by radioiodine uptakes after 35 days. This dose of 

 25 (i.c is equivalent to an irradiation dose of 20,000 r over this 

 time, if the effective half -life is taken as 4 days in the adult 

 rat. This amount of r is equivalent to a human therapeutic 

 dose. 



In summary, normal levels have been established to date 

 for the percentage uptake by rats of a tracer dose of radio- 

 iodine at monthly intervals from birth to two years. This 

 standard can now be used for further studies of thyroid func- 

 tion as affected by age, and of the effects of continuous low- 

 level irradiation on ageing of the individual animal and on 

 litter development. 



The fourth research is the human counterpart of the gum- 

 brushing study in Merion rats. It arose out of a talk with G. A. 

 Cowan, Dental Surgeon to St. Bartholomew's Hospital, and 

 its object was to test a belief, prevalent in many dental circles, 

 that cases of chronic inflammation of the gums can often be 

 relieved by the simple expedient of regularly brushing the gums 

 with a stiff toothbrush. It seemed that a simple and standard- 

 izable test of this kind could also be used for a comparison of 

 reactions at varying ages, and it was decided to proceed 



