liamilton: Page 2 The Pri..-iplies and Applications of Radivuctive Tracers To Tho Medical and Biological Scicnces device, such as a Geiger counter tube, over it and measuring the intensity of the gamma rays emitted from the radioactive atoms which have been stored selcctively in the tissuee The application of this sccond major type of tracer technic is illustrated in the study of the iodine metabolism of the thyroid gland in normal and goitrous human subjectse The selective deposition of the administered radio- jodine by the thyroid is determined by placing a Geiger counter tubo over the neck and measuring tho intensity of the gamma rays emittod from the radio-iodine accumulated in the thyroid tissuee Sincc by this method the necessity of removing tissue for detcrmination of its radioactivity is eliminated, it offers two distinct advantages. First, it enables the observer to follow the cobb and flow of the labolled olement or compound in the same human subject or animal for a concidcrable period of timee The results thus obtained give a continuous record of the fate of the accumulated radioactive atoms or moleculese it makes possible in vivo tracer studies in normal human subjectse Secondly, The third technic mikes use of the photographic action of the radiations from artificial radio-clements, by means of which the distribution of the administered labelled elements or compounds in tissues may bo investigatede Thin sections o° the radion. tive tissues arc olaccd against phoncgraphic filmse After an inter7.1 of time sufficient for adequate exposure, the tilm is removed and developeds The sections ure stuined, ond exch section with its corresponding picce of developed film (rudioeautograoh) is oxamined undur the microscopee arcas of darkening in the film correspond to the regions of che tissues in which the greatest deposition of the racio-elument has taken placce Thus a correlation between the deposition of the labelled element or compound and The the histclogical strusture of the tissue can be established, DO® ARC’