St be drawn from the available data on birds in light of the findings with the postshot rats. In the latter, individual variation between samples was smali enoughthat the differences between organs were significant, except for those of the gastrointestinal tract. In addition to this, ali the postshot rat specimens were collected within an area 50-yd square; so the environmental conditions may be considered identical for practical purposes. Table 4.19 RADIOACTIVITY IN TERNS AND SHORE BIRDS COMPARED BY FEEDING HABITS Terns (Azraanbiru) Shore birds (Rojoa) Ratio of activity Organ or tissue n Activity, d/m/g (x 1000) n Activity, d/m/g (x 1000) of shore birds to terns Skin Muscle Bone Liver Digestive 2 2 2 2 6 1.1 0.36 0.86 0.78 2.1 3 3 3 3 9 17 1.1 4.0 4.5 76 15.45 3.05 4.65 5.77 36.18 wo tract The coefficients of variability for the organs of the postshot rats were determined, and the results are given in Table 4.20. Tabie 4.20 — COEFFICIENTS OF VARIABILITY FOR ORGANS OF RATS, POSTSHOT me. Organ or tissue Mean Coefficient of variation, % Skin Muscle Bone Liver Stomach and contents 9.2 28 0.94 42 17 84 1.9 18 4.3 140 Gut and contents ‘Lung Kidney 12 100 1.0 24 3.1 27 The lack of marked variability in activity between the lungs of the six rats probably de- pends mainly upon the effect of particle size and density as related to deposition within lung tissue, Stokinger et al.,4 working with albino rats, found that particle size greatly affected the amount of deposition in the areolar spaces, with increases as much as 10-fold with a reduction of mass median diameter from 2.6 to 0.45 . Taplin et al. found that in rats lung retention of particles with a mean size of approximately 1 » was strongly dependent upon the density of the particles. Autoradiographs of lungs of rats collected for the present work indicate a diffuse deposition of the radioactive material within the lungs, except for the bronchii, where the activity is more concentrated and irregular (Fig. 4.9). The regults found in the autoradiographs, as well as the lack of appreciable variation in samples, may well be dependent upon the factors of selection and retention of particles by size and density, especially since the particles retained by the lungs are of a small mean diameter and are more nearly the density of the heavier Nevada sand and BaSO,particles than the dye particles Taplin found to be retained to a greater degree in lungs of rats. 58 _ UNCLASSIFD