In a paper published in the Journal of Physiology (vol. xxxv., p. 230), the author pointed out that the percentage weight of the adrenal glands in wild rats was considerably higher than in tame rats of equal weight. This suggested the advisability of ascertaining the effects of captivity on the size of the adrenal glands. Accordingly a series of twenty-five wild rats - nine young and sixteen fully grown - which were obtained at the same time and from the same source as those referred to in the original paper, were kept in confinement for ten weeks, and were fed on a diet of bread soaked in skimmed milk. The animals seemed to thrive well under this regime, but it was observed that for a time the rate of growth of the young rats was retarded. The adrenal glands were, as formerly, weighed out of a 5 per cent, formalin solution. The percentage weight of the wild rats kept in captivity was considerably less than that of the glands of the wild rats which were killed on receipt. The figures are given in the following tables: -

This reduction in weight of the adrenal glands in wild rats in captivity must, it appears to me, be due to one or other of the following causes, cither (a) the diminished muscular activity or (b) the changed diet of the animals. Further investigation is necessary to determine which of these factors has led to the reduction in the amount of adrenal tissue observed.

1 Chalmers Watson, Journal of Physiology, 1908.