Abstract
Regular cyclic variations in behavior have been produced or abolished in experimental animals by surgical interference with the function of one or another of the endocrine glands, and the results indicate that each gland functions with a specific frequency. For example, it has been shown that the regular four to five day cycles of spontaneous activity manifested by rats were definitely associated with the cyclic variations in ovarian function. 1 Further, section of the pituitary stalk in rats often brought out regular cycles in activity and food and water intake, with an average frequency of fourteen days 2 ; removal of the parathyroid glands brought out a twenty day cycle of activity in rats and a forty day cycle of activity and calcium appetite in monkeys 3 ; thyroidectomy brought out a twenty-five to thirty-five day cycle of activity in rats, 4 and removal of one ovary and partial removal of the
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