820 PERIODIC FUNCTIONS IN MAMMALS 



berg, 1953, 1955a; Halberg and Visscher, 1952), may deserve con- 

 sideration as a critical mechanism underlying our adaptation to daily 

 activities; what seems at least equally important, this cycle integrates 

 the periodic functioning of metabolism in different parts of the body, 

 and thus it serves our preparation for activity. Just as the sexual cycle 

 describes endogenously regulated changes preparatory to copulation 

 and subsequent fertilization, the 24-hr adrenal cycle may describe the 

 process of preparation for each activity in the life of animals and man, 

 necessary for day-to-day survival. The many similarities between the 

 eosinophil and the activity rhythm in mice and in man have served to 

 postulate this hypothesis, which is supported by data obtained in docu- 

 mented states of adrenal insufficiency (Fig. 12) (Brown and 

 Dougherty, 1955; Halberg, Visscher, and Bittner, 1953; Halberg et 

 al., 1951; Kaine et al., 1955). Just as the preparation of smears from 

 the fluid content of the vagina enables the observer to follow the cyclic 

 and other changes of the vaginal and cervical epithelium, the deter- 

 mination of endogenous changes in number of circulating blood 

 eosinophils has proved a simple method for the evaluation of a 24-hr 

 adrenal cortical cycle in mice and in man. // judiciously used, this 

 method is dependable. Its use reveals that the 24-hr cycle resembles 

 the sex cycle in being controlled by steroidal hormones. The adrenal 

 cycle differs, however, from the sex cycle, in its persistence under 

 circumstances known to suppress the sex cycle (i.e., a 50% reduction 

 in dietary carbohydrate and fat) (Halberg and Visscher, 1952). 

 Furthermore, the adrenal cycle is already developed, many years be- 

 fore the menstrual cycle starts (Halberg and Ulstrom, 1952). It might 

 be concluded that the 24-hr adrenal cycle is the biologically more 

 essential mechanism. 



The Adrenal Cycle 



In recognizing the adrenal cycle as a physiologic entity concerned 

 with the maintenance of mammalian periodicity, we adopt a view 

 which is at variance with the unqualified assumption that the 24-hr 

 changes of adrenocortical secretion constitute "responses to the stresses 

 of daily life." The latter interpretation does not distinguish adrenal 

 periodicity from other phenomena that are more directly and more 

 immediately dependent upon environmental control. This same inter- 



