864 PERIODIC FUNCTIONS IN MAMMALS 



materials from the two species (see Fig. 33). We err by comparing 

 material from very young infants with that from mice 5 weeks of age. 

 (In view, inter alia, of the species difference in life span, the mice are 

 "children" if not "adolescents.") Therefore, the species diiference in 

 timing of mitotic rhythm in man and mouse may not be regarded as 

 fully established, but the data are quite suggestive of its existence. 

 Finally, when a curve published by Peterson (1957) for daily 

 oxycorticoid changes in human plasma is compared with data for the 

 mouse obtained by the use of the same method (Kliman and Peterson, 

 1958), important differences in the phases of these rhythms with 

 respect to local time come clearly to the fore. It can be concluded 

 that an adrenal cycle does characterize man and mice, but the timing 

 of the rhythmic changes associated with this cycle in predominantly 

 diurnal human beings is different from that in the nocturnally active 

 mouse. The species difference in the timing of these rhythms with 

 respect to local time is of interest to the comparative biologist. In the 

 mouse, the cycle has been traced directly to the adrenal gland itself 

 by corticosterone determinations and by mitotic counts (Fig. 14). 

 The associated hormonal changes have been followed in the blood by 

 direct chemical determinations and more simply, but only indirectly, 

 by eosinophil counts done under standardized circumstances. Among 

 the peripheral consequences of the cycle we find 24-hr periodic effects 

 at the intracellular level of organization in the nucleus and in the 

 cytoplasmic metabolism of hepatic phospholipid. While this cycle 

 constitutes a critical mechanism of adaptation to the environment, 

 the endogenous aspects of its periodicity also may deserve emphasis. 

 In comparing the time course of the daily changes in the blood level 

 of hormone with those in motor activity of the body as a whole, we 

 may infer that the adrenal cortex not only reacts to the activities of 

 daily life, as has been amply demonstrated, but what is equally im- 

 portant, the periodicity of the gland underlies our preparation for 

 daily activity as well (Halberg, 1953 and in press). Thus, as long as 

 optimal physiologic conditions prevail, a new sequence of metabolic 

 events is being initiated in the cell once a day, under at least partial 

 corticoid control. Normally, such metabolic arousal occurs before the 

 reticular activating system (Bremer, 1935; Ingram et ai, 1951; 

 Magoun, 1952), starts us out on our daily activities; in other words. 



