838 PERIODIC FUNCTIONS IN MAMMALS 



may prove to be rather long and the heuristic value of such an ap- 

 proach remains debatable. On the other hand, we should consider the 

 extent to which, under self-determined conditions in the absence of 

 periodic environmental sync signals, maintenance of circadian be- 

 havior in mammals may depend upon feedbacks to the central nervous 

 system from periodic sequences of peripheral metabolic events that 

 are loosely locked in phase with each other via specialized hormonal 

 (and autonomic nervous) controls. If the leads and lags in phase of 

 periodic cellular events should play an important role in this con- 

 nection, they could account for both, our resistance (manifested inter 

 alia by efficiency loss) to the assumption of unusual schedules enforced 

 from without and our but slow adaptation to suddenly instituted 

 acceptable changes of the synchronizer. With this in mind we may 

 now turn back to the role played by the central nervous system with 

 respect to aspects other than phase control, and we shall refrain at 

 this time from regarding any one part of the brain as an endogenous 

 master oscillator underlying all circadian rhythms, an entity which 

 has more often been assumed to exist than documented as to what it 

 does. 



Already by 1933, lores (1933, 1940) had ascribed to a 

 hypophyseo-mesencephalic rhythm the daily changes in urinary 

 volume. Mills (1951) more recently referred to the same struc- 

 tures as possibly underlying the habitual rhythms of "sleepiness," 

 temperature, and urine flow. Moreover, in the course of work unrelated 

 to 24-hr periodicity per se, the feeding rhythm was found to be 

 obliterated in mice made obese by hypothalamic lesions or by goldthio- 

 glucose poisoning (Anlicker and Mayer, 1956; Skinner, 1957), 

 which latter procedure also damages the hypothalamus (for references 

 see Liebelt and Perry, 1957). A hypothalamic osciflator, or this area 

 as a part of an oscillating circuit not necessarily possessing a free- 

 running period of its own, but "driven" from within (by feedbacks) 

 and "triggered" from without, may well control certain 24-hr rhythms 

 of the body as a whole, such as that in motor activity. The hypothal- 

 amus also represents, of course, an added autonomic control of those 

 rhythms that are more intimately dependent upon the endocrines. 



Before leaving the hypothalamus, however, we must introduce a 

 rather ill-defined point into our discussion. Somewhere and somehow 



