PHYSIOLOGICAL RHYTHMS. 33 



either the morning or the early evening. The simple 

 reaction times are shortest in the morning, whereas the 

 others are shortest at the end of the day. 



The determination of a diurnal rhythm for purely 

 intellectual work is complicated by the progressive 

 fatigue of the nervous system as the day advances, and 

 to a large extent by external conditions. It certainly 

 depends on food supply, as all experiments indicate, yet 

 digestion and brain work which call for the maximum 

 blood supply at two different localities are not to be 

 carried on simultaneously. It is difficult to get statistics 

 bearing on the processes in question or to measure 

 them ; for everything, from the orderly marshalling of 

 the mental images in a logical line, to the patient 

 waiting for isolated ideas to take shape, can be classed 

 as intellectual work, and in the face of any statement 

 which might now be made, there arise the shades of 

 dead authors to affirm that it was far otherwise with 

 them. For the present at least the problem must be 

 left as one too complicated for general expression. 



Of the shorter rhythms there are many. As a rule 

 there are three periods of digestion and absorption 

 corresponding with the three meals, associated with 

 changes in the circulation, heat production, muscular 

 strength, and the emotional and mental activity, all of 

 which are evident, even in spite of physiological arrange- 

 ments, the assumed purpose of which is to maintain 

 uniformity in these processes. There are, too, the long 

 slow rhythms of blood pressure extending over many 

 seconds, accompanied^ perhaps, with corresponding varia- 

 tions in the excitability of the central nervous system, 

 while as smaller waves on this ground swell appear the 

 oscillations due to the respiration and the heart beat, all 

 of which are in turn interdependent. 



Passing to the reactions of the nervous system, there 



