PHYSIOLOGICAL RHYTHMS. 299 



have an unequal action, and are highly selective as to 

 the system which they affect. Thus a physiological re- 

 action which, to be normal, demands a subdivision of 

 labour, is assigned in too large measure to one system, 

 which, being unsupported by its neighbours, ultimately 

 suffers harm. 



To return now to the question of daily rhythms in the 

 healthy adult. From morning to evening stature varies; 

 in the morning the stature may be I cm. greater than at 

 night. Perhaps the inability to hold one's self erect 

 has something to do with this, or it may be the result of 

 the general shrinkage of the intervertebral cartilages, for 

 it is in the sitting height that the chief loss occurs. 

 During the day the body-weight increases, the curve 

 being the resultant of several factors, the ingestion of 

 food and the processes of excretion. Along with this 

 goes a regular rhythm in the bodily temperature, as 

 shown in the curve below. 



This curve is fairly well established, but like so many 

 of the other rhythms, it is partly the outcome of the 

 habit of work, and three meals during the day, followed 

 by rest and fasting at night for observation, indicates 

 that those who work at night and rest during the day 

 have this rhythm reversed. 



Several observers have noted a curious periodicity 

 in the capability for muscular work. In one series 

 of experiments the results were the following : On 

 repeatedly raising a weight by the flexion of a 

 finger, it was found that as the finger tired the weight 

 was raised through a smaller and smaller distance, 

 until finally the muscles altogether failed to respond 

 to the strongest voluntary impulse that could be 

 brought to bear on them. If, despite this failure, the 

 effort to raise the weight was regularly repeated, the 

 contractions reappeared and finally resumed their wonted 



