BEHAVIORISTIC STUDY OF THE RAT 
47 
become more and more vigorous with each succeeding period. 
The animal is stimulated in this way only slightly at first but in¬ 
creasingly more intensely until it reaches its maximum of activity 
some twelve hours after the daily meal. At this time the nervous 
system and the large muscles of the body employed in making the 
spontaneous movements become fatigued so that although the 
stimulation from the stomach still continues in its periodic 
fashion these muscles respond only with a very small amount of 
activity. During this period of fatigue the attempts of the 
stomach to gain control of the organism, to start it to activity 
are clearly manifested by the small but regular periods of activity 
shown at this time. This may be looked upon as the period of 
sleep. Actual observation showed that during this period the 
animals are very difficult to arouse they are curled up in balls 
and are as far as can be made out “ sleeping. ” It has already 
been pointed out in Miss Wada’s work on the activity of humans 
that during sleep the period of activity and inactivity come and 
go with great regularity synchronous with the stomach contrac¬ 
tion periods. The stomach continues to function during this 
period of sleep just as vigorously as before. The amount of 
activity becomes larger and larger with each successive period 
until finally, after a condition of sufficient recuperation of the 
nervous system and skeletal muscles is reached, the stomach 
contractions again gain possession of the reactions of the whole 
organism and the individual awakes. The regular anticipation 
of the feeding periods may also be explained on the basis of the 
clock-like functioning of this internal organ. When all external 
conditions are kept fairly constant and when a regular routine of 
activity is gone through each day the accuracy of this time piece 
is quite astonishing. This is seen very well in the activity record 
of individual rats, but it is much better known from numerous 
observations from the great regularity and punctuality of the 
reactions of many farm animals, especially of the braying of the 
mule at a very definite time near noon at which it is accustomed 
to be fed. The ability to carry on other rhythms may be ex¬ 
plained on this basis (9 and 10). 5 
5 A good example of the persistence of a rhythm of activity of an internal organ 
after the removal of the original exciting stimulus is found in the case of the 
