BEHAVIORISTIC STUDY OF THE RAT 
37 
four hours. What mechanism is there within the body which 
functions in this way and at this rate? 
The heart and lungs although very regular and periodic in their 
functioning must be excluded for reason of their rate which is a 
matter of seconds and minutes rather than hours. The liver, 
rectum and bladder and intestines must be excluded for reason 
of their irregularity of periodicity. The sex glands, although 
periodic in their function, must be excluded because of their rate 
which is a matter of days, weeks or even longer intervals. 
There still remains the stomach. This organ has been the 
subject of very thorough investigation by a number of physiol¬ 
ogists, chiefly Cannon (6) and Carlson (7), so that its action is 
rather clearly understood. It has been established in the first 
place that the stomach when empty is not subject to continuous 
contractions as might be supposed, but rather that its activity is 
broken up into more or less clear-cut periods. During the 
intervals between the periods of activity this organ quite relaxed 
and almost completely inactive. During the activity periods it 
undergoes a series of contractions which may involve usually 
the greater part of the musculature of this organ. The rate of 
the recurrences of these periods differs in different species of 
animals, but also in members of the same species but of different 
ages. The average rate for most mammals when the stomach is 
empty is ten to fifteen per day. Under carefully controlled and 
regulated conditions these periods of activity may come with an 
astonishing regularity. 
These contractions of the stomach when this organ is empty 
must not be confused with the contractions that occur during the 
process of digestion when the stomach is full. These latter 
contractions are known as digestion contractions, while the 
former are known as hunger contractions. They differ in a num¬ 
ber of ways, first as to parts of the stomach involved, and secondly 
their relation in time to the last meal. The digestion contrac¬ 
tions begin almost immediately after an ordinary meal and con¬ 
tinue until the stomach is empty. Just as soon as any of the food 
is prepared for the assimilative processes in the alimentary canal 
below the stomach it is slowly expelled through the pylorus. The 
