6 
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obtained from an animal under these con¬ 
ditions is shown in figure 2. It is seen that 
the rat is alternately active and inactive 
and that the regularity of the recurrence 
of the periods is quite striking. This 
regularity is brought out even better in 
figure 23 on page 50. Frequently when 
the conditions of the experiments are 
particularly well controlled the differences 
in the intervals between the periods does 
not vary more than just a few minutes. 
The rate of the periods was found to 
vary with the age of the animals. In very 
young animals this rate is most rapid, 
averaging about fifteen for the twenty-four 
hours. In old animals the rate averages 
about ten. The length of the activity 
| periods also shows some variation with 
-2 age, being longer in younger animals and 
'2 becoming progressively shorter in older 
.§ animals. 
Szymanski (1) has recently also studied 
the activity of the rat along with the 
activity of many different animals, from 
the simplest insects to the human infant. 
He used an “Aktograph” for his work on 
rats—a device very similar to the activity 
cage described above, except that it is 
supported on springs and registers activity 
directly with a lever rather than pneumat¬ 
ically. Szymanski found that the activity 
of the rat is divided into ten periods per 
• twenty-four hours. What was found in 
the experiment above seems to agree fairly 
well with this result, except that Szymanski 
makes no statement as to the age of the 
animals that he used for his experiments. 
He did not notice the great regularity of 
