104 C. M. JACKSON 



were fed an amount just sufficient to hold them constant at the 

 initial body-weight. Of course slight fluctuations in the gross 

 weight were unavoidable, but they rarely exceeded one gram 

 above or below the initial weight. The food used was in all 

 cases whole wheat (Graham) bread soaked in whole milk. Pre- 

 vious experience has shown that, at least up to the age of one 

 year, albino rats thrive and develop normally upon this simple 

 diet. Water ad libitum was also supplied. 



It is a curious fact that under these circumstances the amount 

 of food necessary for maintenance of body-weight apparently 

 decreases as the experiment proceeds. Thus rats of about 25 

 grams gross body-weight when three weeks of age at the begin- 

 ning of the experiment will at ordinary room temperature re- 

 quire about 5 grams of milk-soaked bread daily for maintenance. 

 Later, this will usually decrease to an average of about 3 grams 

 toward the age of ten weeks. ^ This is the opposite of what might 

 be expected: (1) because at later periods the amount of avail- 

 able food supply stored in the body has been greatly diminished; 

 and (2) because the animals held at constant body-weight al- 

 most invariably become much more active, requiring a greater 

 expenditure of energy. Possibly the smaller amount of food 

 required to maintain the animals at the later periods may be 

 due to a greater absorption of water, thus maintaining a bod}^- 

 weight which would otherwise dechne with the given amount of 

 food. It is well known that during inanition in general the 



^ Two examples may be cited. Six rats of litter No. 12 were held at constant 

 body-weight (within a range of 1 gram) from the age of three weeks on June 21, 

 1914, average body-weight 23.6 grams, for seven weeks to the age of ten weeks on 

 August 6, 1914, at which time the average gross body-weight w^as 23.8 grams. The 

 average daily food-supply of whole wheat (Graham) bread soaked in whole milk 

 for the seven consecutive weeks of the experiment was as follows: 5.1, 3.9, 3.7, 

 3.5, 3.3, 2.7, 2.7 grams. Similarly, six rats of litter No. 13, average weight 23.1 

 grams at three weeks of age on June 28, 1914, were held at constant weight for seven 

 weeks until August 12, 1914, when at ten weeks of age their average gross weight 

 was 22.6 grams. Their average daily food-supply for the seven consecutive weeks 

 was as follows: 5.3, 5.0, 4.1, 3.9, 3.3, 3.2, 2.9 grams. In all cases water (city 

 supply, from the Mississippi river) was supplied ad lihitum. The diminishing 

 amount of food necessary for maintenance cannot be explained as due to increas- 

 ing temperature, as this was fairly constant. Moreover, a similar condition has 

 been found in other litters at all seasons of the year. 



