INTEGRATION OF MOVEMENTS IN LEARNING IN THE RAT 389 



All the rats used in the experiment with the latch-box problem 

 were 50 days old. The usual method of feeding for a day or two 

 in the problem was thought essential. Sometimes with very 

 timid rats a longer time was given. This period of feeding was 

 regarded as the beginning of learning the environment of the prob- 

 lem and careful observation of the behavior of the rats was made 

 at this time. 



For a stimulus to activity, food was used. Because so many 

 organic changes are dependent upon the taking of food and for 

 nutrition, the food stimulus is no doubt the most satisfactory 

 one to produce activity. Physiologically, it may be regarded the 

 most convenient, and the most important one to produce the 

 responses which the rat's organization is capable of making. Ac- 

 cording to Carlson (5', 6) augmented irritability sweeps over the 

 nervous system during gastric hunger contractions of the ali- 

 mentary canal. It must, as a consequence, put in a state of 

 readiness, so to say, many reflexes which require only the external 

 senses to produce progression and to solve the problem. 



Obviously, the selection of food and its control are important 

 considerations. Bread soaked in unskimmed milk was used. 

 Such a mixture contains sufficient proteins, carbohydrates, and 

 fats to maintain growth and activity. The physical condition 

 of each rat was carefully watched in respect to the amount of food 

 required for healthy metabolism. Usually the amount of food 

 required could be discovered by exerting slight pressure on the 

 abdomen of the rat before and after feeding. Pressure on the 

 abdomen before feeding should not reveal a condition of relaxa- 

 tion of the abdominal muscles which indicates emptiness of the 

 alimentary canal, and after feeding an exaggerated extended con- 

 dition of the stomach and abdomen. Only sufficient food should 

 be taken by a rat to maintain metabolism and growth. The 

 exact amount needed by each rat can be determined only by 

 experience. 



Very early in experimentation the problem frequently remained 

 unsolved after several hours time. So a given period of 30 min- 

 utes was established for each rat. If the problem at the end of 

 this period was unsolved, the door of the problem box was opened 



