190 PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



reflex is first established it may be obtained also with neighboring 

 tones, but with training the reflex becomes more and more special- 

 ized, so that finally as small an interval as one-eighth of a tone 

 may be differentiated a sound of 800 vibrations causes a flow 

 of saliva while a sound of 812 vibrations is ineffective. The 

 process of differentiation is explained as being due to an internal 

 inhibition, as shown by the following experiment. If a tone of 812 

 vibrations is sounded no saliva is obtained, and if immediately 

 afterward the effective tone, 800 vibrations, is sounded no effect 

 follows. Only after a wait of fifteen to twenty minutes does the 

 tone of 800 again cause a flow of saliva. He argues from this that 

 the tone of 812 vibrations is accompanied by a process of inhibi- 

 tion, whose effect passes off only after some minutes. By these 

 objective methods Pawlow* has brought to light some interesting 

 peculiarities in the action of the cortex, and has established a 

 method by means of which it may be possible to study the nature 

 and development of the mechanisms elaborated in the plastic 

 cortical centers by education and experience during the life of the 

 individual. 



Extirpation of the Cerebrum. One of the methods used in 

 physiology to determine the general functional value of the cerebral 

 hemispheres has been to remove them completely, by surgical 

 operation, and to study the effect upon the psychical responses of 

 the animal. Upon the cold-blooded animals and the birds the 

 operation may be performed with ease, but in these animals the 

 positive results are not striking and the experiments are valuable 

 chiefly for their negative results. If the cerebral hemispheres are 

 removed from the frog, for example, the animal after recovering 

 from the immediate effects of the operation that is, the effects 

 of the anesthetic and the shock shows surprisingly little difference 

 from the normal animal. It maintains a normal posture and shows 

 no loss at all in its power of equilibration. When placed on its 

 back it quickly regains its usual position. If thrown into water 

 it swims to a solid support and crawls out like a normal animal. 

 It jumps when stimulated and is careful to avoid obstacles placed 

 in its way, showing that its visual reflexes are not impaired. It 

 is said, however, that the more complicated reactions that depend 

 upon the memory of past experiences or the instincts are absent or 

 imperfect. This latter peculiarity is manifested most impressively 

 in birds (pigeons) after removal of a part or all of the cerebrum. As 

 a result of such an operation, the nervous, active animal is changed 

 at once to a stupid, lethargic creature which reacts only when 



* Pawlow, "Livre jubilaire du Professeur Ch. Richet," 1912, p. 325. See 

 also Morgulis, "The Journal of Animal Behavior," 4, 362, 1914. 



