178 PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



psychical and motor responses of a much more intricate and in- 

 direct character. 



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 experien ces 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 

 stimulated. It sits in a drowsy attitude, with its head drawn in 

 to the shoulders, its eyes closed, and its feathers slightly erected; 

 occasionally it will open its eyes, stretch the neck, gape, preen 

 its feathers perhaps, and then sink back into its somnolent attitude. 

 The animal in this condition maintains its equilibrium perfectly, 

 flies well if thrown into the air and perches comfortably upon a 

 narrow support. It may be kept alive apparently indefinitely by 

 appropriate feeding and so long as it is well fed retains its stupid 

 and impassive appearance. If allowed to starve for a while it 

 becomes restless from the effects of hunger, may walk to and fro, 

 and peck aimlessly at the ground. If surrounded by grain it may 

 peck at the separate grains, but never actually seizes one in its 

 beak and- swallows it. The striking defect in these animals is the 

 loss of those responses that depend upon memory of past or in- 

 herited experiences. Its motor reactions are all of a simple kind. 

 If placed upon a hot plate it will, for a time, lift first one foot, then 

 the other, and finally squat, but never flies away. When dosing 

 a loud noise awakens it, but it exhibits no signs of fear, and 

 quickly relapses into somnolence when the auditory stimulus ceases. 

 The one positive conclusion that we may draw from the behavior 



