636 THE NERVOUS SYSTEM 



hunger and thirst, apparently do not affect the brainless frog. In other 

 words, the operation has reduced the animal to the condition of an autom- 

 aton capable of carrying on complex activities, but only upon receiving 

 some definite stimulus. This condition contrasts sharply with that resulting 

 from the removal of the entire brain, leaving only the spinal cord. In this 

 spinal cord frog only the simpler reflex actions can take place. The frog 

 does not breathe. It lies flat on the table instead of sitting up. When 

 thrown into a vessel of water it sinks to the bottom. When its legs are 

 pinched it kicks out, but does not leap away as in the normal. 



If the cerebrum of the frog be removed, taking special care not to interfere 

 with the optic nerves or the thalami, then it acts somewhat differently. 

 Whereas with the entire cerebrum removed it makes no effort to take food, in 

 this instance it is said to attempt to catch flies or other insects, and will show 

 other signs of spontaneous activity. It will avoid an object and shows signs of 

 responding to visual sensations, such as the attempt to feed just mentioned. 



The cerebral lobes of the frog, however, are very low in the scale of de- 

 velopment as compared with other vertebrates. The cortex is a simple 

 layer of rather small cells, and the total volume of the cortex as compared 

 with other portions of the brain is small. 



The case of the pigeon, which represents a higher animal in the scale, 

 has been extensively studied by Flourens and others. They have shown 

 that when the cerebrum is carefully removed, leaving the basal nuclei un- 

 disturbed, and the animal has recovered from the immediate effects of the 

 shock, it is able to carry on many co-ordinate activities. In the first place it 

 can stand or perch without difficulty. If placed on its back it immediately 

 regains its equilibrium. If tossed in the air it flies until it comes in contact 

 with a firm support. If disturbed on its perch it will walk away, showing 

 the power to co-ordinate not only wing muscles, but the leg muscles. If left 

 undisturbed, such a pigeon will occasionally make motions, i.e., open its 

 eyes, move its head, preen its feathers, or even take a step or two. It 

 spends most of its time, however, sitting quietly as though asleep. If aroused, 

 the animal shows little or no signs of excitement or fright. 



After several months such pigeons are usually said to increase the motions 

 of spontaneity or take short flights, avoiding obstacles in the way and alight- 

 ing definitely on the perch. They will pick around among food for definite 

 articles, apparently attempting to select the food. Early after the operation 

 the pigeon will pick at objects indiscriminately, but does not take food un- 

 less it is placed in the mouth. 



Apparently the main effect produced here is to diminish the complexity 

 and efficiency of those activities which we call spontaneous. The surprising 

 thing is that there is as little disturbance among the motor functions as is 

 found. 



In mammals it is difficult to remove the cerebral hemispheres, but in those 



