BY DR. MICHAEL FOSTER. 417 



sides of the animal, he will utter a single distinct, sharp, short 

 croak, and as often as the movement is repeated the croak will 

 be heard. 



This again is very different from the behavior of the entire 

 frog. The entire frog, when thus stroked, may or may not 

 croak ; for a single stroke he may croak several times, or not 

 at all. The frog without the cerebral hemispheres, but posses- 

 sing the optic lobes, and otherwise in good condition, croaks 

 at every stroke, and croaks once only for each stroke. 



One seems driven to regard this behavior as the result of a, 

 so to speak, croaking mechanism ; and not as the act of a con- 

 scious intelligence. 



06s. III. The cerebral hemispheres having been carefully re- 

 moved, in such a way as to leave intact the optic nerves, the 

 student may attempt the following experiment of Goltz to test 

 the persistence of any visual sensations. 



Place the frog on the table, with his head towards the 

 window, and some six or eight inches in front of him place a 

 large book, or other thoroughly opaque mass. Gently pinch 

 him behind, in any spot which is exactly in the median line of 

 his body. Under ordinary circumstances, he would spring for- 

 ward in a straight line, and, in the absence of all vision, would 

 strike his head against the book. It will be found in this case, 

 however, if the experiment be successful, that instead of spring- 

 ing forward in a straight line, he turns a little to the right or 

 to the left, so as to avoid the book. 



If he turns to the left, shift the book to the left and then 

 repeat the experiment. He will now move in a straight line or 

 to the right. In the same way, if the book be to the right he 

 will incline to the left. 



The student will do well to try this experiment, but it fre- 

 quently fails. Care should be taken to have the light coming 

 into the room as directly in front of the animal as possible, in 

 order to exaggerate the shadow cast by the book. Apparently 

 the image of the opaque book produces some sort of visual im- 

 pression sufficient to guide the movements of the animal. But 

 it would be hazardous to * ay that the animal sees, for it is diffi- 

 cult, or rather impossible, to obtain any other evidence of the 

 influence of vision in a frog in such a condition, 

 i These observations are introduced to illustrate the fact that, 

 in the absence of the cerebral hemispheres, whether the optic 

 lobes be present or no, the frog possesses no volition. He exe- 

 cutes none of those so-called spontaneous movements which we 

 are in the habit of attributing to volition. This leads us to infer 

 the absence of at least that amount of consciousness which we 

 find inseparably connected with volition. At the same time, 

 we learn that the presence of certain parts of the brain lying 

 behind the cerebrum, determines the nature of the movements 

 27 



