THE FUNCTIONS OF THE BRAIN. 17 



board. As liis optic nerves are destroyed by the usual 

 operation, it is impossible to say whether he will avoid 

 obstacles placed in his path. 



When, finally, a frog's cerebral hemispheres alone are cut 

 off by a section between them and the thalami which pre- 

 serves the latter, an unpractised observer would not at first 

 suspect anything abnormal about the animal. Not only is 

 he capable, on proper instigation, of all the acts already 

 described, but he guides himself by sight, so that if an 

 obstacle be set up between him and the light, and he be 

 forced to move forward, he either jumps over it or swerves 

 to one side. He manifests sexual passion at the proper 

 season, and, unlike an altogether brainless frog, which em- 

 braces anything placed between his arms, postpones this 

 reflex act until a female of his own species is provided. 

 Thus far, as aforesaid, a person unfamiliar with frogs 

 might not suspect a mutilation ; but even such a person 

 would soon remark the almost entire absence of spontane- 

 ous motion — that is, motion unprovoked by any present in- 

 citation of sense. The continued movements of swimming, 

 performed by the creature in the water, seem to be the 

 fatal result of the contact of that fluid with its skin. They 

 cease when a stick, for example, touches his hands. This 

 is a sensible irritant towards which the feet are automatic- 

 ally drawn by reflex action, and on which the animal re- 

 mains sitting. He manifests no hunger, and will suffer a 

 fly to crawl over his nose unsnapped at. Fear, too, seems 

 to have deserted him. In a word, he is an extremely com- 

 plex machine whose actions, so far as they go, tend to 

 self-preservation ; but still a machine, in this sense — that it 

 seems to contain no incalculable element. By applying 

 the right sensory stimulus to him we are almost as certain 

 of getting a fixed response as an organist is of hearing a 

 certain tone when he pulls out a certain stop. 



But now if to the lower centres we add the cerebral 

 hemisplieres, or if, in other words, we make an intact ani- 

 mal the subject of our observations, all this is changed. In 

 addition to the pre^nous responses to present incitements 

 of sense, our frog now goes through long and complex acts 

 of locomotion spontaneously, or as if moved by what in our- 



