CH. L.] REMOVAL OF THE CEREBRUM 723 



damage the primary centres of vision (the optic lobes, which corre- 

 spond to the corpora quadrigemina of the mammal), and in these 

 fishes the eye is the most important sense organ. 



A shark, however, subjected to the same operation, is reduced to 

 a condition of complete quiescence ; this is due to the circumstance 

 that in this fish the principal sense organ is that of smell, and sever- 

 ance of both olfactory tracts produces the same result as removal 

 of the entire cerebrum. In either case the path between the 

 olfactory bulbs and the centres that control the cord are interrupted. 



Going higher in the animal scale to the frog, we find that re- 

 moval of the hemispheres only does not entirely abolish it's apparent 

 spontaneity; it still continues to feed itself, for instance, by catching 

 passing insects. It is not until the optic thalami are removed also 

 that it becomes the purely reflex animal described on p. 709. If 

 the brain and the anterior end of the bulb are removed the lower 

 centres of the cord are set free, and the result is incessant movement 

 provoked by slight stimuli. 



A bird treated in the same way remains perfectly motionless, 

 sleepy, and unconscious, unless it is disturbed (see fig. 446). When 



FIG. 446. Pigeon after removal of the hemispheres. (Dalton.) 



disturbed in any way it will move ; for instance, when thrown into 

 the air it will fly ; but these movements are, as in the frog, purely 

 reflex in character ; when the animal is made to fly its movements 

 are directed by the sense of sight, the optic lobes being still intact, 

 and it will select a perch to settle on in preference to the floor. 

 It will start at a noise ; it will not eat voluntarily ; it exhibits no 

 emotions such as fear, sexual feeling, or maternal instincts. 



In mammals the operation of extirpation of the brain is attended 

 with such severe haemorrhage that the animal dies very rapidly, but 

 in some few cases where the animals have been kept alive, the 



