CHAP. n. ] THE BRAIN. 1003 



639. In warm-blooded animals the removal of the cerebral 

 hemispheres is attended with much greater difficulties than in the 

 case of the frog. Nevertheless, in the bird the operation may be 

 carried out with approximate success. Pigeons for instance have 

 been kept alive for five or six weeks after complete removal of the 

 cerebral hemispheres, with the exception of portions of the crura 

 and corpora striata immediately surrounding the optic thalami ; 

 these parts were left in order to ensure the intact condition of 

 the latter bodies. 



When the immediate effects of the operation have passed off, 

 and for some time afterwards, the appearance and behaviour of 

 the bird are strikingly similar to those of a bird exceedingly sleepy 

 and stupid. It is able to maintain what appears to be a completely 

 normal posture, and can balance itself on one leg, after the fashion 

 of a bird which has in a natural way gone to sleep. Left alone in 

 perfect quiet, it will remain impassive and motionless for a long 

 time. When stirred it moves, shifts its position ; and then, on 

 being left alone, returns to a natural, easy posture. Placed on 

 its side or its back it will regain its feet; thrown into the air, 

 it flies with considerable precision for some distance before it 

 returns to rest. It frequently tucks its head under its wings, 

 and at times may be seen to clean its feathers; when its beak 

 is plunged into corn, it eats. It may be induced to move not 

 only by ordinary stimuli applied to the skin, but also by sudden 

 loud sounds, or by flashes of light ; in its flight it will, though 

 imperfectly, avoid obstacles, and its various movements appear 

 to be to a certain extent guided not only by touch but also by 

 visual impressions. 



In a certain number of cases this sleepy, drowsy condition 

 passes off and is succeeded by a phase in which the bird, apparently 

 spontaneously, without the intervention of any obvious stimulus, 

 moves rapidly about. It does not fly, that is to say, it does not 

 raise itself from the ground in flight, but walks about incessantly 

 for a long while at a time, periods of activity alternating with 

 periods of repose. It seems, from time to time, to wake up and 

 move about, and then to go to sleep again; and it has been 

 observed that during the night it appears to be always asleep. 

 It is obvious, therefore, that the sleepy, quiescent condition is 

 not due simply to the absence of the cerebral hemispheres, but 

 is a temporary effect of the operation, and that spontaneous 

 movements, that is to say, movements not started by any obvious 

 stimulus, may occur after removal of the cerebral hemispheres. 

 But the movements so witnessed differ from those of an intact 

 bird. They are, it is true, varied ; and the variations are in part 

 dependent on external circumstances, the bird being guided by 

 tactile, and as we have said, visual sensations, or, to be more 

 exact, by impressions made upon the sensory nerves of the skin 

 and on the retina ; but they do not shew the wide variations of 



