504 PHYSIOLOGY CHAP. 



the periods of activity alternate regularly with those of rest and 

 quiet sleep during the night. 



From the outset these spontaneous movements are guided by 

 visual sensations, for the animals are capal >le of avoiding obstacles 

 of any kind as perfectly as normal pigeons. The movements 

 are regulated perfectly by tactile sensations, and all changes of 

 equilibrium are exactly compensated. Sounds and noises, on the 

 contrary, have no influence on the course of the movements, 

 although hearing is not lost, since the sound of striking a match 

 makes the pigeon start. 



The brainless pigeon can easily be inhibited in its movements. 

 If it is touched lightly, or taken up and set down again, it will at 

 once throw its head back, ruffle its feathers, and sleep. 



By special experiments it has been shown that the decerebrated 

 pigeon is capable of making definite purposeful movements. 

 When, for instance, it is set on a perch that hardly supports its 

 feet, 6 feet above the floor of an empty room, it decides after 

 long hesitation to fly, and drops to the ground in a gentle curve. 

 If, again, a horizontal support is placed at the same height a 

 few yards away, the bird much sooner resolves to leave its uncom- 

 fortable perch, and flies to the firmer support. If a stool is then 

 set a yard away from the bough, the pigeon drops first on to the 

 stool and then to the ground. But while capable of flying down, 

 it never attempts to fly up. It seems doubtful whether it is able 

 to feed itself. 



The brainless pigeon shows by its voice and movements that 

 it is capable of sexual excitation, but it is indifferent to the 

 presence of the female. Nor does she in turn trouble about the 

 young birds that surround her and follow her. Decerebrated 

 pigeons are equally destitute of the sense of fear; their move- 

 ments are governed by the size, form, situation of surrounding 

 objects, but to these themselves they remain entirely indifferent, 

 whether they be animate or inanimate, friend or foe. 



In conclusion, it can be affirmed from Schrader's observations 

 that the fore-brain of the pigeon is neither a sensory nor a motor 

 centre, since its total absence causes neither loss of movement nor 

 of sensation. But the decerebrated, as compared with the normal, 

 pigeon shows marked defects which are most readily explained 

 as the loss of memory impressions of previous sensations, owing 

 to loss of intelligence properly so-called. All the actions of 

 pigeons without fore-brains, however varied and complex, show a 

 regular and definite direction. They have the character of the 

 responsive movements of Goltz, that is, they are to a large extent 

 determined reflexly by the excitations which come from the 

 periphery to the sensory centres of the thalami, optic lobes, and 

 medulla oblongata. As a whole, they give an idea of the very 

 important functions dependent on the remaining portions of the 



