500 



PHYSIOLOGY 



CHAP. 



spontaneously, sees quite well, avoids obstacles, responds ade- 

 quately to the impressions it receives, and behaves in all respects 

 like the normal tortoise. The only difference is that it moves 

 slower, is less lively, and shows less initiative. Still its actions 

 are certainly not merely reflex, and probably arise in the optic 

 thalami. Fano, in fact, demonstrated not only by removal of 

 the hemispheres, but also by stimulation of the thalami, that 

 the latter play a considerable part in the evolution of the 

 voluntary acts. The optic thalami, like the cerebral hemispheres, 

 react to electrical stimulation by groups of locomotor movements 

 which have all the character of voluntary movements, which 

 is not the case on electrical stimulation of other parts of the 

 nervous system. 



When the optic thalami as well as the hemispheres are excised 

 in a tortoise,- the constant result is that spontaneous activity 



ceases, and the animal becomes absolutely 

 motionless. It remains for days in the 

 position in which it is placed without 

 manifesting any reaction. Its spon- 

 taneous activity is not abolished, but 

 merely inhibited, by the mid-brain, for 

 we have seen that on removing the latter 

 automatic locomotion reappears. 



On the strength of these results, 

 which were confirmed by Bickel, Fano 

 credited the mid-brain with a continuous 

 inhibitory action upon the automatic 

 the medulla. According to 

 central mechanism of the 

 movements of the tortoise 



the 



.A 



Fin. '25'>. Brain of Kmys 



seen in situ, after removing the 

 top of the skull. (Fano.) A, 

 prosencephalon ; B, inescnceplia- 

 lon ; (.', metencephalon ; D, my- 

 elencephalon. 



centres of 



Fano, the 



voluntary 



consists in inhibition by the fore- and 'tween -brain of 

 constant tonic inhibition exerted by the mid-brain. So soon as 

 the fore-brain depresses mesencephalic inhibition in voluntary 

 activity, the automatic activity of the bulb spreads along the 

 efferent paths determined by heredity or acquired by habit, 

 making use of the spinal mechanisms and the energy accumulated 

 therein. This schematic concept undeniably agrees with the 

 phenomena exhibited by the marsh tortoise , but its applicability 

 to other amphibia and reptiles, and still more to other classes of 

 vertebrates, is very doubtful. 



V. Eolando (1809) first observed the effects of removing the 

 cerebral hemispheres in birds, but he confined himself to observa- 

 tions made shortly after the operation. Flourens (1822) used 

 his experiments on pigeons as the basis of his general theory of 

 the functions of the cerebral hemispheres. He experimented on 

 other classes of vertebrates with the single object of controlling 

 and generalising from the data acquired on pigeons, which 



