CHAPTER XXXVII. 

 THE PHYSIOLOGY OF THE BRAIN. 



The Functions of the Brain in General. The brain, at 

 least in man and the higher animals, is the seat of conscious- 

 ness and intelligence ; these disappear when its blood-supply 

 is cut off, as in fainting ; pressure on parts of it, as by a tumor 

 or by an effusion of blood in apoplexy, has the same result ; 

 inflammation of it causes delirium; and when the cerebral 

 hemispheres are unusually small idiotcy is observed. The 

 brain has, however, many other important functions ; it is the 

 seat of many reflex, automatic, and co-ordinating centres, 

 which can act as entirely apart from consciousness as those 

 of the spinal cord. It is also traversed by many paths of con- 

 duction, some uniting it with the spinal cord and numerous 

 others putting its own parts in anatomical connection. 



The psychical activities, at least in man, seem to be depend- 

 ent on the forebrain, the rest of the complex mass having 

 other non-mental functions or at most being only concerned 

 in very simple mental states. After the cerebral hemispheres 

 have been removed from a frog it is still able to perform every 

 movement as before, but. it no longer performs any spontane- 

 ously. Suitably stimulated it will leap, swim, crawl, climb, 

 turn off its back to its normal position ; and if the optic thalami 

 have not been injured will in leaping forward avoid an ob- 

 stacle placed between it and the light. Its whole essential 

 mechanism of movement is clearly intact, and can be thrown 

 into action and to a certain extent be guided by afferent 

 nervous impulses. Quite similar phenomena may be observed 

 in pigeons ; they not only can stand, but walk, fly if thrown 

 into the air, and preen their feathers, after removal of the 

 cerebral hemispheres; and if carefully tended will live for 

 months. Mammals bear badly extensive operations on the 

 forebrain and usually die before fully recovering from the 

 shock of the operation; but rats survive some hours, and 

 then exhibit very similar phenomena. However it has been 



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