vin] 



THE NERVOUS SYSTEM 



99 



The cerebrum which occupies a large part of the cranium is 

 divided into the two hemispheres. The surfaces of these are 

 convoluted, the extent of the convolutions varying considerably 

 in different species of animals. The gre}' matter is external to 

 the white matter. The cerebral hemispheres are especially well- 

 developed in man, being the seat of the intelligence. An animal 

 without the cerebrum is incapable of conscious sensation and can 

 perform no voluntary movement. It can respond appropriately 

 to every kind of stimulus from outside, but it cannot originate, 

 and it cannot associate its sensations. It is quite devoid of all 

 reasoning power. A frog deprived of its central hemispheres will 

 swim to land if put into water but once it has reached a position 

 of rest it wdll remain in that position until interfered with by any 



Th.E 



M.h 



Fig. 47. Diagi'ammatic longitudinal section through brain 

 (after Huxley, from Halliburton), Olf. olfactory lobe, 

 Hmp. hemisphere, Th.E. thalamencephalon, Pn pineal 

 body, P(/ pituitary body, Th. optic thalamus, i)/.6. mid- 

 brain, CC. crura cerebri, Ch. cerebellum, PV. Pons Varolii, 

 MO. medulla oblongata, I — IX, cranial nerves, 1 — 4, cavi- 

 ties or ventricles. 



fresh stimulus. A dog so deprived Avill eat and perform all its 

 essential bodily functions, but it w ill never recognise its master, 

 nor carry out any act implying intelligence. The hemispheres 

 contain certain areas associated with particular functions and 

 any injury to these areas destroys or deleteriously affects the 

 discharge of the function in question. Thus there is a visual 

 and an auditory area, and areas for taste and smell, as well as 

 for tactile and muscular sensibility, for speech, for the association 

 of ideas, and various other functions. 



The two hemispheres are connected by a transverse com- 

 missure called the corpus callosum. The hemispheres are con- 

 nected with the medulla or hind brain by two large bands of 

 nerve fibres called the crura cerebri. The olfactory lobes project 

 forward from beneath the front end of the hemispheres. 



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