X REFLEX AND VOLUNTARY ACTION 173 



Moreover, it can be shown by experiment that the process 

 of originating voluntary impulses is not performed by the 

 whole brain, but is confined to the cerebral hemispheres. 

 If the- hemispheres and optic lobes are removed so as to 

 leave nothing but the bulb and cerebellum, the frog no 

 longer lies in any position in which it may be placed, ex- 

 hibiting no movements beyond the beating of the heart, as 

 is the case' when the whole brain is removed. It sits up in 

 the ordinary attitude, breathes, swallows food placed in the 

 mouth — while making no attempt to feed itself, turns over 

 and sits up if placed on its back, and swims if placed in 

 water. . If left alone, however, it remains in the sitting 

 posture until it dies. Hence the bulb and cerebellum 

 are evidently concerned with the co-ordination of muscular 

 movements, but have no power of originating impulses. If 

 the optic lobes as well as the medulla oblongata and cere- 

 bellum are left, the animal is affected by light,, is able to per- 

 form complex balancing movements, and will even croak 

 when stroked in a particular way. There is still, however, 

 no voluntary action ; without the application of stimuli, the 

 animal sits motionless until it dies. 



To sum tip in the language of the illustration with which 

 this chapter was begun, comparing the frog with an engine 

 of human construction : — the grey matter of the brain may 

 be compared with the engineer ; much of the work of the 

 engine may go on without him — certain levers, valves, &c., 

 acting automatically ; but it is only by his controlhng intel- 

 ligence that the whole mechanism is adapted to the circum- 

 stances of the moment. 



So far, we have considered the nervous system only in its 

 relations to the skin or general surface of the body and to 

 the muscles or organs of movement. The other parts of the 

 body are, however, under nervous control. 



