x REFLEX AXD VOLUNTARY ACTION 173 



impulses originate and exercise a controlling effect upon 

 the cells of the spinal cord, so that these latter do not 

 constitute, as in the brainless frog, a machine every 

 movement of which can be accurately predicted. 



Moreover, it can be shown by experiment that the 

 process of originating voluntary impulses is not per- 

 formed 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, exhibiting 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 till it dies. Hence the bulb and cerebellum 

 are evidently concerned with the co-ordination of mus- 

 cular movements, but have no power of originating 

 impulses. If the optic lobes as well as the medulla 

 oblongata and cerebellum are left, the animal is affected 

 by light, is able to perform complex balancing move- 

 ments, 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 up 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 etc., acting automatically ; but it is only 

 by his controlling intelligence that the whole mechanism 

 is adapted to the circumstances of the moment. 



