THE NERVOUS SYSTEM 



171 



5. Lowest of all are some irregular cells with a similar 

 disposition of processes. (These are not shown in Fig. 

 92.) 



This arrangement is considerably modified in certain 

 regions of the cortex, the large pyramidal cells being best 

 developed in those parts from which the great mass of fibres 

 pass down to the spinal cord. 



From the various fibres collaterals come off which connect 

 different parts of the cortex of the same side, and which also 

 connect the cortex of one side with that of the other, and 

 with the basal ganglia (Fig. 93). 



The diagram opposite shows the more important con- 

 volutions of the cortex cerebri (Fig. 94). 



FIG. 95. A, Pigeon with the Cerebellum destroyed to show struggle to maintain 

 the balance ; JB, Pigeon with Cerebrum removed to show balance maintained, 

 but the animal reduced to a somnolent condition. 



Functions of the Cerebrum 1. General Consideration. 



The functions of the cerebrum may be best understood by 

 first contrasting the condition of animals with, and of animals 

 without, this part of the brain. 



(1) In the frog the cerebral lobes may easily be removed. 

 The animal sits in its characteristic attitude. When touched 

 it jumps, when thrown into water it swims. It is a perfect 

 reflex machine, with the power of balancing itself unimpaired. 

 But it differs from a normal frog in moving only when 

 directly stimulated, and in showing no signs of hunger or of 

 thirst. A worm crawling in front of it does not cause the 

 characteristic series of movements for its capture which are 

 seen in a normal frog. 



(2) In the pigeon (Fig. 95, B), removal of the cerebral 

 hemispheres reduces the animal to the condition of a 



