236 THE HUMAN SPECIES 



ing the lowest intelligence. The grey matter of the cerebral 

 cortex is, as Huschke expressed it, " the true brain ". It is in 

 the cortex that the " soul " rises superior to the sensations and 

 the physical perceptions of pain and pleasure to the formation 

 of " concepts " and " ideas ". In the nerve-cells of the cortex is 

 concentrated all the machinery of the " psychic " existence, 

 and from them all its impulses proceed. Every nerve-fibre 

 is connected centrally with a nerve-cell, whether afferent or 

 efferent. The whole nervous system is composed of nerve- 

 units (neurons). Every ganglion-cell has its protoplasmal 

 extensions (dendrites) with delicate end-branches, and its nerve- 

 extension (neuron) each of which ultimately ramifies among 

 the end-branches of its collaterals. 



Thus the vast multitudes of neurones are brought in contact 

 and cohesion. 



Until forty years ago it was generally held, with Flourens 

 and Longet, that the various functions of the brain were not 

 localised but were produced by the activity of the cerebrum as 

 a whole. Since then, however, it has been convincingly de- 

 monstrated that both in man and animals the motor and sensory 

 functions are situated in definite regions of the cerebral cortex. 

 Repeated experiments have shown that the motor centres lie 

 on the anterior portions of the cortex, the sensory centres on 

 the posterior. 



Paul Flechsig has mapped out the distribution of the four 

 central sense-organs in the grey matter of the cerebral cortex 

 as follows : 



Tactile centre in the parietal lobe 

 Olfactory frontal 

 Visual ,, occipital 



Auditory ,, temporal 



Between these four perception-areas lie the four thought- 

 centres. Upon the integrity of the cerebral cortex depends the 

 " intelligence " in man and animals. 



As to what constitutes the brain an organ of perception, 

 thought and volition, -Humboldt has spoken in his characteristic 

 manner : " The invisible weapon of defence given to certain fishes 

 and evoked by contact so that the shock is spread through the 

 whole body of the disturber, the lightning's flash in the sky, the 



