THE NERVOUS SYSTEM 337 



animal ; its segments of equal value ; its nervous reactions 

 unisegmental, although linked in functional sequence. If it 

 starts to walk, owing to stimulation of one of its sense-organs, 

 the impulse to walk spreads from segment to segment. Com- 

 paring the latest product of evolution with the earliest, we 

 find that nervous tissue has concentrated at the anterior 

 end of the body. The double chain of ganglia, now condensed 

 into the axis of the brain and the spinal cord, still contain 

 all the effector neurones by which muscles are called into 

 action. Sensory nerves still arborize in the axis, providing 

 the mechanism for actuating motor neurones. But the vast 

 majority of intermediate or intercalated neurones have been 

 attracted to the two huge brain-masses the cerebellum and 

 cerebrum. In the former all sensations (not conscious) con- 

 nected with tone, position, orientation and equilibrium are 

 worked into appropriate impulses for the regulation of the 

 muscular system. In the latter all sensations which convey 

 information regarding the relation of the environment, in- 

 cluding the body, to the ego the not-me to the me are 

 transformed into motor discharges which set a-going the move- 

 ments (and the thoughts) by means of which the purposes of 

 life are fulfilled ; for in the cortex of the great brain alone is the 

 passage of nerve-currents accompanied by consciousness. 

 Concentration of nerve-tissue allows of the combination of 

 sensations. It also facilitates the no less important effect of 

 mutual influence, interference. Sensations are suppressed, and 

 therefore the multitude of reactions to which they would give 

 rise are inhibited, in the interests of restricted and sustained 

 movement or thought. 



The Cerebellum. Sharks and other swift-swimming fishes 

 have large, deeply fissured cerebella, for the cerebellum is the 

 part of the brain which has gathered into itself most of the 

 grey matter associated with balancing, attitude, posture. 

 The cerebellum is in birds large and deeply folded. Developed 

 from the ganglia to which the auditory nerve distributes im- 

 pulses from the semicircular canals, it has established connec- 

 tions with all the other nervous tissues concerned with sen- 

 sations of position, strain, or pressure, including the eyes, 

 which afford information regarding the position of our limbs 

 relatively to the trunk, and of the whole body relatively to 



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