CHAPTER X 



SEM1DECUSSATION OF FIBRES AND FORCED 



MOVEMENTS 



It is apparent from the foregoing that in the central 

 nervous system of Vertebrates only segmental gan- 

 glia and only segmental reflexes appear. Superior 

 centres, a " centre of coordination " for instance, can- 

 not exist. Irritability and conductivity suffice to pro- 

 duce coordination in Medusae, in the heart, in the 

 respiration of Limulus, and in the movements of the 

 earthworm and the salamander. Schrader has rightly 

 expressed it : the nature of the nervous connections 

 alone determines the cooperation of different seg- 

 ments in a common activity. Some of these connec- 

 tions require special mention, for instance, those in 

 which decussation and semidecussation of fibres ap- 

 pear. Possibly the most familiar example of semi- 

 decussation is found in the optic nerves. Here the 

 fibres cross, so that while each eye has its own special 

 nerve each tract contains fibres from both eyes. The 

 fibres of the temporal half of the retinae pass through 

 the chiasma uncrossed (that is, remain on the same 

 side of the head and brain), the fibres which come 



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