CHAPTER IX 



THE SEGMENTAL THEORY IN VERTEBRATES 



i. The segmental arrangement of the central nerv- 

 ous system of Vertebrates is suggested by the ar- 

 rangement of the spinal nerves. The number of 

 segmental ganglia present in the head exceeds the 

 number of cranial nerves. The auditory nerve and 

 the vagus, for instance, originate from more than one 

 segment each. Dohrn, Locy, and others have shown 

 this. Locy states that there are originally fourteen 

 segments in the head of the embryo of the shark, 

 while there are only twelve cranial nerves. Physi- 

 ology is more interested in the decision of this quest- 

 ion than morphology, because upon it depends the 

 theory of coordinated movements. The question 

 of segmentation may also be of importance indi- 

 rectly in connection with the idea of localisation in 

 the cerebral hemispheres, for the so-called centres of 

 the cerebral cortex are merely the places where the 

 fibres from single segments of the central nervous 

 system enter. 



The spinal nerves originate in the spinal cord, and, 



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