CHAPTER X 

 THE NERVOUS SYSTEM 



" Indeed, while Nature is wonderfully inventive of new 

 structures, her conservatism in holding on to old ones 

 is still more remarkable. In the ascending line of de- 

 velopment she tries an experiment once exceedingly 

 thorough, and then the question is solved for all time. 

 For she always takes time enough to try the experi- 

 ment exhaustively. It took ages to find how to build 

 a spinal column or brain, but when the experiment was 

 finished she had reason to be, and- was, satisfied." 



JOHN TYLER, The Whence and Whither of 



Man, p. 173. 



THE central nervous system begins its history as a straight 

 tube lying along the mid-dorsal line just beneath the integ- 

 ument. Anteriorly this tube ends blindly and exhibits a series 

 of three vesicular enlargements, the beginnings of the brain ; 

 posteriorly it ends blindly also and tapers to a point, although 

 there are certain mysterious indications in the embryonic 

 record of a former connection with the lumen of the alimentary 

 canal, indications which have not as yet received any satis- 

 factory explanation, and which may be after all merely de- 

 velopmental necessities, without historic significance. Through 

 modification of this simple neural tube without the addition of 

 extraneous elements save as auxiliary to this, there arise in all 

 vertebrates the brain and spinal cord, which, even in their 

 highest and most complicated form, appear to the morpholo- 

 gist as still tubular; the walls, enormously thickened in places 

 and often folded, give rise to such solid masses as the cere- 

 bellum or the cerebral hemispheres, the lumen persists as the 

 ventricles of the brain, and their continuation through the 

 spinal cord as the canalis centralis. 



All nervous systems have arisen in the beginning in response 

 to stimuli from without, and hence developed originally upon 



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