CHAPTER IX 



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THE NEEVE-NET 



THE nerve-net of the lower animals contains the germ 

 out of which has grown the central nervous systems of 

 the higher forms. As a definite type of structure the 

 nerve-net has been recognized only for a few years. Its 

 discovery was brought about by the repeated attempts 

 made since the declaration of the cell theory to resolve 

 nervous tissue into its component cells. Although the 

 cell theory as applied to animal tissues was enunciated by 

 Schwann as early as 1839, it was not till more than half 

 a century later that a clear and consistent idea of the 

 nerve cell was arrived at. 



Nerve fibers seem to have been first really seen and 

 figured by the Florentine physician Felix Fontana in 

 1781, but it was not till 1833 that Ehrenberg in the pre- 

 liminary announcement of a monumental work on the 

 fibrous structure of the central nervous organs, described 

 certain corpuscles that proved to be what later investi- 

 gators called ganglion cells. The connection of these two 

 elements, vaguely intimated in 1838 by Eemak and sur- 

 mised in 1840 by Hannover, was first really demonstrated 

 for invertebrates in 1842 by Helmholtz and for verte- 

 brates in 1844 by Kolliker, who showed that fibers with 

 a medullary sheath, and therefore unquestionably nerv- 

 ous, were directly connected with ganglion cells. From 

 the time of these discoveries, it became necessary to 

 assume that in some way or other ganglion cells were 

 integral parts of the nervous system. 



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