70 THE ORIGIN OF THE NERVOUS SYSTEM 



in organisms, which was briefly discussed in the preced- 

 ing chapters, indicates that these gradients originate as 

 such excitation gradients and become more or less per- 

 manently fixed or established in the protoplasm through 

 the persistence for a certain length of time of the dif- 

 ferential exposure, the region of activity or excitation 

 determined by it and the resulting excitation gradient. 



THEORIES OF EXCITATION AND TRANSMISSION 



As a basis for further consideration of the problem 

 some discussion of excitation and transmission in 

 general is necessary. Various theories of excitation 

 and transmission have been advanced in the past, but 

 most of them are either highly speculative or too exclu- 

 sively concerned with the process as it occurs in nerve. 1 

 These theories differ both as regards the cause of 

 the electromotor phenomena characteristic of excitation 

 and transmission and their role in the process of trans- 

 mission. According to some authorities the electrical 

 changes are merely an incidental feature of the process, 

 a result or a by-product, so to speak, while others hold 

 that they are the chief factors in transmission. Again, 

 according to some, the electrical changes have a purely 

 physical basis, while others believe that chemical re- 

 actions are concerned in the excitation process and in 

 bringing about the electrical changes. Some twenty 

 years ago Waller (1897, 1903) maintained that excita- 

 tion is associated with metabolic activity and more 

 recently Tashiro (191 7 and earlier papers) has shown 



1 For discussion of various theories of excitation and transmission 

 see Biedermann (1896, 1903), Bose (1902, 1906, 1907, 1913), Morat 

 (1906), Vcrworn (1913), Bayliss (1918, chaps, xiii and xxii and refer- 

 ences), also the various textbooks of physiology. 



