CHAPTER IV 



EXCITATION AND TRANSMISSION IN RELATION TO 



AXIATE PATTERN 



THE EXCITATION GRADIENT 



The determination of a region of greater activity in 

 a cell or cell mass by differential exposure is merely 

 the first step in the establishment of a physiological 

 gradient. Living protoplasm is irritable or excitable, 

 and excitation in a broad sense is conceived to consist 

 in an acceleration of the fundamental activities of pro- 

 toplasm or, more particularly, those concerned with 

 energy liberation. A region, the activity of which has 

 been increased by the action of some external factor, 

 whether such factor be oxygen or a stimulus in the 

 stricter sense, is a region of excitation as compared with 

 other regions. Protoplasm also possesses the capacity 

 to transmit excitation to a greater or less degree or, 

 more strictly speaking, to transmit some sort of dynamic 

 change resulting from excitation at one point, which 

 induces excitation in adjoining regions. These may in 

 turn excite still other regions, and so the process of 

 excitation spreads, irradiates, or is transmitted. In the 

 absence of highly specialized conducting paths in pro- 

 toplasm we find, however, that this capacity for trans- 

 mission is apparently limited by distance. In other 

 words, the process of excitation undergoes a decrement 

 in intensity or effectiveness with increasing distance 

 from the point of origin and at a greater or less distance 



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