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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