CHAPTER XII 
CERTAIN FEATURES OF TRANSMISSION AND CON- 
DUCTION IN RELATION TO THE PHYSIO- 
LOGICAL GRADIENTS 
In chapter iv the part played by excitation and trans- 
mission in originating and establishing the physiological 
gradients was discussed. In chapters vi-xi it was 
shown that the physiological gradients afford a general 
physiological basis for the localization, the course of 
differentiation and the general plan of structure of the 
nervous system as a whole, as well as for tentative con- 
clusions concerning the origin and development of 
neuron pattern. Since there can be no doubt concern- 
ing the close relation between nervous structure and 
nervous function, i.e., the conduction of impulses, and 
since the facts indicate that the physiological gradients 
originate in excitation and its protoplasmic transmis- 
sion, there is every reason to believe that these gradients, 
once established, must play a part in the development 
of the more or less specialized forms of transmission, 
which are usually called conduction. The gradients 
provide, first of all, a mechanism for giving conduction 
a more or less definite, or at least a preferential, direction 
and thereby for determining in some degree the physio- 
logical relations to each other of various parts. The pur- 
pose of the present chapter is to call attention to certain 
facts in connection with the transmission or conduction 
of impulses in paths which do not show any distinguish- 
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