10 VERTEBRATE ZOOLOGY 



and proceeding laterally, so that, in the case of such structures as the 

 paired appendages, the last parts to develop are the terminal elements. 



The Nervous System as a Mechanism for Maintaining the 

 Relations of Dominance and Subordination in the Organism.— 

 The method of maintaining the dominance of the apical parts over 

 basal parts is the method of conduction over nerve paths. There 

 seems to be little doubt now that communications between an apical 

 part and a basal part are electroid in character, so that changes in the 

 chemical state of the apical part are transmitted speedily to the basal 

 part and excite it to functional activity or inhibit its action, according 

 to the character of the apical change. Thus a stimulated condition 

 of a particular segment of the central nervous system involves a 

 change of electric potential between it and the basal organ with which 

 it is connected. The reaction of the basal part is in the nature of an 

 adjustment to meet the change in the apical part so as to bring about 

 a return to equilibrium between the two terminal points of the system. 

 Thus, throughout the whole body there is a very intricate system 

 of controlling and controlled parts, depending on the relative inten- 

 sity or relative rate of metabohsm of the various parts. It has been 

 shown by the most exact types of apparatus, e. g., the Tashiro biom- 

 eter, which measures exceedingly minute differences in carbon dioxide 

 production in small objects such as nerves, that the apical part of a 

 nerve has a higher rate of metabolism than the basal part. 



Relative Susceptibility to Inhibiting Agents of the Apical 

 and Basal Parts of the Axes. Child has demonstrated differ- 

 ences in rate of metabolism between apical and basal parts of 

 an organism by the so-called "susceptibility method." He finds 

 that apical parts are more susceptible than basal parts to agents 

 that retard metabolic activity (such as anaesthetics, potassium 

 cyanide, etc.). If lethal concentrations are used, the apical part 

 dies first and there is a progression of death changes down the 

 axes until the last parts to retain hfe are the basal parts. These 

 experunents demonstrate that the axial gradient is essentially a 

 gradient in the rate or intensity of metabolic activity. From this we 

 may conclude that the relationship of functional dominance and sub- 

 ordination is one depending on the relative rates of metabolism, the 

 part with the more rapid or intense metabolism stimulating through 

 conduction regions of less rapid metabolism, and exciting them to per- 

 form their peculiar functions. 



