168 INHIBITORY NERVES AND CO2 PRODUCTION 



structive or anabolic processes. The latter processes are typical of 

 arrest,^ but mere cessation of action, such as occurs in blocking of 

 impulses, is not to be confused with true inhibition which must be 

 conceived as synonymous with depression of the chemical processes 

 concerned in CO2 production in the nerve cells. When this con- 

 ception of inhibition is applied to other types of cells as well as to 

 nerve cells, it will do much to clear up conflicting and confusing 

 ideas concerning the true nature of the inhibitory mechanism. 



SUMMARY AND CONCLUSIONS. 



It has been shown in this paper that stimulation of the inhibitory 

 nerves of the neurogenic heart of Limulus, which correspond to the 

 vagus nerves of the vertebrate heart, results in a marked diminution 

 of CO2 production in the heart ganglion, while stimulation of the 

 ganglion, leading to increased activity of the heart, leads also to in- 

 creased CO2 production by the ganglion. This shows that inhibition 

 of the automaticity of this ganglion by the action of its inhibitory 

 nerves consists, not in a process of blocking, but in a diminution 

 of those chemical reactions in the ganglion cells which give rise to 

 the production of CO2. 



•° Arrest may be quite independent of inhibition and due to secondary causes 

 such as blocking or removal of stimulating impulses. This is exemplified upon 

 stimulation of the vagus fibers to the turtle heart; the auricles are inhibited 

 (true depression), the ventricles merely arrested, and the latter only show the 

 after-augmentation. 



