THE PROCESS OF EXCITATION 91 



forms of stimuli and their action. It cannot be denied that the 

 degree of dissociation of an electrolyte can be altered by very 

 different factors, such as heat, light, chemical processes, etc., 

 and in that the surfaces of the protoplasm, acting as semi- 

 permeable membranes, bring about a selective action on the pas- 

 sage of the ions, there arises the opportunity for the development 

 of difference of electrical potential on both sides, and for further 

 chemical alterations in the protoplasm. These observations, how- 

 ever, require further experimental investigations in many fields, 

 before we are justified in extending the Nernst theory of the 

 manner of action of the electric stimuli to a general explanation 

 of the primary alterations produced by all stimuli in the living 

 substance. For the present we must confine our observations to 

 those alterations which are known to be responses to an exci- 

 tating stimulus; these are the chemical alterations in the metab- 

 olism of rest in the living substance. 



If it is asked, which members of the entire metabolic chain 

 are increased primarily by the stimulating excitation of a vital 

 system, we should not be able to answer this question gen- 

 erally for all living systems. To begin with, it appears highly 

 probable that the various forms of vital substances in this respect 

 act quite differently. It is to be regretted that, up to the present, 

 this question has not been treated from a comparative stand- 

 point. This inquiry should be extended to the greatest possible 

 number of organisms. Still there is enough material at hand, 

 obtained from the muscles, glands, ganglion cells, nerve fibers 

 and plants, to show that the complexity is by no means so great 

 as one might at first assume. 



In considering the two stages of metabolism, assimilation and 

 dissimilation, in their entirety, it appears as a very remarkable 

 fact, that nearly all stimuli produce primarily a dissimilative 

 excitation. We are only acquainted with a primary assimi- 

 lative excitation, that is, an augmentation of the building up 

 processes, in short, the formation of living substance, occurring 

 as a primary result of stimulation, following increased intro- 

 duction of foodstuffs extending over a prolonged length of time. 

 With this exception it cannot be proved that any other stimuli. 



