52 



THE CELL 



5. Those agents which evoke a response as a rule also alter the excitability 

 of the living substance i. e., under their influence a given stimulus pro- 

 duces a stronger or weaker response than if it were acting alone. We must 

 therefore make a distinction between excitation and the alterations of excita- 

 bility (positive or negative). An excitation may be said to have taken place 

 if a given stimulus can be shown to have started a dissimilative process. If, 

 however, the stimulus produces no effect of this kind, but a second stimulus 

 under the influence of the first produces a response stronger or weaker than 

 it otherwise would, then the first stimulus has increased or diminished the 

 excitability of the cells stimulated. If the stimulus becomes too strong, the 

 functional powers of the living substance may be either reduced or destroyed. 



B. AUTOMATIC EXCITATION 



When protoplasm is protected from all possible external influences, it 

 still exhibits the functions which we have learned to regard as essential: 

 absorption of food, motility, digestion, heat formation, etc. There must be 

 therefore inside the cell something which calls forth its activities, and from 

 all that is known to us on this subject, we may assume with a high degree 

 of probability, that the excitation is caused by the products of metabolism 

 formed in the activity of the cell. 



The significance of stimuli arising in the body itself appears from dis- 

 coveries which have been made concerning the activity of the central nervous 

 system in the higher animals. If for example a rabbit be choked by compression 

 of the trachea, within a short time there appear powerful respiratory move- 

 ments, convulsions of the whole body musculature, contractions of the vascular 

 walls, etc. In this case t*he decomposition products normally eliminated in the 

 expired air are retained in the body and bring about the powerful stimulation 

 of the central nervous system in the manner observed (cf. Chapter XXII). 

 Similar phenomena appear when by extirpation of the kidneys the fluid decom- 

 position products otherwise removed from the body by them, are allowed to 

 collect in the body in large quantity. 



The direct excitation produced by metabolic products is called automatic 

 excitation, because the exciting substances are formed by the activity of the 

 protoplasm itself. That is to say, in this case the cells develop within them- 

 selves the stimuli which rouse them to continued activity. 



C. CHEMICAL STIMULATION 



Automatic excitation, as it is here defined, is a kind of chemical stimu- 

 lation, and fundamentally is, so far as we can judge, of exactly the same 

 nature as the excitation which we can produce artificially by various kinds of 

 chemical substances. 



Unicellular organisms, Amoebae and other Ehizopoda, are made to con- 

 tract by contact with a one- to two-per-cent sodium chloride solution, 0.1-per- 

 cent hydrochloric acid, one-per-cent potassium hydrate or weak solutions of 

 other acids, alkalies and salts; they draw in their pseudopodia and assume 

 a spherical form. The same substances quicken the movements of the flagel- 



