90 IRRITABILITY 



or fatigue. This is apparent when it is understood that the 

 amount of the alterations produced by the stimulus must ascend 

 from the value zero to a certain height before the changes are 

 perceptible, and that under various conditions this amount is, on 

 the one hand, attained in different lengths of time and, on the 

 other, must reach a varying amount before it is perceptible by 

 means of the indicator. 



The facts concerning the whole latent period and its depend- 

 ence on various factors would be incomprehensible if it were 

 assumed that no alterations whatever take place during the latent 

 period although the stimulus is already operative. In reality, the 

 alterations following a stimulus occur with imperceptible rapidity 

 in the form of a molecular interchange, and the latent period is 

 simply an expression of the fact that the primary alterations, 

 being limited in nature, are not registered by our indicators. 



The question first arises, In what do these first imperceptible 

 alterations consist? Nernst^ has evolved the theory for electric 

 stimulus, that the primary effect produced by the electric current 

 is an alteration in the ion concentration on the surface of the 

 living substance. In fact, we know that the surfaces of all proto- 

 plasm possess the property of semi-permeable membranes and that 

 changes in the concentration of ions invariably occur when an 

 electric current flows through two electrolytes separated by a 

 semi-permeable membrane, in which the anions and cations have 

 a different rapidity of movement. It is apparent, therefore, that 

 such an alteration in the ion concentration must be followed by 

 further chemical processes in the living substance. According 

 to the theory of Nernst the first impetus for all further altera- 

 tions, which the electrical stimulus brings about in the metabolism 

 of rest, is the alteration in the concentration of the ions on both 

 sides of the semi-permeable membrane, which represents the sur- 

 face of the protoplasm. In view of the present findings of physi- 

 cal chemistry, objections can hardly be made to this theory of 

 Nernst's. It is a question, however, in how far this theory, espe- 

 cially established for the electric stimuli, can be applied to other 



1 Nernst: "Zur Theorie der electrischen Reizung." Xachrichten der Konigl. 

 Gesellsch. d. Wissensch. zu Gottingen. Math, physik. Klasse 1899. 



