OF MICRO- ORGANISMS. 107 



the most highly developed, as well as the most ele- 

 mentary; for upon ultimate analysis every psychical 

 manifestation consists in a response to an excitation. 

 Evidently, it is not in this general and somewhat 

 common sense that M. Richethas intended to employ 

 the word. For a more exact definition, let us consult 

 his work, of which, a whole chapter, the first, is de- 

 voted to this subject; the author enumerates and de- 

 velops at length the laws of irritability: 



1. Every action that modifies the actual condition 

 of a cell is an irritant of that cell. 



2. Every external force, provided it has a certain 

 intensity, is capable of inducing cellular irritability. 



3. The movement in response to irritation is pro- 

 portional to the excitation. 



4. The movement in response to iiritation is, for 

 equal irritations, stronger in proportion as the equilib- 

 rium of the cell is less stable; in other words, stronger 

 in proportion as the cell is more excitable. 



5. The response to the irritation, is a movement in 

 the form of a wave, which has a very short latent pe- 

 riod, a period of ascent, correspondingly brief, and a 

 very long period of descent. 



6. The movement of the cell upon irritation is, for 

 equal irritations, stronger in proportion as the irrita- 

 tion has been more sudden. 



7. The movement in response to a brief irritation 

 lasts much longer than the irritation has lasted. 



8. Forces which, alone, appear impotent, become 

 effective when repeated; for they have, in spite of 

 their apparent inefficacy, increased the excitability of 

 the organism. 



The statement of these various laws, gives the term 

 irritability a precision which it lacked. M. Richet had 



