40 IRRITABILITY 



tain temperature; the existence of a certain light intensity; the 

 existence of a definite static pressure ; and finally the presence of 

 an equal osmotic pressure. The stimulus according to its quality 

 can be differentiated into chemical, thermal, photic, mechanical 

 and osmotic varieties. To these must be added other forms of 

 stimuli not ordinarily operative, for instance, many uncommon 

 chemicals, and certain kinds of rays. The form of stimulation, 

 par excellence, which has acquired the greatest importance for the 

 experimental investigation of life, is electricity. In its manifold 

 forms it permits, as no other, of such fine gradations of inten- 

 sity and duration that it has become in the hand of the physiolo- 

 gist an invaluable means of research. 



Alterations in those factors which act as vital conditions com- 

 pose the great mass of physiological stimuli which act continu- 

 ously on every living organism. The first point to be considered 

 in every alteration is its direction. The alterations produced by 

 stimuli may be of two different kinds, either positive or negative. 

 The quantity of foodstuffs, water or oxygen, in the surrounding 

 medium, can undergo an increase or diminution; as may the 

 temperature, intensity of light, the atmospheric and osmotic pres- 

 sure. The strength of the electric current, which may be applied, 

 can also be regulated. In accordance with the definition of stimu- 

 lation already referred to, we must consider these alterations, 

 whether negative or positive, as forms of stimulation. Now the 

 question arises: Is this point of view justifiable? Should one 

 also consider, for example, the lessening or total removal of a 

 vital condition as a stimulus? Should one consider the removal 

 of water or oxygen, cooling or darkening, as a stimulus ? It has, 

 in point of fact, been occasionally attempted not to regard these 

 negative deviations as forms of stimuli. These observers per- 

 mitted themselves to be led by the dogma, that only that which 

 produces an excitation, that is, an increase of the processes in the 

 living substance, should be regarded as a stimulus. Such a limi- 

 tation of the conception of stimuli would only result from the 

 one-sided consideration of an all too limited circle of facts. Con- 

 sidered from the point of view which results from a broader 

 range of experience, this narrow view becomes untenable. 



