266 PROTOPLASMIC ACTION AND NERVOUS ACTION 

 GENEEAL CONDITIONS OF STIMULATION 



It is well known that different living systems may 

 vary widely in their sensitivity to the same stimuli, 

 and also that irritability is often specialized with reference 

 to particular physical agents. Sensory elements wdth 

 special sensitivity to light, contact, slight changes of 

 temperature, or chemical substances, are found in all 

 higher animals. These differences are referred to special 

 features of chemical and structural organization. 

 Chemical sensitivity in particular is often minutely 

 specialized; and such instances as the special sensitivity 

 exhibited by the sensitized smooth muscle of guinea-pigs 

 in anaphylaxis indicate clearly that many forms of 

 specific chemical irritability are dependent on the 

 presence of specific chemical compounds (apparently 

 in this case proteins) in the irritable cell, probably in 

 the protoplasmic surface layer. Similarly, photo-sensi- 

 tive elements like the retinal rods and cones contain 

 compounds of definite photochemical properties (visual 

 purple and related substances) upon which the special 

 responsiveness undoubtedly depends.^ We must recog- 

 nize, therefore, in addition to the general susceptibility 

 to mechanical or electrical stimuli possessed by all forms 

 of protoplasm, a variety of specific or selective forms of 

 irritability depending on special features of structure or 

 organization. Selective irritability is shown especially 

 by the sensory nerve-termini or receptors of higher 

 animals; these are classified, according to the agents to 



* Thus Hecht and Williams have recently shown that the curve 

 of absorption of visual purple is almost identical with tlie curve of 

 visual sensitivity for different wave-lengths {Jour. Gen. Physiol., IV 

 [1922], i). 



