STIMULATION AND TRANSMISSION 267 



which they are specially responsive, as chemo-receptors, 

 thermo-receptors, photo-receptors, etc. 



Such special sensitivity may be described as consisting 

 in a lowering of the threshold of stimulation for a particu- 

 lar agent, and need not affect the general sensitivity to 

 mechanical and electrical stimulation. Some specific 

 irritability is superposed upon the general or non- 

 specific irritability. Thus a nerve or muscle may be 

 stimulated by mechanical, thermal, chemical, osmotic, 

 and electrical stimuli; similarly, a highly specialized 

 receptor such as a retinal element may be stimulated 

 by these agents as well as by light of a definite wave- 

 length. In all cases, however, the response following 

 stimulation has a specific character which is dependent 

 on the special structure or organization of the irritable 

 system or on its relations with other systems. In the 

 field of sensory stimulation this generalization is known 

 as the ''law of specific energies." 



LOCAL CHANGE AND PROPAGATED EFFECT 



We have seen that an irritable system with a highly 

 developed general sensitivity, e.g., a muscle or a nerve, 

 may be excited by a variety of stimulating agents, and 

 the question arises why such physically dissimilar agents 

 produce the same physiological effect. The general 

 sequence of events when such a tissue is stimulated may 

 be briefly described as follows. Some local change, 

 whose precise nature is determined by the nature of the 

 stimulating agent, occurs at the site of stimulation; a 

 state of ''excitation" is there initiated which, however, 

 does not remain confined to this region, but spreads or 

 is propagated to a distance, often at a high velocity. 



