STIMULATION AND TRANSMISSION 265 



activity to changes in bodily conditions, and so on. 

 Evidently the effects of external stimuli upon the sense 

 organs cannot be regarded as forming a significantly 

 different class from these phenomena, so that from the 

 standpoint of general physiology the foregoing distinc- 

 tion is a purely formal one and has little objective 

 importance. 



Various terms are applied to special processes which 

 may be included under the general conception of stimula- 

 tion, as just defined. The term ''activation" is used 

 with reference to the initiation of development in a 

 resting egg cell by a spermatozoon or a parthenogenetic 

 agent; acceleration, or simple increase in the rate of an 

 already existing process, is a frequent form of response 

 (e.g., secretion, the heart-beat, or other regular muscular 

 movement, growth, etc.); retardation or inhibition is 

 perhaps equally frequent. In cases of automatism, like 

 that of the heart, the rhythm may be regarded as deter- 

 mined by periodic stimuli furnished by processes within 

 the cell. Since all of these phenomena may occur, or 

 undergo modification, in response to changes of environ- 

 mental condition, all are to be considered under the 

 general conception of stimulation. 



The most general features of the stimulation-process 

 are best studied in those irritable tissues or cells which 

 give a prompt and definite response to electrical or 

 mechanical stimulation, such as the nerves and muscles 

 of higher animals; and the majority of investigations, 

 on stimulation, especially those of a quantitativ^e kind, 

 have been carried out with these tissues, usually after 

 isolation. The results gained have, however, a general 

 applicability to other irritable living systems. 



