24 HUMAN PHYSIOLOGY 



to be less by means of the nervous system than by the 

 passage of chemical substances through the circulation. 

 Such chemical messengers, arising in certain parts of the 

 body and perhaps affecting very remote parts, are com- 

 monly referred to as hormones. The very slow processes 

 of growth and development are known to be greatly 

 dependent upon the interchange of hormones. 



Stimuli. If objective life consists in the adjustment of 

 internal to external conditions we shall do well to con- 

 sider somewhat more fully than we have done the nature 

 of the external factors. Any external condition which 

 modifies the activities of a living organism may be called 

 a stimulus. One naturally thinks of contacts, changes of 

 temperature, chemical applications, and electric shocks. 

 Light must not be left out of our list. A stimulus is best 

 thought of as a change rather than a continued environ- 

 mental state. A moment's thought will help one to real- 

 ize that a sustained condition may favor the preservation 

 of the plant or the animal, but it is the shifting of outward 

 conditions which cause it to exhibit its capacity for reac- 

 tion. Generally speaking, the more suddenly a change 

 occurs the more marked is its effect upon living matter. 



It would not be quite accurate to describe a stimulus as 

 a force. A force may stimulate but so may the discon- 

 tinuance of a force which has been operative for some time. 

 Positive effects from negative factors are common enough. 

 Silence may constitute a stimulus when it succeeds an 

 accustomed sound. (Note, for instance, how one wakes 

 at sea when the measured throb of the engine is inter- 

 rupted.) We cannot deny that the reduction of the tem- 

 perature of the skin is a source of stimulation, though this 

 is a subtraction of energy from the tissues rather than a 

 contribution. 



