Stimulus

(stim'-u-lus). Anything exciting the animal economy, or any part thereof, to increased functional activity. Stimuli, Adequate or Homologous, those for whose action the sense organs are specially adapted. S., Chemic, irritation caused by a chemic agent. S., Heterologous, one acting upon the nervous elements of the sensory apparatus along their entire course. S., Homologous, a stimulus only acting upon the end-organ. S., Mechanical, irritation caused by mechanical means, as by pinching or striking. S., Summation of. See Summation .