348 GENERAL PHYSIOLOGY 



I. The Nature of Stimulation 



In accordance with the foregoing statements, a stimulus may be 

 define as every change of the external agencies that act upon an 

 organism. If a stimulus comes in contact with a body that 

 possesses the property oi irritability , i.e., the capability of reacting 

 to stimuli, the result is stimidation. It is necessary to examine 

 somewhat in detail the general characteristics of the process of 

 stin:iulation. 



A. THE RELATION OF STIMULI TO VITAL CONDITIONS 



1. The Varieties of the Stimulus 



If every change of the agencies that act upon the organism 

 frora without is able to stimulate, it is evident that innumerable 

 kinds of stimuli exist. JSTot only may every existing condition of 

 life be changed, but new conditions may appear and affect the 

 organism. Notwithstanding this possibility, stimuli may be 

 classified according to their qualities into a few large groups. A 

 natural classification is possible in accordance with the forms of 

 energj' which the different stimuli represent ; for the operation of 

 every external agent upon a body depends upon a transformation 

 of energy. 



In accordance with this principle all influences of a chemical 

 nature may be grouped as chemical stimuli, including not only 

 changes in the income of food, water, and oxygen, but other 

 chemical changes which ordinarily do not come into contact with 

 the organism. Among chemical stimuli belong also the processes 

 by which in the animal cell-community the nervous system 

 influences the tissue-cells dependent upon it ; for every nerve 

 stimulation has at its foundation a chemical transformation 

 of nerve-substance, which is transmitted to the cells of the 

 tissues and acts towards the latter as a chemical stimulus. In 

 accordance with our modern ideas upon the metabolism of living 

 substance, the old conception that nerve stimuli are merely 

 electrical stimuli, and that nerves behave as copper wires, can find 

 credence no longer. 



All purely mechanical influences that affect the organism va&j 

 be termed mechanical stimuli, including those that consist in 

 changes of jaressure, such as pushing, shaking, pressing, pulling, 

 and sound-vibrations, those that manifest themselves by molecular 

 attractions, such as cohesion or adhesion in the surrounding 

 medium, and those that depend on the action of gravitation. 



Thermal stimuli comprise changes of the temperature that 

 surrounds the organism. 



