714 * Dynamic Theory. 



organs are connected with the brain by nerves of sensation. These 

 nerves and the cerebral organs with which they connect, we have inherited 

 from our ancestors as much as we have inherited our other bodily parts, 

 and they exist in us at birth. They were developed together pari passu, 

 the bodily parts, the sensory nerves, and the cerebral receiving organs. 

 Their harmony and correspondence at birth are perfect. The process 

 by which this correspondence has been brought about, has gone on 

 during past ages in our ancestry and consisted in the constant associa- 

 tion of stimuli of a certain class and degree of force applied upon the 

 periphery of the bod} 7 , with their corresponding cerebral centers. The 

 development of the cerebral centers of sensation is such as necessary to 

 bring them into correspondence with an average of the stimulations to 

 which they are subjected. Having become so developed, any stimulations 

 which are within this normal average will harmonize with the cerebral 

 organ arid tend to confirm and strengthen its function of sensitiveness 

 to such sort of stimulation, while any stimulation in slight excess of 

 such average will tend to change the organ and to make it sensitive 

 to the new tone of sensation. When this process is going on we have a 

 sensation of effort which is a more or less subdued uneasiness. But if 

 the stimulation be too great to be followed by a further differentiation 

 of the cerebral organ it is painful, because not having been built up by 

 such stimulation it is not in harmony with it. If the skin be violently 

 scratched with a pin it is painful, because the corresponding cerebral 

 organ has not been built up by stimuli as violent as that. Nor, neither 

 has the skin itself as an organ of sense been developed by such stimuli. 

 The} r are both protoplasm, and what develops one will develop its cor- 

 respondent. They are complementary of each other and rise or fall 

 together. 



As observed heretofore, the intrinsic constitution of protoplasm allows 

 of its differentiation only by stimuli within certain limits of violence ; 

 with reference to temperature for example, the limits for the simplest 

 organisms are the boiling point of water on one hand and its freezing 

 point on the other. With the higher organisms the limits of endurable 

 violence are greatly circumscribed. The blood of but few mammals can 

 with safety vary in temperature more than ten or fifteen degrees F. The 

 same thing is true of other sorts of stimuli. In order to affect us at all 

 they must be of a certain degree of force. If the force be too great the 

 effect is destruction instead of differentiation or health}^ sensation. Sen- 

 sation can be aroused only by stimuli within the limits of the integrity 

 of the tissues involved. Gentle stimulus within those limits gives satis- 

 faction or pleasure according to the state of the tissues ; forcible stimulus 

 begets a sense of uneasiness or disagreeableness, more forcible stimulus 

 of pain, violent stimulus of intense pain. The most violent stimulus. 



