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Responsiveness— Tlie Sense 



o 



r^ans 



Sense organs are gateways to the mind. All that we know of our surround- 

 ings is brought to us through them. It is difficult to imagine our existence 

 without them. What would it be? 



Receptors. Receptors are cells or parts of cells that are especially sensitive 

 to particular conditions in their surroundings. Sense organs include receptors 

 and associated cells. Those that are affected by external things are most 

 familiar for they include sight and hearing, taste and smell, touch and tem- 

 perature. Other receptors are sensitive to situations within the body, the 

 stretch and pressure of muscles, the movements of internal organs. We feel 

 comfortable in one chair and not in another, we feel thirsty, or we know 

 that we have eaten too much. We make hundreds of adjustments of the body 

 without being aware of any of the sensory signals concerned with them 

 although such signals are constantly being given by these active internal 

 receptors. 



A receptor responds to stimuli only when they are of a certain kind and a 

 particular intensity called the threshold of sensation. Within limits, the inten- 

 sity of a given sensation increases with the stimulus to a certain point, then 

 there is a sensory adaptation and the sensation decreases, and a limit may be 

 reached when there is no sensation at all. The first piece of candy is the sweet- 

 est. The stronger the odor, the more quickly it fades. A solution of camphor 

 can be smelled for about five minutes. 



External Tactile Senses — Touch, Pain 



Protoplasm is in general sensitive to slight differences in pressure whether 

 sensory structures are present or not. Some of the nerve fibers end without 

 any coverings and these probably react to any stimulus from muscular cells 



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