CHAPTER XLIII 



THE SENSE ORGANS 



Vocabulary 



Irritability, response of simple organs to environment. 

 Papillae, minute projections supplied with nerve endings. 

 Pigment, color substance. 

 Concentrate, to bring to one point, to focus. 

 Competent, able. 



The chief function of the nervous system mentioned in the pre- 

 vious chapter was that of control. It has another equally im- 

 portant use, namely to keep us in touch with our surroundings by 

 what we call sensation. 



Irritability. All living things respond more or less to their en- 

 vironment. Plants react to light, moisture, contact, and gravita- 

 tion, and thus have a very simple sort of sensation, usually called 

 " irritability." These responses are sufficient for their needs, as 

 our experiments have shown, and enable plants to reach food and 

 water supplies, to turn leaves toward light, to climb by means of 

 tendrils and to perform certain movements concerned in pollena- 

 tion and seed dispersal. 



Touch. Even the simplest animals are affected by actual con- 

 tact with surroundings. The amceba recoils from hard or hot 

 particles, absorbs food when in contact with it, and thus may be 

 said to exhibit a primitive sense of touch. 



In higher forms, the whole body surface possesses this sense 

 more or less. It is often especially developed in tentacles, hairs, 

 or papillae in various animals. In man the sense of touch is com- 

 mon to all parts of the skin, especially the finger tips, forehead, and 

 tongue. The human skin also possesses special nerves that receive 

 temperature, pressure, and pain sensations. If we gently touch 



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