CHAPTER XXII 



THE SPECIAL SENSES 



Evolution of the special senses. In the simple 

 forms of animal life, the special senses of taste, sight, 

 smell and hearing are either lacking entirely or are 

 most rudimentary. The sense of touch, on the contrary, 

 is found in all animals, however simple in structure, and 

 may therefore be considered the fundamental sense from 

 which all the others have been developed. In the next 

 higher forms of animal life, the senses of taste and smell 

 are added to touch, as they require but a fairly simple 

 nervous structure. It is difficult to say at what stage 

 of animal evolution sight and hearing begin. Rudi- 

 mentary organs are found which are probably capable 

 of perceiving noise and light. Hearing and sight of 

 the perfect kind which the higher animals possess is 

 clearly impossible for them, however, since the structure 

 of their ears is too simple to enable them to perceive 

 pitch and quality of sound, nor can their rudimentary 

 eyes build up perfectly formed and colored images of 

 external objects. 



Special senses. Five distinct or special senses, 

 namely, touch, taste, smell, sight and hearing, have 

 always been recognized. The sensations included under 

 touch have been shown by experiment, however, to con- 

 sist of at least four distinct and different kinds, those 

 of contact or touch, of pain, of heat and of cold. 



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