CHAPTER XI 



THE FROG (continued} : THE ORGANS OF SPECIAL SENSE 



IN the previous chapter you have learnt how the 

 nervous system controls the various functions of the 

 body, and how voluntary action is absolutely dependent 

 upon the connection of the brain, through the spinal 

 cord, with the nerves. Obviously, in order that the 

 power of voluntary action should be of full use to its pos- 

 sessor, some means of communication with the external 

 world is not only desirable but necessary ; the frog, in 

 order to adjust its actions to the circumstances in 

 which it from time to time finds itself, must be able to 

 distinguish friends from enemies, suitable from unsuit- 

 able food, darkness from light, heat from cold. 



The avenues of communication between the animal 

 and its surroundings are, as in ourselves, the senses of 

 touch, taste, smell, sight, and hearing. 



The sense of touch, including that of temperature, is 

 lodged in the whole extent of the skin, which, as you* 

 have already learnt, is abundantly supplied with sensory 

 nerves. Many of the nerves terminate in connection with 

 (what are known as tactile cells large flattened cells 

 arranged in groups just below the epiderm and around 

 which the ultimate fibres of the sensory nerves are distri- 

 buted. Stimuli applied to the skin, either by direct 



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