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 con- 

 nection 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 possessor, 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 unsuitable 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 distributed. Stimuli applied to 

 the skin, either by direct touch or by the heat of the sun, 



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