STRUCTURE OF THE HUMAN BODY 59 



Common Origin of Sense Organs. — We have seen that all the 

 sense organs agree with one another in the important point that 

 their essential parts, i.e. those containing end-organs, are really 

 modifications of part of the skin, though this is not apparent at 

 first sight, and reference must be made to the facts of development 

 in order to prove it. But as the different sense organs are 

 adapted to respond to very different agents, it is only natural 

 that they should be modified for the purpose, so that the ear, 

 for example, possesses an elaborate sound-conducting apparatus, 

 while the eye consists largely of refracting structures. 



