Extra 2: External material

If you want to know more about this subject, follow the following links to external study material:

 

 

 

http://www.bbc.co.uk/schools/gcsebitesize/science/ocr_gateway_pre_2011/ourselves/3_keeping_in_touch1.shtml

http://leavingbio.net/the%20senses_files/the%20senses.htm