How to dismantle / clean the microsoft basic optical mouse

Hi,

 

I've got the basic microsoft optical mouse and i have troubles with it as it is full of dirt which seems to have got in between the two buttons and is now making them stick when I try to use it.

 

Problem is I just cannot figure how to dismantle the mouse.  I have searched everywhere but have drawn a blank.  Nothing seems to unclip and there are no screws anywhere to be found.

 

Does anyone know of a way that I can take this mouse apart so that I can give it a clean??  Any help appreciated.