Iris aperture shape

Mike Jones mkjones at
Thu Nov 5 20:29:59 EST 1998

Does anyone have an explanation to a non-biologist why the aperture in 
my iris is circular whether open up or shut down, whereas cats, sheep, 
and donkeys have a slitted iris opening, which in some cases become 
circular under low light levels. Also what is the reason for varying the 
axis of slit orientation, ie. vertical in cats - horizontal in other 

Mike Jones

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