Ophthalmologist visit

I went to see the ophthalmologist yesterday.  What's with the device that you rest your head and chin on so the doctor can examine you?  Why do they have the tear away sheets for the chin rest but not the forehead rest?  I feel like foreheads are dirtier than chins.  I put my head into the machine and my forehead is always confronted with a greasy piece of plastic while the tear-away chin rest piece taunts me with its cleanliness.