Embedded in the tarsal plates are tarsal glands (see Fig. 8.77), which empty onto the free margin of each eyelid. These glands are modified sebaceous glands and secrete an oily substance that increases the viscosity of the tears and decreases the rate of evaporation of tears from the surface of the eyeball. Blockage and inflammation of a tarsal gland is a chalazion and is on the inner surface of the eyelid.