Brown stains on the inside corner of the eve is normal in dogs and cats. The reddish brown stain is caused by the presence of porphyrins a substance normally found in their tears. When tears mix with any mucus flushed from the eye’s surface and then dry, the crusty deposit can have a dark brown to black color. This, too, is completely normal. Unless the discharge seems excessive to you or is accompanied by signs of discomfort, ocular tearing and slight discharge are usually normal. You should be concerned, for example, if your pet is blinking or winking the eve, or if the white area of the eyeball looks pink or the inside of the eyelids look red. See if your pet is rubbing paws or face against furniture or carpet. In addition to these signs, if your pet avoids bright light, seems withdrawn and is less hungry than usual, see a veterinarian immediately. The eyes are delicate and can be affected by a long list of disorders that fortunately often respond to early treatment.