Liz Buchanan BVSc MRCVS
Eyes have a very delicate surface, the cornea, and keep it clear with a film of tears. Impurities such as dust particles or bacteria are balled up into something sticky and carried to the corner as shown. This happens all the time on a low level but sometimes the speed or thickness of it can be increased, for example if the dog goes somewhere with a lot of particulate dust in the air, or the eye gets dry, or they suffer an upper respiratory infection eg a 'cold.' Often this does the job and is self limiting, but it can also be a sign or more serious problems eg an ulcer or erosion to the cornea or some underlying illness.