SCIENCE HAS SO far not been able to fathom entirely why horses have a sinus system — a complicated set of empty air-filled chambers that extend off the nasal cavity and through which they normally drain — but basically if they didn't and their heads were made up entirely of bone they would be impossibly heavy. It is thought, too, that the sinuses may have an immune function, plus they protect vital structures from trauma.
The lining of the sinuses are constantly secreting small amounts of fluid and, provided this is clear, it is normal and nothing to worry about. Indeed, in a healthy horse with no sinus problems, on a cold day he might, as in humans and dogs, have a slight watery discharge, but otherwise the sinuses won't give notice that they are there unless something is amiss.
All animals have sinus cavities, but compared to other creatures, such as dogs and humans, the cavities in the horse are particularly large, probably due to his significant head size. They are sited at the front of the head, and around and just below
