The nasolacrimal duct is a a duct which allows tears to drain from the eyes to the back of the nose. A close examination of the eye will reveal two small holes in the corner close to the nose. These holes connect to the nasolacrimal duct, which travels down the nasolacrimal canal to the back of the nose. People who have wondered why they get runny noses when they cry need wonder no more: their runny noses are actually just caused by tears flooding the back of the nose.
Under normal conditions, the eye is constantly being bathed in tears which are swept away by the nasolacrimal duct. Drainage of the eyes ensures that they don't overflow, which can look unsightly, and it also reduces the risk of infection of the eyes. When people cry, they typically overload the duct's ability to drain, causing tears to spill from the eye and from the back of the nose.
In some people, the nasolacrimal duct becomes blocked by debris or inflammation caused by an infection, causing epiphora, a situation in which tears flow out of the eyes rather than down the duct designed to drain them away. This is common in people with chronic allergies, which explains why people who suffer from hay fever and environmental allergies often have runny eyes. If the blockage persists, the eyes can also develop a thick, pus-like discharge. Clearing the blockage can be accomplished by removing the allergen or taking medications to reduce inflammation.
In some cases, it may be necessary to perform a dacryocystoplasty, in which a blockage is surgically removed. Infants who are born with congenital blockages of their nasolacrimal ducts may require surgery. This is classically done by inserting a small inflatable balloon into the duct to push it open and hold it open so that the patient's eyes will drain properly. Sometimes, congenital blockages will clear up on their own, often with the help of gentle massage and eyewashes.
There's another fact about the nasolacrimal duct which people may find of interest: because it drains down the back of the nose and throat, any pathogens introduced to the eyes will eventually find their way into the body. This is why people should avoid touching their eyes, and should always wash their hands first if they do need to handle their eyes or eyelids, as they can spread infection into their bodies by accident.