Rheum is thin mucus naturally discharged as a watery substance from the eye

Human eye

The human eye is an organ which reacts to light for several purposes. As a conscious sense organ, the eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth...

s, nose

Human nose

The visible part of the human nose is the protruding part of the face that bears the nostrils. The shape of the nose is determined by the ethmoid bone and the nasal septum, which consists mostly of cartilage and which separates the nostrils...

 or mouth

Mouth

The mouth is the first portion of the alimentary canal that receives food andsaliva. The oral mucosa is the mucous membrane epithelium lining the inside of the mouth....

 during sleep (contrast mucopurulent discharge

Mucopurulent discharge

Mucopurulent discharge is the emission or secretion of fluid containing mucus and pus from the eye, nose, cervix, vagina or other part of the body due to infection and inflammation....

). Rheum dries and gathers as a crust in the corners of the eyes or mouth, on the eyelids, or under the nose. It is formed by a combination of mucus

Mucus

In vertebrates, mucus is a slippery secretion produced by, and covering, mucous membranes. Mucous fluid is typically produced from mucous cells found in mucous glands. Mucous cells secrete products that are rich in glycoproteins and water. Mucous fluid may also originate from mixed glands, which...

 (in the case of the eyes, consisting of mucin

Mucin

Mucins are a family of high molecular weight, heavily glycosylated proteins produced by epithelial tissues in most metazoans. Mucins' key characteristic is their ability to form gels; therefore they are a key component in most gel-like secretions, serving functions from lubrication to cell...

 discharged from the cornea

Cornea

The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Together with the lens, the cornea refracts light, with the cornea accounting for approximately two-thirds of the eye's total optical power. In humans, the refractive power of the cornea is...

 or conjunctiva

Conjunctiva

The conjunctiva covers the sclera and lines the inside of the eyelids. It is composed of rare stratified columnar epithelium.-Function:...

), nasal mucus

Dried nasal mucus

Dried nasal mucus, pieces of which are colloquially known as bogeys in English and boogers in American, is commonly found in the nose and is a result of drying of the normally viscous colloidal mucus .-Formation:...

, blood cell

Blood cell

A blood cell, also called a hematocyte, is a cell normally found in blood. In mammals, these fall into three general categories:* red blood cells — Erythrocytes* white blood cells — Leukocytes* platelets — Thrombocytes...

s, skin cell

Skin

-Dermis:The dermis is the layer of skin beneath the epidermis that consists of connective tissue and cushions the body from stress and strain. The dermis is tightly connected to the epidermis by a basement membrane. It also harbors many Mechanoreceptors that provide the sense of touch and heat...

s, or dust. Rheum from the eyes is particularly common, and is called gound (ˈ; from Old English gund, gound) or, in common usage: sand, eye gunk, sleepydust, sleepysand, sleepyseed, sleep, sleepys, eye goop, or eye boogers.

In the waking individual, blinking of the eyelid causes gound to be washed away with tears via the nasolacrimal duct

Nasolacrimal duct

The nasolacrimal duct carries tears from the lacrimal sac into the nasal cavity. Excess tears flow through nasolacrimal duct which drains into the inferior nasal meatus...

. The absence of this action during sleep, however, results in a small amount of dry gound accumulating in corners of the eye, most notably in children.

A number of conditions can cause an increase in the production of gound in the eye. In the case of allergic conjunctivitis

Allergic conjunctivitis

Allergic conjunctivitis is inflammation of the conjunctiva due to allergy. Although allergens differ between patients, the most common cause is hay fever. Symptoms consist of redness , oedema of the conjunctiva, itching and increased lacrimation...

, the buildup of rheum can be considerable, many times preventing the sufferer opening the eye upon waking without prior cleansing of the eye area. The presence of pus

Pus

Pus is a viscous exudate, typically whitish-yellow, yellow, or yellow-brown, formed at the site of inflammatory during infection. An accumulation of pus in an enclosed tissue space is known as an abscess, whereas a visible collection of pus within or beneath the epidermis is known as a pustule or...

 in an instance of heavy rheum buildup can indicate dry eye or conjunctivitis

Conjunctivitis

Conjunctivitis refers to inflammation of the conjunctiva...

, among other infections.

Occasionally, in infants in their first year, the tear ducts fail to open. This causes epiphora

Epiphora (medical)

Epiphora is overflow of tears onto the face. A clinical sign or condition that constitutes insufficient tear film drainage from the eyes in that tears will drain down the face rather than through the nasolacrimal system.-Etiology:...

, that is, with non-patent nasolacrimal ducts, tears have no egress from the body and rheum is released onto surrounding skin.