Tears

Tears (ˈtir) or lacrima are a clear liquid secreted by the lacrimal glands (tear ducts) in the eyes of all land mammals (except for goats and rabbits). Their functions include lubricating eyes (basal tears), removing irritants (reflex tears), and aiding the immune system. Tears also occur as a part of the pain response. Humans are the only mammals known to produce tears as part of an emotional response and tears have cultural significance among humans (see crying). Emotional tears may serve a biological function by excreting stress-inducing hormones that build up during times of emotional distress. Tears are made up of water, electrolytes, proteins, lipids, and mucins that form layers on the surface of eyes. The different types of tears—basal, reflex, and emotional—vary significantly in composition. Tears (ˈtir) or lacrima are a clear liquid secreted by the lacrimal glands (tear ducts) in the eyes of all land mammals (except for goats and rabbits). Their functions include lubricating eyes (basal tears), removing irritants (reflex tears), and aiding the immune system. Tears also occur as a part of the pain response. Humans are the only mammals known to produce tears as part of an emotional response and tears have cultural significance among humans (see crying). Emotional tears may serve a biological function by excreting stress-inducing hormones that build up during times of emotional distress. Tears are made up of water, electrolytes, proteins, lipids, and mucins that form layers on the surface of eyes. The different types of tears—basal, reflex, and emotional—vary significantly in composition. Tears are made up of three layers-- lipid, aqueous, and mucous. Tears are composed of water, salts, antibodies and lysozymes (antibacterial enzymes), though composition varies among different tear types. The composition of tears caused by emotion differs from that of tears as a reaction to irritations, such as onion fumes, dust or allergy. Emotional tears contain higher concentrations of stress hormones such as adrenocorticotropic hormone, and leucine enkephalin (a natural pain killer), which suggests that emotional tears play a biological role in balancing stress hormone levels. The lacrimal glands secrete lacrimal fluid, which flows through the main excretory ducts into the space between the eyeball and lids. When the eyes blink, the lacrimal fluid is spread across the surface of the eye. Lacrimal fluid gathers in the lacrimal lake which is found in the medial part of the eye. The lacrimal papilla is an elevation in the inner side of the eyelid, at the edge of the lacrimal lake. The lacrimal canaliculi open into the papilla. The opening of each canaliculus is the lacrimal punctum. From the punctum, tears will enter the lacrimal sac, then on to the nasolacrimal duct, and finally into the nasal cavity. An excess of tears, as caused by strong emotion, can cause the nose to run. Quality of vision is affected by the stability of the tear film. There are three very basic types of tears: basal, reflex, and psychic. The trigeminal V1 (fifth cranial) nerve bears the sensory pathway of the tear reflexes. When the trigeminal nerve is cut, tears from reflexes will stop while emotional tears will not. The lacrimal nerve from cranial nerve VII innervates the lacrimal gland, which is located within the orbit. It is responsible for the production of much of the aqueous portion of the tear film. In nearly all human cultures, crying is associated with tears trickling down the cheeks and accompanied by characteristic sobbing sounds. Emotional triggers are most often sadness and grief but crying can also be triggered by anger, happiness, fear, laughter or humor, frustration, remorse, or other strong, intense emotions. Crying is often associated with babies and children. Some cultures consider crying to be undignified and infantile, casting aspersions on those who cry publicly, except if it is due to the death of a close friend or relative. In most Western cultures, it is more socially acceptable for women and children to cry than men, reflecting masculine sex-role stereotypes. In some Latin regions, crying among men is more acceptable. There is evidence for an interpersonal function of crying as tears express a need for help and foster willingness to help in an observer. Some modern therapy movements such as Re-evaluation Counseling teach that crying is beneficial to health and mental well-being, encouraging it positively. An insincere display of grief or dishonest remorse is sometimes called crocodile tears in reference to an Ancient Greek anecdote that crocodiles would pretend to weep while luring or devouring their prey. In addition, in medical terms, someone is said to have crocodile tears syndrome as an uncommon consequence of recovery from Bell's palsy, in which faulty regeneration of the facial nerve causes sufferers to shed tears while eating. 'Crocodile tears syndrome', also known as Bogorad's syndrome, is an uncommon consequence of nerve regeneration subsequent to Bell's palsy or other damage to the facial nerve in which efferent fibres from the superior salivary nucleus become improperly connected to nerve axons projecting to the lacrimal glands, causing one to shed tears (lacrimate) during salivation while smelling foods or eating. It is presumed that this would cause salivation while crying due to the inverse improper connection of the lacrimal nucleus to the salivary glands, but this would be less noticeable. Keratoconjunctivitis sicca, known as dry eye, is a very common disorder of the tear film. However, sufferers can experience watering of the eyes, which is, in fact, a response to irritation caused by the original tear film deficiency. Lack of Meibomian gland secretion can mean the tears are not enveloped in a hydrophobic film coat, leading to tears spilling onto the face.