The Normal Eye - Tear Film
The aqueous component of tears is largely produced and drained by the lacrimal gland and its associated structures. The lacrimal gland resides in the upper orbit of the eyelid and consists of a superior and inferior lobe.
Tears, which are produced in both parts of the lacrimal gland, are secreted into the upper fornix via ducts from the inferior lobe and thereby cover the whole eye. In addition to a moisturising function, the tears contain antibacterial agents such as lysozyme, lactoferrin and immunoglobulins. The rate of production is low in new-born infants and also shows a reduction in old age. Complications arising from lack of tear production (e.g. dry eye).
Sympathetic and parasympathetic systems contribute to innervation of the lacrimal gland. Normal tear production by the lacrimal gland is controlled by sympathetic innervation, which is intimately associated with the blood supply. It is generally thought that sympathetic stimulation regulates the gland's blood flow in response to a dry atmosphere or irritation. Parasympathetic innervation is derived from branches of the VII cranial nerve in the form of the lacrimal nerve. Excessive tear production (lacrimation) during weeping (a unique human trait) is under parasympathetic control under the direction of many centres of the brain, including the frontal cortex, thalamus and hypothalamus.
Tears drain by an active process along the lacrimal accessory structures consisting of superior and inferior openings (puncta), canaliculi and large ducts. Drainage is achieved by a pump mechanism, which requires closure of the eyelids thereby producing a suction effect in the ducts that in turn forces the tears into the puncti and down the caniculi. The tears are subsequently drained into the nasolacrimal ducts and into the nose.