TEARS, a name for the limpid fluid secreted by the lachrymal glands, and flowing on the surface of the eye ; either in consequence of local irritation, or the emotions of grief. Some part of this aque ous fluid is dissipated in the air ; but the greatest part, after having performed its office, is propelled by the orbicular mus cle, which so closely constringes the eye. lid to the ball of the eye as to leave no space between, unless in the internal an gle, where the tears are collected.
From this collection the tears are ab sorbed by the orifices of the punctx tacit rymalim ; from thence they are propelled 'through the lachrymal canals into the la chrymal sac, and flow through the ductus nasalis into the cavity of the nostrils, un der the inferior concha nasalis. The tears have no smell, but a saltish taste. The uses of the tears are these : 1. They continually moisten the surface of the eye and eye-lids, to prevent the transparent cornea from drying and becoming opaque, or the eye from concreting with the eye lids. 2. They prevent that pain which would otherwise arise from the friction of the eye-lids against the ball of the eye from continually winking. 3. They wash away dust, or any thing acrid, that may ' have fallen into the eye. This liquid is transparent and colourless, has no per ceptible smell, but a saline taste. It com
municates to vegetable blues a perma nent green colour. When it is evaporat ed nearly to dryness, cubic crystals are formed, which are muriate of soda. So da is in excess, because vegetable blues are converted by it to a green colour. A portion of mucilaginous matter, which be comes yellow as it dries, remains after the evaporation. This liquid is soluble in water, and in alkalies. Alcohol pro duces a white flaky precipitate, and when it is evaporated, muriate of soda and so. da remain behind. By burning the resi• duum, some traces of phosphate of lime and of soda are detected. The compo nent parts of tears are, therefore, Water, Muriate of soda, Mucilage, Phosphate of limes Soda, Phosphate of soda.
The mucilage of tears absorbs oxygen from the atmosphere, and becomes thick, viscid, and of a yellow colour. It is then insoluble in water. Oxymuriatic acid produces a similar effect. It is converted into muriatic acid, so that it has been de prived of its oxygen. The mucus of the nose consists of the same substances as the tears ; but being more exposed to the air, it. acquires a greater degree of vis cidity from the mucilage absorbing oxy gen.