Most vertebrate animals that live in air have a gland in connection with the surface of their eyes, which secretes a thin fluid, to moisten the conjunctiva. This fluid commonly passes from the eye into the nasal cavity, and supplies the inspired air with moisture.
The lachrymal fluid is clear and colorless, with a distinctly salty taste and alkaline reaction. It contains only about I per cent, of solids, in which can be detected some albumin, mucus, and fat (1 per cent.), epithelium (1 per cent.), as well as sodium chloride and other salts (.8 per cent.).
The secretion is produced continuously in small amount, but is subject to such considerable and sudden increase, that at times it cannot all escape by the nasal duct, but is accumulated in the eyes until it overflows to the cheek as tears. This excessive secretion may be induced by the application of stimuli to the conjunctiva, the lining membrane of the nose, or the skin of the face, or by strong stimulation of the retina, as when one looks at the sun. A similar increase of secretion follows certain emotional states consequent on grief or joy. These facts show that the secretion of the gland is under nervous control, the impulses stimulating secretion commonly starting either from the periphery, and passing along the sensory branches of the fifth or along the optic nerve, or from the emotional centres in the brain, and arriving at the gland in a reflex manner. The amount of secretion can also be augmented by direct stimulation of the lachrymal nerves, so that in all probability these are the efferent channels for the impulse.