*The Lachrymal Apparatus.*—Seeing requires that the light penetrate to the retina. For this reason all the structures in front of the retina are transparent. One of these structures, the cornea, on account of its exposure to the air, is liable to become dry, like the skin, and to lose its transparency. To preserve the transparency of the cornea, and also to lubricate the eyelids and aid in the removal of foreign bodies, a secretion, called _tears_, is constantly supplied. [Fig. 163] Fig. 163—*Diagram of irrigating system of the eye.* After wetting the eyeball the tears may also moisten the air entering the lungs. The lachrymal, or tear, glands are situated at the upper and outer margins of the orbits. They have the general structure of the salivary glands and discharge their liquid by small ducts beneath the upper lids. From here the tears spread over the surfaces of the eyeballs and find their way in each eye to two small canals whose openings may be seen on the edges of the lids near the inner corner (Fig. 163). These canals unite to form the _nasal duct_, which conveys the tears to the nasal cavity on the same side of the nose. When by evaporation the eyeball becomes too dry, the lids close reflexively and spread a fresh layer of tears over the surface. Any excess is passed into the nostrils, where it aids in moistening the air entering the lungs. HYGIENE OF THE EYE *Defects in Focusing.*—The delicacy and complexity of the sense organs of sight render them liable to a number of imperfections, or defects, the most frequent and important being those of focusing. Such defects not only result in the imperfect vision of objects, but they throw an extra strain upon the nervous system and may render the process of seeing exceedingly painful. A normal eye is able, when relaxed, to focus light accurately from objects which are twenty feet or more away and to accommodate itself to objects as near as five inches. An eye is said to be _myopic_, or _short-sighted_, when it is unable to focus light waves from distant objects, but can only distinguish the objects which are near at hand. In such an eye the ball is too long for the converging power of the lenses, and the image is formed in front of the retina (_C_, Fig. 164). [Fig. 164] Fig. 164—*Diagrams illustrating long-sightedness and short-sightedness*, and method of remedying these defects by lenses. _A._ Normal eye. _B._ Long-sighted eye. _C._ Short-sighted eye.