From this sac there passes a channel, the nasal duct, about one-half of an inch long, leading into the lower portion of the nostril.  The fluid which has flowed over the eye is drained off by these canals into the nose.  During sleep this secretion is much diminished.  When the eyes are open the quantity is sufficient to moisten the eyeball, the excess being carried into the nose so gradually that the attention is not attracted to it.

The lacrymal canals are at times blocked by inflammation of the nasal duct, and the fluid collects in the corners of the eyelids and overflows down the cheeks, producing much inconvenience.  The lining membrane of the eyelids through these canals is continuous with that of the nostrils.  Hence, when the lining membrane of the eye is red and swollen, as during a cold, the nasal passages are also irritated, and when the nasal membrane is inflamed, the irritation is apt to pass upwards and affect the eyelids.

336.  The Tears.  The lacrymal or tear gland is under the control of the nervous system.  Thus, if anything irritates the eyelids, the sensory nerves are stimulated and the impression is carried to the brain.  Thence the nerve impulses travel to the lacrymal glands, leading to an increased flow of their secretion.  The irritation of the sensory nerves in the nasal passages by smelling such substances as onions, or pungent salts, often causes a copious flow of tears.

[Illustration:  Fig. 135.—­Lacrymal Gland and Ducts.

  A, lachrymal gland, the size of a small almond lodged in a shallow
     depression in the bones of the orbit;
  B, lachrymal ducts (usually seven), which form a row of openings into
     the conjunctival fold.
]

Various mental emotions, as joy and grief, may produce similar results.  In these cases the glands secrete the fluid in such quantities that it cannot escape by the lacrymal canals, and the excess rolls over the cheeks as tears.  Excessive grief sometimes acts on the nerve centers in exactly the opposite manner, so that the activity of the glands is arrested and less fluid is secreted.  This explains why some people do not shed tears in times of deep grief.

  Experiment 155.  Gently turn the inner part of your lower eyelid
  down.  Look in a mirror, and the small lacrymal point, or opening into
  the nasal duct, may be observed.

337.  Color-blindness.  There is an abnormal condition of vision called color-blindness, in which the power of discrimination between different colors is impaired.  Experiment shows that ninety-six out of every one hundred men agree as to the identity or the difference of color, while the remaining four show a defective perception of color.

The first may be said to have normal vision; the second are called color-blind.  It is a curious fact that ten times more men than women are color-blind.