mpression 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.

In its true sense, color-blindness is always congenital, often
hereditary. This condition of abnormal vision is totally incurable. A
person may be color-blind and not know it until the defect is accidentally
revealed. The common form of defective color-vision is the inability to
distinguish between _red_ and _green_. As green lights mean safety, and
red lights danger, on railroads, on shipboard, and elsewhere, it becomes
of paramount importance that no one who is color-blind should be employed
in such service. Various tests are now required by statute law in many
states to be used for the detection of such defects of vision among
employees in certain occupations.

338. School Life and the Eyesight. The eyes of children need more
care than thos