he inferior channel of
the nose, where it terminates by an expanded orifice.

[Illustration: Fig. 141. 1, The lachrymal gland. 2, Ducts leading from
the lachrymal gland to the upper eyelid. 3, 3, The puncta lachrymalia. 4,
The nasal sac. 5, The termination of the nasal duct.]

916. The fluid (tears) secreted by the lachrymal gland, is conveyed to
the eye by the small ducts before described. It is then imbibed by the
puncta lachrymalia, and carried by the lachrymal canals into the
lachrymal sac, from which it is passed to the nasal cavities by the
nasal ducts.

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What are they called? With what do they communicate? 915. Describe the
nasal duct. 916. How are the tears conveyed from the lachrymal gland
to the nose?

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CHAPTER XLV.

PHYSIOLOGY OF THE ORGANS OF VISION.

917. To comprehend the theory of vision, it is not sufficient to know
the structure of the eye. We must be familiar with some of the
properties of a subtile fluid, which is constantly emanating from all
luminous bodies, called _light_.

918. It is the province of natural philosophy, rather than physiology,
to enter minutely upon the properties of light. It may be observed,
however, that, when light passes through any medium of the same
density, the rays are in straight lines; but, when it passes from one
medium into another of different density, it is refracted, or
turned from a straight course, unless it strikes the medium in a
perpendicular direction--then light passes through without a change
of direction.

919. When a ray of light meets with a body, it either passes through
it, or is reflected by it, or it may be absorbed. Again, in proportion
as the rays of light become distant from the body from which they
emanate, they diverge one from the other. In accordance with the laws
of optics, the rays of light, in passing through an optical instrument
like the eye, must cross each other, and thus produce an inverted
image of the object from which the rays proceed. With the general
view of the structure of the eye, we will now examine the use of each
part in the function of vision.

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917-933. _Give the physiology of the organs of vision._ 917. What is
necessary in order to understand the theory of vision? 918. When light
passes through a medium of the same density, in what direction will be
its rays? Of a different density? What exception? 919. When light
me