ted;
thus, if odors are agreeable, we inspire forcibly, to enjoy them; but,
if they are offensive, our inspirations are more cautious, or we close
our nostrils. This sense is likewise modified by habit; odors which,
in the first instance, were very offensive, may not only become
endurable, but even agreeable.

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886. What is the use of the sense of smell? Can this sense be
improved by cultivation? What is said respecting this sense in some
individuals? 887. What is said of this sense in the bloodhound?
Mention an instance of astonishing acuteness of smell in some of
the higher orders of animals. 888. Show that smell is somewhat under
the control of the will.

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889. Acuteness of smell requires that the brain and nerve of smell be
healthy, and that the membrane that lines the nose be thin and moist.
Any influence that diminishes the sensibility of the nerves, thickens
the membrane, or renders it dry, impairs this sense.

_Observations._ 1st. _Snuff_, when introduced into the nose, not only
diminishes the sensibility of the nervous filaments, but thickens the
lining membrane. This thickening of the membrane obstructs the passage
of air through the nostrils, and thus obliges "snuff-takers" to open
their mouths when they breathe.

2d. The mucous membrane of the nasal passages is the seat of chronic
catarrh. This affection is difficult of removal, as remedial agents
cannot easily be introduced into the windings of these passages. Snuff
and many other articles used for catarrh, produce more disease than
they remove.

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889. On what does acuteness of smell depend? What effect has snuff
when introduced into the nose? What is said of chronic catarrh?

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CHAPTER XLIV.

SENSE OF VISION.

890. This sense contributes more to the enjoyment and happiness of man
than any other of the senses. By it we perceive the form, color,
volume, and position of objects that surround us. The eye is the organ
of sight, or vision, and its mechanism is so wonderful, that it not
only proves the existence of a great First Cause, but perhaps, more
than other organs, the design of the Creator to mingle pleasure with
our existence.

ANATOMY OF THE ORGANS OF VISION.

891. The apparatus of vision consists of the _Op'tic Nerve_, the
_Globe_ and _Muscles_ of the eye, and its _Protecting Organs_.

892. The OPTIC NERVE arises by