Part 57 (2/2)

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

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

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 _Optic Nerve_, the _Globe_ and _Muscles_ of the eye, and its _Protecting Organs_.

892. The OPTIC NERVE arises by two roots from the central portion of the base of the brain. The two nerves approach each other, as they proceed forward, and some of the fibres of each cross to the nerve of the opposite side. They then diverge, and enter the globe of the eyes at their back part, where they expand, and form a soft, whitish membrane.

893. The GLOBE, or ball of the eye, is an optical instrument of the most perfect construction. The sides of the globes are composed of _Coats_, or membranes. The interior of the globe is filled with refracting _Humors_, or _medi-ums_.

890. Which sense contributes most to the enjoyment of man? What do we perceive by this sense? What is said of the mechanism of the eye?

891-916. _Give the anatomy of the organs of vision._ 891. Of what does the apparatus of vision consist? 892. Describe the optic nerve. 893.

Describe the globe of the eye.

894. The COATS are three in number: 1st. The _Scle-rotic_ and _Corne-a_. 2d. The _Choroid_, _Iris_, and _Cilia-ry processes_. 3d.

The _Reti-na_.

895 The HUMORS are also three in number: 1st. The _Aque-ous_, or watery. 2d. The _Crystal-line_, (lens.) 3d. The _Vitre-ous_, or gla.s.sy.

[Ill.u.s.tration: Fig. 137. The second pair of nerves. 1, 1, Globe of the eye: the one on the left is perfect, but that on the right has the sclerotic and choroid coats removed, to show the retina. 2, The crossing of the optic nerve. 5, The pons varolii. 6, The medulla oblongata. 7, 8, 9, 10, 11, 12, 13, The origin of several pairs of cranial nerves.]

896. The SCLEROTIC COAT is a dense, fibrous membrane and invests about four fifths of the globe of the eye. It gives form to this organ, and serves for the attachment of the muscles that move the eye in various directions. This coat, from the brilliancy of its whiteness, is known by the name of ”the white of the eye.” Anteriorly, the sclerotic coat presents a bevelled edge, which receives the cornea in the same way that a watch-gla.s.s is received by the groove in its case.

894. Name the coats of the eye. 895. Name the humors of the eye.

Explain fig. 137. 896. Describe the sclerotic coat.

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