882. The s.p.a.ce that intervenes between the superior and middle spongy bone, is called the _superior me-atus_, or channel; the s.p.a.ce between the middle and inferior bone, is the _middle meatus_; and that between the inferior bone and the floor of the fossa, is the _inferior meatus_.

[Ill.u.s.tration: Fig. 135. A vertical section of the middle part of the nasal cavities. 7, The middle spongy bones. 8, The superior part of the nasal cavities. 10, The inferior spongy bones. 11, The vomer. 12, The upper jaw. 13. The middle channel of the nose. 14, The lower channel of the nose. 17, The palatine process of the upper jaw-bone. 18, The roof of the mouth covered by mucous membrane. 19, A section of this membrane.]

883. The MEATUSES are pa.s.sages that extend backward, from the nostrils, into which are several openings. They are lined by a mucous membrane, called the _pi-tui-ta-ry_, or _schneiderian_, from Schneider, who first showed that the secretion of the nasal foss proceeded from the mucous membrane, and not from the brain.

882. What terms are applied to the s.p.a.ces between these processes?

What does fig. 135 represent? 883. Define the meatuses. By what are they lined?

884. Upon the mucous membrane of the nasal pa.s.sages, the olfactory nerve ramifies, and also a branch of the fifth pair of nerves. This membrane is of considerable extent in man; and in those animals whose sense of smell is very acute, it is still more extensive.

[Ill.u.s.tration: Fig. 136. A side view of the pa.s.sage of the nostrils, and the distribution of the olfactory nerve. 4, The olfactory nerve. 5, The fine and curious divisions of this nerve on the membrane of the nose. 6, A branch of the fifth pair of nerves.]

PHYSIOLOGY OF THE ORGANS OF SMELL.

885. The sense of smell enables us to discern the odor or scent of any thing. When substances are presented to the nose, the air that is pa.s.sing through the nostrils brings the odoriferous particles of matter in contact with the filaments of the olfactory nerves, that are spread upon the membrane that lines the air-pa.s.sages, and the impression is then transmitted to the brain.

884. What nerves ramify upon this membrane? What is represented by fig. 136? 885-899. _Give the physiology of the organs of smell._ 885.

How does the mind become sensible of odoriferous particles?

886. This sense, with that of taste, aids man as well as the inferior animals, in selecting proper food, and it also gives us pleasure by the inhalation of agreeable odors. The sense of smell, like that of taste and touch, may be improved by cultivation. It likewise varies in different persons.

_Observation._ Sometimes this sense seems to possess a morbid degree of acuteness in respect to odors, which is highly inconvenient and even dangerous. With some individuals, the smell of certain fruits, flowers, cheese, &c., produce nausea and even convulsions.

887. In the inferior animals generally, the sense of smell is more acute than in man. Thus the bloodhound will track the hare over the ground for miles, guided only by the odor that it leaves in its flight. He also traces the progress of his master through thickly-crowded streets, distinguishing his footsteps from those of a thousand others, and amidst the odorous particles emanating from a thousand sources.

_Observation._ In some of the higher orders of the inferior animals, there is an astonishing acuteness of smell in regard to effluvia that come from living animals. To these animals, it possesses an importance in them far beyond what it has in man, by making them acquainted with the presence of their enemies or their prey, when the eye and ear are incapable of acting. It is related by travellers in Africa, that they were always apprised of lions in their vicinity during the night, by the moans and tremblings of their horses.

888. Smell is somewhat under the control of the will. That is, we have the power of receiving or rejecting odors that are presented; 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.

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.

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 pa.s.sage of air through the nostrils, and thus obliges "snuff-takers" to open their mouths when they breathe.

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.