Good artificial light is much to be preferred to insufficient sunlight.

The artificial light should be sufficiently bright and steady; a fickering light is always bad. Riding against a strong wind, especially on a bicycle, may prove hurtful, at least for eyes that are inclined to any kind of inflammation. The light reflected from snow is a common source of injury to the eyes. It is a wise caution in pa.s.sing from a dark room to avoid looking immediately at the sun, an incandescent light, the glistening snow, or other bright objects.

The eyes should never be rubbed, or the fingers thrust into them,[46] and much less when they are irritated by any foreign substance. The sooner the offending substance is removed the better.

[Ill.u.s.tration: Fig. 137.--Lacrymal Ca.n.a.ls, Lacrymal Sac, and Nasal ducts, opened by their Anterior Portion.]

340. Effect of Alcohol upon the Eye. The earlier and slighter forms of injury done to the eye by the use of intoxicants are quite familiar: the watery condition of the eye and of the lids, and the red and bleared aspect of the organ. Both are the result of chronic inflammation, which crowds the blood into the vessels of the cornea, making them bloodshot and visible. The nerves controlling the circulation of the eye are partially paralyzed, and thus the relaxed vessels become distended.

But more serious results ensue. Long use of intoxicants produces diseases of the retina, involving in many cases marked diminution of acuteness as well as quickness of vision, and at times distorted images upon the surface of the retina. In other instances, the congestion of the optic nerve is so serious as to involve a progressive wasting of that organ, producing at first a hazy dimness of vision which gradually becomes worse and worse, till total blindness may ensue.

It is beyond question that a wide comparison of cases by careful observers proves that a large fraction of those who indulge in strong drink suffer from some form of disease of the eye.

341. Effect of Tobacco upon Vision. Tobacco, in its distribution of evil effects, does not neglect the senses and especially the eye. A variety of vicious results is produced. The pungent smoke inflames the lids. The narcotic dilates the pupil, causing dimness and confusion of vision. A diseased condition occurs with severe pain in the eye followed by impaired vision.

Oculists speak impressively of the ill effects of tobacco, and especially of cigarettes, upon the eyes of the young. They mention a well-known disease, tobacco blindness, usually beginning with color-blindness, and progressing occasionally with increasing dimness of vision to entire loss of sight.[47]

342. The Sense of Hearing. The structure of the human ear is much more complicated than is generally supposed. It is an apparatus constructed to respond to the waves of sound. As a whole, it may be considered a peculiar form of nerve-ending.

The external ear forms only a part of a most elaborate apparatus whereby sound waves may be transmitted inwards to the real organ of hearing. The really sensitive part of the ear, in which the auditory nerve ends, is buried for protection deep out of sight in the bones of the head; so deep that sounds cannot directly affect it. Some arrangement, therefore, is required for conducting the sounds inwards to this true organ.

[Ill.u.s.tration: Fig. 138.--The Pinna, or Auricle.]

In studying the structure of the ear, and how it is fitted to respond to sonorous vibrations, we may divide it into three parts: the sound-conducting part, known as the external ear, the middle ear, and the deeply placed nerve portion, the inner ear.

343. The External Ear. The external ear consists of an expanded portion known as the pinna or _auricle_, and of a pa.s.sage, the auditory ca.n.a.l or _meatus_, leading inwards from it. The surface of the auricle is convoluted to collect and transmit the vibrations of air by which sound is produced the auditory ca.n.a.l conducts these vibrations to the tympanic membrane. Many animals move the auricle in the direction of the sound. Thus the horse p.r.i.c.ks up its ears when it hears a noise, the better to judge of the direction of sounds.[48]

The external auditory meatus, the pa.s.sage to the middle ear, is curved and is about an inch and a quarter long. Near its outer portion are a number of fine hairs slanting outwards to prevent the entrance of insects.

Embedded in the deeper parts of the ca.n.a.l are glands which secrete the _cerumen_, or ear-wax, which keeps the ca.n.a.l moist, and helps to protect it against foreign bodies and insects. As the result of a cold, this wax may collect in sufficient quant.i.ties to block the pa.s.sage, and to diminish to a considerable extent the power of hearing.

344. The Middle Ear. At the inner end of the outer ear pa.s.sage is the tympanum, known as "the drum of the ear." It is a thin, oval membrane, stretched at an angle across the deep end of the pa.s.sage, which it completely closes. The tympanum is thus a part.i.tion between the pa.s.sage of the outer ear and the cavity of the middle ear. On its inner side is a small air chamber in the petrous portion of the temporal bone, called the cavity of the tympanum. Its bony walls are lined with mucous membrane similar to that lining the nose, mouth, and throat. On the inner wall of the tympanum are two openings, the round window, or _foramen rotundum_, and the oval window, or _foramen ovale_.

The tympanic cavity communicates with the back part of the throat, by the Eustachian tube. This tube is about one and a half inches long and lined with mucous membrane similar to that of the tympanic chamber and the throat. This pa.s.sage is usually closed, but is opened in the act of swallowing. In health there is no communication between the chamber of the middle ear and the outside, except by the Eustachian tube. Thus a throat cold, with redness and swelling of the mucous membrane, is usually accompanied with some degree of deafness, because the swelling may block the lumen of the tube, and thus prevent the free pa.s.sage of air to and fro.

[Ill.u.s.tration: Fig. 139.--General View of the Organ of Hearing.

A, pinna; B, cavity of the concha, showing the orifices of a great number of sebaceous glands; C, external auditory meatus; D, membrana tympani; F, incus; H, malleus; K, handle of malleus applied to the internal surface of the membrana tympani; L, tensor tympani muscle; between M and K is the tympanic cavity; N, Eustachian tube; O, P, semicircular ca.n.a.ls; R, internal auditory ca.n.a.l; S, large nerve given off from the facial ganglion; T, facial and auditory nerves.

A most curious feature of the ear is the chain of tiny movable bones which stretch across the cavity of the middle ear. They connect the tympanic membrane with the labyrinth, and serve to convey the vibrations communicated to the membrane across the cavity of the tympanum to the internal ear. These bones are three in number, and from their shape are called the malleus, or _hammer_, incus, or _anvil_; and stapes, or _stirrup_.

The hammer is attached by its long handle to the inner surface of the drum of the ear. The round head is connected with the anvil by a movable joint, while the long projection of the anvil is similarly connected with the stirrup bone. The plate of the stirrup is fixed by a membrane into the oval window of the inner wall of the tympanic chamber.

These little bones are connected with each other and the tympanum by ligaments and moved by three tiny muscles. Two are attached to the hammer, and tighten and relax the drum; the other is attached to the stirrup, and prevents it from being pushed too deeply into the oval window.

[Ill.u.s.tration: Fig. 140.--Ear-Bones. (Anterior View.)

1, malleus, or hammer; 2, incus, or anvil; 3, stapes, or stirrup.

345. The Internal Ear. This forms one of the most delicate and complex pieces of mechanism in the whole body. It is that portion of the organ which receives the impression of sound, and carries it directly to the seat of consciousness in the brain. We are then able to say that we hear.

The internal ear, or bony labyrinth, consists of three distinct parts, or variously shaped chambers, hollowed out in the temporal bone,--the vestibule, the semicircular ca.n.a.ls, and the cochlea, or snail's sh.e.l.l.

[Ill.u.s.tration: Fig. 141.--A Cast of the External Auditory Ca.n.a.l.

(Posterior view)]

The vestibule is the common cavity with which all the other portions of the labyrinth connect. It is an oval-shaped chamber, about ? of an inch in diameter, occupying the middle part of the internal ear. It is on the inner side of the oval window, which was closed, as we have seen, by the stirrup bone. From one side of this vestibule, or central hall, the three semicircular ca.n.a.ls pa.s.s off, and from the other side, the cochlea.

The three semicircular ca.n.a.ls, so called from their shape, are simply bony tubes about 1/20 of an inch in width, making a curve of about 1/4 of an inch in diameter. They pa.s.s out from the vestibule, and after bending around somewhat like a hoop, they return again to the vestibule.

Each bony ca.n.a.l contains within it a membranous ca.n.a.l, at the end of which it is dilated to form an _ampulla_.

Experiment 157. _To vibrate the tympanic membrane and the little ear-bones._ Shut the mouth, and pinch the nose tightly. Try to force air through the nose. The air dilates the Eustachian tube, and is forced into the ear-drum. The distinct crackle, or clicking sound, is due to the movement of the ear-bones and the tympanic membrane.

The cochlea, or snail's sh.e.l.l, is another chamber hollowed out in the solid bone. It is coiled on itself somewhat like a snail's sh.e.l.l. There is a central pillar, around which winds a long spiral ca.n.a.l. One pa.s.sage from the cochlea opens directly into the vestibule; the other leads to the chamber of the middle ear, and is separated from it by the little round window already described.

The cochlea contains thousands of the most minute cords, known as the fibers or _organ of Corti_.[49] Under the microscope they present the appearance of the keyboard of a piano. These fibers appear to vibrate in sympathy with the countless shades of sounds which daily penetrate the ear. From the hair-like processes on these tightly stretched fibers, auditory impulses appear to be transmitted to the brain.

The tubes and chambers of the inner ear enclose and protect a delicate membranous sac of exactly the same shape as themselves. Between the bony walls of the pa.s.sages and the membranous bag inside is a thin, clear fluid, the _perilymph_. The membranous bag itself contains a similar fluid, the _endolymph_. In this fluid are found some minute crystals of lime like tiny particles of sand, called _otoliths_, or ear-stones. Every movement of the fluid itself throws these grains from side to side.

[Ill.u.s.tration: Fig. 142.--Bony internal Ear of Right Side. (Magnified; the upper figure of the natural size.)

A, oval window (foramen ovale); B, C, D, semicircular ca.n.a.ls; * represents the bulging part (ampulla) of each ca.n.a.l; E, F, G cochlea, H, round window (foramen rotundum).

The auditory nerve, or nerve of hearing, pa.s.ses to the inner ear, through a pa.s.sage in the solid bone of the skull. Its minute filaments spread at last over the inner walls of the membranous labyrinth in two branches,--one going to the vestibule and the ampullae at the ends of the semicircular ca.n.a.ls, the other leading to the cochlea.

346. Mechanism of Hearing. Waves of sound reach the ear, and are directed by the concha to the external pa.s.sage, at the end of which they reach the tympanic membrane. When the sound-waves beat upon this thin membrane, it is thrown into vibration, reproducing in its movements the character of the air-vibrations that have fallen upon it.

Now the vibrations of the tympanic membrane are pa.s.sed along the chain of bones attached to its inner surface and reach the stirrup bone. The stirrup now performs a to-and-fro movement at the oval window, pa.s.sing the auditory impulse inwards to the internal ear.

Every time the stirrup bone is pushed in and drawn out of the oval window, the watery fluid (the perilymph) in the vestibule and inner ear is set in motion more or less violently, according to the intensity of the sound. The membranous labyrinth occupies the central portion of the vestibule and the pa.s.sages leading from it. When, therefore, the perilymph is shaken it communicates the impulse to the fluid (endolymph) contained in the inner membranous bag. The endolymph and the tiny grains of ear-sand now perform their part in this marvelous and complex mechanism. They are driven against the sides of the membranous bag, and so strike the ends of the nerves of hearing, which transmit the auditory impulses to the seat of sensation in the brain.

It is in the seat of sensation in the brain called the _sensorium_ that the various auditory impulses received from different parts of the inner ear are fused into one, and interpreted as sounds. It is the extent of the vibrations that determines the loudness of the sound; the number of them that determines the pitch.

Experiment 158. Hold a ticking watch between the teeth, or touch the upper incisors with a vibrating tuning-fork; close both ears, and observe that the ticking or vibration is heard louder. Unstop one ear, and observe that the ticking or vibration is heard loudest in the stopped ear.

Experiment 159. Hold a vibrating tuning-fork on the incisor teeth until you cannot hear it sounding. Close one or both ears, and you will hear it.

Experiment 160. Listen to a ticking watch or a tuning-fork kept vibrating electrically. Close the mouth and nostrils, and take either a deep inspiration or deep expiration, so as to alter the tension of the air in the tympanum; in both cases the sound is diminished.

Experiment 161. With a blindfolded person test his sense of the direction of sound, _e.g._, by clicking two coins together. It is very imperfect. Let a person press both auricles against the side of the head, and hold both hands vertically in front of each meatus. On a person making a sound in front, the observed person will refer it to a position behind him.

347. Practical Hints on the Care of the Ear. This very delicate and complicated organ is often neglected when skilled treatment is urgently needed, and it is often ignorantly and carelessly tampered with when it should be let alone.