This section is from the book "Health", by W. H. Coefield.
What have we two eyes for ? Why will not one eye do ? Well, it is astonishing what people having but one eye are able to do with it, but that is a matter of great practice. Having two eyes, we are able to judge distances, and we judge the distance of an object partly by our knowledge of its real size, and partly by an involuntary estimation of the angle between the optic axes, when both eyes are directed towards the object; and we are able also, by means of our two eyes, to see bodies stereoscopically; to "tell, for instance, whether their surface is concave or convex.
When I look at this bottle the image on the retina of my right eye includes more of the right hand side of the bottle, and that formed on the retina of my left eye includes more of the other side; so that between these images, which are different in the two eyes, I am able to understand that the bottle has a convex surface.
How is it that if the images are produced at the back of our eyes upside down, we see things as they are ? There have been a lot of ingenious theories to explain this; one was that it had something to do with the junction of the optic nerves; but it is the simplest possible thing in the world, a pure matter of experience. We are accustomed, from our earliest infancy, to regard images that are produced on the lower part of the retina as coming from objects high up, and we are accustomed to regard images produced on the upper part of the retina as coming from bodies low down; and so, when we have an upside down image produced upon the retina, we see it the right way up, because the lower part corresponds, and the upper part corresponds with the whole experience of our lives.
The eyeball is moved in its cavity or orbit by four muscles called the recti, or straight muscles, which are attached to the sclerotic coat, and, passing backwards, are fixed to the bones at the hinder part of the orbit; a superior one above and an inferior one below, an external one outside and an internal one inside. When the upper one contracts it makes the eye look up, when the lower one contracts it makes the eye look down, when the external one contracts it makes the eye look outwards, and when the internal one contracts it makes the eye look towards the nose. There are two others ; they start on the outer side of the eyeball, one above and one below. They are called the oblique muscles. One of these, the upper one (the superior oblique muscle) starts on the outer side, its tendon passing round a little notch in the bone, which forms a kind of pulley, and then it lies back along with the straight muscles. The inferior oblique muscle is attached below to the outer side of the eyeball, and by its other end to the floor of the orbit When either of these contracts it pulls the eye round, so that between these six muscles the eye can be turned in all directions, and they are supplied by the nerves named in the last lecture.
The eyelids are lined by a mucous membrane which we call the conjunctiva, which secretes fluid which continually moistens the front part of the eye. That mucous membrane is not continued over the transparent cornea, but only its epithelial lining. The eye is kept still moister by means of the secretion of a gland on the upper side of the eyeball, called the lachrymal gland or tear gland, which secretes watery fluid continually, keeping the front part of the eyeball moist. The excess of fluid is conveyed by some small ducts into a tube which leads into the nose, and which is a kind of drainage tube for the eye-socket. When that secretion is excessive, that drainage tube is not able to carry it all off, and it rolls out on to the surface of the face in the form of tears.
The colour of the eye, as we call it, as due to the colour of the iris. That colour is caused by the pigment in the iris and by the blood in the blood-vessels of the iris, and in that way we get all the varieties of black, grey, blue, brown, etc. that we see in people's eyes. Persons who have no pigment in their eyes and no pigment in their hair are called albinos, and they have pink eyes, the pink colour being due to the colour of the blood in the iris.
Such persons always have weak eyes, and can only bear a very small amount of light, and that shows you that one of the principal objects of the pigment in the iris and in the choroid membrane, but especially of the pigment in the iris, is to absorb light which is not required for the purposes of clear vision.
The organ of hearing consists of three parts-the external ear. the middle ear, and the internal ear, the two latter being contained in cavities in the hard part of the temporal bone. The external ear is more or less trumpet-shaped, and has a passage leading through a hole in the temporal bone to the middle ear. Across the end of this passage, separating the external ear from the middle ear, is stretched a membrane called the tympanic membrane.
The cavity of the middle ear, on the inner side of this membrane, is called the tympanum, or drum of the ear, and communicates with the pharynx by a tube called the Eustachian tube. On the inner side of this cavity, opposite to the tympanic membrane, are two openings in the bone closed by membranes which form separations between the middle and the internal ear. A chain of three small bones jointed together stretches between the tympanic membrane, to which the first of them is attached, and the membrane closing one of these two apertures on the other side of the tympanum, to which the third bone is attached. The internal ear or labyrinth is contained in complicated cavities in the bone, called the vestibule, the semicircular canals, and the cochlea (or shell, from its shape). Inside these cavities is a closed sac, prolongations from which extend through the winding passages of the bone. Outside of this sac, between it and the walls of the cavities, is a watery liquid called perilymph, and inside of the sac is a liquid called endolymph. On the inner wall of the sac, and, therefore, in the endolymph, there are in some parts fine hair-like bodies, and in others small calcareous particles, looking something like grains of sand, or little rod-like bodies placed side by side like the keys of a pianoforte (called fibres of Corti). The fibres of the seventh pair of cranial nerves-the auditory nerves- are distributed to the walls of the membranous labyrinth, as this complicated sac is called, and their extremities are in connection with the little bodies just mentioned.
When the waves of sound, caused by vibrations of the particles of the ear, being collected by the external air and directed along the passage leading towards the middle ear strike the tympanic membrane or drum, it vibrates, and its vibrations shake the little chain of bones that cross the cavity of the middle ear, and so shake the membrane closing the aperture on the other side of the tympanum to which the last bone of the chain is attached. At the same time the vibrations of the air in the tympanum cause the membrane covering the other aperture leading into the internal ear to vibrate.
The vibration of these two membranes sets the perilymph in motion, and its vibrations are transmitted to the walls of the sac or membranous labyrinth in the internal ear, and so the little hair-like bodies, calcareous particles, and fibres of Corti are shaken, and their movement irritates the extremities of the auditory nerves, by the fibres of which the stimuli produced are transmitted to the brain.
No doubt the different parts of the labyrinth have different duties to perform. One part, for instance, has to do with the quantity or loudness of sounds, another with the quality or tone; but what I want you specially to note is, that the extremities of the optic nerve are not themselves directly affected by the vibrations of the air, but that, as in the case of the other senses, the stimulus which comes from outside the body affects a structure belonging to the epithelial tissues.