"Keep me as the apple of thine eye; hide me under the shadow of thy 969. Why is the pupil of the eye larger sometimes than at others? Because the iris, a ring of extremely fine muscles which surround the pupil, contracts when too much light falls upon the retina, and dilates when the light is feeble. It therefore enlarges or diminishes the size of the pupil to regulate the admission of light. Fig. 62.-SECTION OF THE EYE SEEN FROM BEHIND A. The pupil of the eye through which the light enters. B. The iris, which dilates or contracts, and thereby increases or lessens the size of the pupil. C C. The three coats of the eye, called the sclerotic, choroid, and retina. D. The ciliary processes, or hair-like muscles, which have a slight vibratory motion which they impart to the fluids of the eye. E. The dark coat of the choroid, the coat forming the retina removed. 970. Why have we two eyes? Because the field of vision is thereby much extended; the intensity of sight is also increased, the impressions upon the brain being clearer and better defined, just as in a stereoscope the effect of vision is heightened by a double picture; the sense of sight being more constantly exercised than any other sense during our waking moments, one eye is frequently called upon to give rest to the other; and the important faculty of vision, being endangered by the necessary exposure of some parts of the eye, and the equally 66 The eyes of the Lord are upon the righteous, and his ears are open unto their cry."-PSALM XXXIV. necessary delicacy of an organ formed to receive impressions from so ethereal an element as light, is rendered the more secure to us, since though one eye may become enfeebled, diseased, or wholly lost, the other eye will retain the blessing of sight. 971. Why, having two eyes, and each eye receiving a reflection upon its retina, does the brain experience only one impression of an object? Because, besides those optical laws which bring upon the two retinas the exactly corresponding images of the same objects, the optic nerves meet before they reach the brain, and blend the impulses which they convey. 972. Why are the eyes provided with eye-lids? Because the eyes require to be defended from floating particles in in the air, and to be kept moist and clean. The eye-lids form the shutters of the eye, defending it when waking, by closing upon its surface whenever danger is apprehended, moistening its surface when it becomes dry, and covering it securely during the hours of sleep. 973. Why are the eye-lids fringed with eye-lashes? Because the eye-lashes assist to modify the light, and to protect the eye, without actually closing the eye-lids. When the eye-lids are partially closed, as in very sunny or dusty weather, the eye-lashes cross each other, forming a kind of shady lattice-work, from the interspaces of which the eye looks out with advantage, and sees sufficiently for the guidance of the body. 974. Why are we able to see at long or short distances? Because the crystalline lens of the eye is a moveable body, and is pushed forward, or drawn back by fine muscular fibres, acccording to the distances of the objects upon which we look. By these means its focus becomes adjusted. 975. Why do we wink? Because, by the repeated action of winking, the eye is kept moist and clean, and the watery fluid secreted by little glands in the eye-lids, and at the sides of the eye, is spread equally over the surface, instead of being allowed to accumulate. But the action of "And the eye cannot say unto the hand, I have no need of thee; nor again the head to the feet, I have no need of you."-CORINTH. XII. winking, or brightening the eye, is so instantaneous that it does not impede the sight. 976. Whence are the humours and secretions of the eye derived? From the blood, which flows abundantly to the eyes, and is circulated in capillary vessels that are spread out upon the membranous coats of the eye-balls. A and B. The sclerotic, choroid, and retina, the three layers or coats which form the walls of the globe of the eye, and enclose its humours. C C. The iris. D. The front chamber of the eye, filled with watery humour. E. The pupil, through which the rays of light pass to F. The crystalline lens. G G. The vitreous humour enclosed in cells formed by the hyaloid membrane. H. Au artery which supplies blood to the crystalline lens, and which passes through the centre of the optic nerve. G. The optic nerve, showing the sheath in which the nerve is enclosed. 977. Why do tears form in the eyes? Because, under the emotions of the mind, the circulation of blood in the brain, and in its nearest branches, becomes considerably quickened. The eyes receive a larger amount of blood, and the secretion of the lachrymal glands being increased, the fluid overflows, and tears are formed. The use of tears is probably to keep the eyes cool during the excitement of the brain. They are formed also during laughing, but less frequently. If the whole body were an eye, where were hearing? if the whole were hearing where were smelling ?"-CORINTHIANS XII. 978. Why do we feel inconvenienced by sudden light? Because an excess of light enters the eye before the iris has had time to adjust the pupil to the amount of light to be received. 979. Why if we look upon a very bright light, and then turn away, are we unable to see? Because the iris has so reduced the pupil while we were looking at the bright light, that immediately upon turning to a darker object, the pupil is too small to admit sufficient rays to enable us to see. Fig. 64.-CAPILLARY BLOOD-VESSELS OF THE EYE. A A. Capillary veins distributed over the sclerotic coat. B. One of the trunks of the optic nerve. C. A nerve communicating with the ciliary processes. D. A vein running parallel with the nerve to the ciliary processes. 980. Why do we see better after a short time? Because the iris has relaxed and enlarged the pupil, therefore we receive more rays of light from the comparatively dark object, and are enabled to see it more clearly. 981. Why do cats, bats, owls, &c., see in the dark? Because their eyes are made highly sensitive to small quantities of light. It is also believed that there are certain properties of light which affect their eyes, but do not affect ours. In other words, "He that hath ears to hear, let him hear."-MATTHEW XI. www that there are some rays which are luminous to them which are not luminous to us. Hence they find light in what we call darkness. 982. Why does the pupil of a cat's eye appear nearly closed by day? Because the cat's eye is so sensitive to light that the iris closes the pupil almost entirely to shut out the too powerful light. CHAPTER XLIX. 983. Why do we hear? Because the tympanum of the ear receives impressions from sounds, and transmits those impressions to the brain in a similar manner to that in which the retina of the eye transmits the impressions made upon it by light. 984. Why is one part of the ear spread out externally ? The external ear is a natural ear-trumpet, and serves to collect the vibrations of sound, and to conduct them towards the internal ear. 985. Why is the ear allowed to project, whilst the eye carefully enclosed? is Because the external ear, being formed of tough cartilaginous substance, and being very simple in its organisation, is but little liable to injury. 986. Why do hairs grow across the entrance of the ears? Because they prevent the intrusion of insects, and of particles of dust, by which otherwise the faculty of hearing would be impaired. The insect called the ear-wig is popularly supposed to be so named from its tendency to get into the human ear, and cause pain and madness by penetrating to the brain. An earwig, however, is no more likely to get into the ear than any other insect whose habit it is to penetrate the corollas of flowers; and should an insect enter the ear, i could get no further than the membrane of the tympanum, which spreads all over the auditory passage, just as the parchment of a drum spreads over the entire circumference of that instrument. The fact is, that the wing of the insect, when spread, resembles the external ear in shape. It is similar to the wing of the stag beetle (see illustration), and this fancied resemblance of the wing of the insect to the ear of man may have given rise to the name of ear-wing, which became corrupted to ear-wig. |