The coating consists of three layers. The outer layer is formed by the porcelain-white and firmly fixed hard skin (sclerotic) and is a protective covering for the inner parts of the eyeball. A part of its anterior is known as the 'White of the eye'. <Callout type="important" title="Important">The sclerotic layer must be kept clean to prevent infections.</Callout> The middle layer is the choroid, a tender tissue, some parts of which are coloured black on its inner surface, in which the blood vessels running to the eyeball branch out. The inner layer is the retina, a fine very soft network of the filaments of the optic nerve. On the anterior surface of the sclerotic is a circular slightly convex transparent division, the cornea through which the light falls into the interior of the eye, as through a window. <Callout type="tip" title="Tip">Regularly clean your eyes to maintain clear vision.</Callout> The part of the choroid lying behind does not touch the cornea but is stretched like a curtain between the space formed by its convexity — the anterior chamber — and the interior of the eye. This part of the choroid is called the iris because it is differently coloured in different persons. According to its colour we speak of gray, blue, brown or black eyes. The iris has in its centre a round aperture, the pupil of the eye, which appears as the 'black in the eye'. <Callout type="risk" title="Risk">Prolonged exposure to bright light can damage your retina.</Callout> According as the pupil dilates or contracts, it allows more or less light to fall into the interior of the eye. The iris therefore represents a contrivance, which is able to soften or to lessen a too strong light by contraction of the pupil. Behind the pupil, immediately in front of the vitreous body lies the crystalline lens, a body formed of transparent, strong tissue, which is curved in front and behind like a magnifying-glass. The crystalline lens unites the rays of light, passing through the cornea and the pupil on the background of the eye into an image, which is received (retained) by the retina. By the ordinary convexity of the lens only rays of light falling parallel into the eye are combined on the back ground of the eye, while rays entering obliquely only unite behind the back of the eye. As only rays coming from an immensely great distance strike the eye in parallel lines, the lens possesses the faculty of increasing its convexity by muscular activity so as to focus also the rays coming obliquely from a near distance on the background of the eye. <Callout type="gear" title="Gear">Wearing protective goggles can prevent damage to your eyes from bright light or debris.</Callout> There are however eyes of such a small length diameter, that the lens must increase its convexity in order to focus even parallel rays on the retina, and cannot focus oblique rays at all on the back of the eye, so that the image, produced on the retina is rendered indistinct. We call such eyes oversighted. Their power of vision can be improved by an artificial lens, which supplements the action of the crystalline lens in the shape of a double-convex eyeglass placed before the eye. Other eyes, which are constructed with such a long diameter that the focussing of the parallel rays takes place already before they reach the back of the eye are only able to perceive clearly near images, as rays coming from near objects strike the eye obliquely and are therefore focussed at a greater distance from the lens than parallel rays. We call such eyes near-sighted, and their visual capacity is improved by the use of eyeglasses, which are grounded double concave and which scatter the rays of light before they can reach the eye. With higher age the power of the lens of focussing itself for rays coming from near gradually decreases. The near-point of sight i.e. the smallest distance at which the eye can see an object plainly, recedes more and more; the eye can — relatively speaking — only perceive clearly very distant objects; it becomes longsighted. In popular 'parlance' this term is also used — not quite appropriately — for oversighted eyes. A grayish dimness of the lens, arising from injury to the eye or from sickness and particularly from very old age diminishes or destroys the visual power of the eye and is called cataract. By a removal — through an operation — of the lens, which has become opacous (opaque) persons attacked by star can again recover their eye-sight; but they must afterwards constantly wear highly convex spectacles instead of the removed lens. The eyeballs can be moved in several directions by the muscles, lying with them in the cavity of the eyes and therefore can be directed towards different objects in quick succession. A wider vision around is made possible by the movements of the head. As soon as both eyes at the same moment are directed towards a near object, they perceive it from different sides, and thus the form of the object perceived can easier be imaged. When looking straight ahead the muscles of the eyes are in a state of counteraction; i.e. the action of the muscles on the inner-side of the eyeball is counterbalanced by the action by those acting from outside. A disturbance of this counterpoise, which may be due to various causes produces squinting. If for instance the outer muscle of the eye is in a state of weakness, or if the inner muscle is shortened, the direction of the affected eye will be more inside, which causes squinting inward. By certain protective arrangements the eyes are preserved from external injuries. The eyelids especially protect the eyeballs from the intrusion of foreign bodies (for instance insects) and prevent by means of the thin hairs (hanging) on their edges (eyelashes) dust or other foreign bodies getting into the eye. The surface of the lids, next to the eye is covered by a membrane, the so called conjunctiva, which is continued over the anterior surface of the eyeball. <Callout type="warning" title="Warning">Avoid rubbing your eyes as it can introduce bacteria and cause infections.</Callout> The lachrymal fluid serves for the removal of particles of dust, which notwithstanding the protection of the eyelids and eyelashes have penetrated into the space between the lids and the eyeball, called the conjunctiva sack. iy Wh Fig. 18. Section of the Fur. #2, Outer auditory canal, if. 'Tympanuin, ph. Tympanic cavity. of, Eustachian Tube. i. Mal~ 8. a. Incus. », Stapes. bg. Semi-circular Canals, vh, Vestibule. sp. Cochlea. a, Temporal bone. The fluid is secreted by the lachrymal glands, which are also lying in the cavities of the eyes and gets into the conjunctiva sack whence it ordinarily flows into through the lachrymal duct into the cavity of the nose. In weeping an increased secretion of the lachrymal fluid takes place. Also when the conjunctiva becomes red from inflammation of the eye or when it swells up, or discharges freely 'pus' and matter, a 'running of the eyes' takes place, as the thin apertures, which lead to the lachrymal duct become more or less impassable, by which the flowing off of the lachrymal fluid into the nose is prevented.
Key Takeaways
- The sclerotic layer must be kept clean to prevent infections.
- Regularly cleaning your eyes can maintain clear vision.
- Prolonged exposure to bright light can damage the retina.
- Wearing protective goggles can prevent eye damage from bright light or debris.
Practical Tips
- Regularly clean your eyes using a sterile solution to avoid infections and maintain clear vision.
- Wear protective eyewear when working in environments with potential hazards, such as dust or bright sunlight.
- Avoid rubbing your eyes to reduce the risk of introducing bacteria and causing infections.
Warnings & Risks
- Prolonged exposure to bright light can damage the retina and cause permanent vision loss.
- Rubbing your eyes can introduce bacteria and lead to eye infections, especially in dusty or dirty environments.
- Injuries to the eye from foreign objects can result in serious vision impairment if not treated promptly.
Modern Application
Understanding the structure and function of the eye is crucial for survival in various scenarios. While modern technology has improved many aspects of ocular health, such as corrective lenses and surgical techniques, the basic principles outlined in this chapter remain relevant. Proper hygiene practices, protective gear, and awareness of potential hazards can help prevent injuries and maintain vision during emergencies.
Frequently Asked Questions
Q: What is the function of the iris in the eye?
The iris controls the amount of light entering the eye by adjusting the size of the pupil. It contracts or dilates to allow more or less light, protecting the retina from damage.
Q: How can I prevent infections in my eyes during survival situations?
Regularly clean your eyes using a sterile solution and avoid rubbing them to reduce the risk of introducing bacteria. Keep your hands clean and use protective eyewear when necessary.
Q: What are some signs that my vision might be deteriorating?
If you notice difficulty seeing objects clearly, especially at close range, or if your vision becomes blurry, these could be signs of vision deterioration. Regular eye exams can help detect issues early.