domingo, 21 de agosto de 2011
Blepharitis is classified into two types:
Anterior blepharitis occurs at the outside front edge of the eyelid where the eyelashes are attached.
Posterior blepharitis affects the inner edge of the eyelid that comes in contact with the eyeball.
Individuals with blepharitis may experience a gritty or burning sensation in their eyes, excessive tearing, itching, red and swollen eyelids, dry eyes, or crusting of the eyelids. For some people, blepharitis causes only minor irritation and itching. However, it can lead to more severe signs and symptoms such as blurring of vision, missing or misdirected eyelashes, and inflammation of other eye tissue, particularly the cornea.
In many cases, good eyelid hygiene and a regular cleaning routine can control blepharitis. This includes frequent scalp and face washing, using warm compresses to soak the eyelids, and doing eyelid scrubs. In cases where a bacterial infection is the cause, various antibiotics and other medications may be prescribed along with eyelid hygiene.
Two types of Blepharitis:
-Anterior blepharitis is commonly caused by bacteria (staphylococcal blepharits) or dandruff of the scalp and eyebrows (seborrheic blepharitis). It may also occur due to a combination of factors, or less commonly may be the result of allergies or an infestation of the eyelashes.
-Posterior blepharitis can be caused by irregular oil production by the glands of the eyelids (meibomian blepharitis) which creates a favorable environment for bacterial growth. It can also develop as a result of other skin conditions such as acne rosacea and scalp dandruff.
Blepharitis can be diagnosed through a comprehensive eye examination. Testing, with special emphasis on evaluation of the eyelids and front surface of the eyeball, may include:
Patient history to determine any symptoms the patient is experiencing and the presence of any general health problems that may be contributing to the eye problem.
External examination of the eye, including lid structure, skin texture and eyelash appearance.
Evaluation of the lid margins, base of the eyelashes and meibomian gland openings using bright light and magnification.
Evaluation of the quantity and quality of tears for any abnormalities.
A differentiation among the various types of blepharitis can often be made based on the appearance of the eyelid margins:
Staphyloccal blepharitis patients frequently exhibit mild sticking together of the lids, thickened lid margins, and missing and misdirected eyelashes.
Seborrheic blepharitis appears as greasy flakes or scales around the base of eyelashes and a mild redness of the eyelids.
Ulcerative blepharitis is characterized by matted, hard crusts around the eyelashes that when removed, leave small sores that ooze and bleed. There may also be a loss of eyelashes, distortion of the front edges of the eyelids and chronic tearing. In severe cases, the cornea, the transparent front covering of the eyeball, may also become inflamed.
Meibomian blepharitis is evident by blockage of the oil glands in the eyelids, poor quality of tears, and redness of the lining of the eyelids.
Using the information obtained from testing, your optometrist can determine if you have blepharitis and advise you on treatment options.
By educating yourself about the symptoms and risk factors for Acanthamoeba keratitis, you can help protect yourself from this potentially sight-threatening infection. The best defense against Acanthamoeba keratitis infection is proper contact lens hygiene.
-A red, (frequently) painful eye infection—especially if the discomfort does not improve with treatment.
-Foreign body sensation, tearing, light sensitivity, and blurred vision.
-Red, irritated eyes lasting for an unusually long period of time after removal of contact lenses.
-Use of tap water in cleaning and disinfecting contact lenses—including the lens case.
-Swimming with contact lenses in the eyes, especially in fresh water lakes and rivers. Acanthamoeba keratitis has also been isolated from virtually all water sources—from pools to hot tubs to showers.
What you should do:
-Always wash hands before handling contact lenses.
-Rub and rinse the surface of the contact lens before storing.
-Use only sterile products recommended by your optometrist to clean and disinfect your lenses. -Saline solution and rewetting drops are not designed to disinfect lenses.
-Avoid using tap water to wash or store contact lenses.
-Contact lens solution must be discarded upon opening the case, and fresh solution used each time the lens is placed in the case.
-Replace lenses using your doctor’s prescribed schedule.
-Do not sleep in contact lenses unless prescribed by your doctor and never after swimming.
-Never swap lenses with someone else.
-Never put contact lenses in your mouth.
-See your optometrist regularly for contact lens evaluation.
-If you experience RSVP (redness, secretions, visual blurring or pain), return to your optometrist immediately!
Anyone with noncorrectable reduced vision is visually impaired, and can have a wide range of problems.
The World Health Organization uses the following classifications of visual impairment. When the vision in the better eye with best possible glasses correction is:
20/30 to 20/60 is considered mild vision loss, or near-normal vision
20/70 to 20/160 is considered moderate visual impairment, or moderate low vision
20/200 to 20/400 is considered severe visual impairment, or severe low vision
20/500 to 20/1,000 is considered profound visual impairment, or profound low vision
less than 20/1,000 is considered near-total visual impairment, or near total blindness
no light perception is considered total visual impairment, or total blindness.
There are also levels of visual impairment based on visual field loss (loss of peripheral vision).
In the United States, any person with vision that cannot be corrected to better than 20/200 in the best eye, or who has 20 degrees or less of visual field remaining, is considered legally blind.
Visual impairments take many forms and exist in varying degrees. It is important to understand that visual acuity alone is not a good predictor of the degree of problems a person may have. Someone with relatively good acuity (e.g., 20/40) can have difficulty functioning, while someone with worse acuity (e.g., 20/200) might not be having any real problems.
They are not able to enjoy everyday moments or special events. Eye problems can also be very costly to individuals. The need for eye wear, eye surgeries, and other needs can greatly affect a person or a family is income. Some people are not even able to work because of their eye problems. There are several steps everyone can take to help prevent eye diseases and the problems related to them. It starts with knowledge of keeping the eye healthy and following good guidelines for maintaining good eye health.
There are many eye diseases that can be prevented or whose progression can be slowed down with proper precautions and healthy eating. Eating a lot of Vitamins A, E, C, and some Zinc and Selenium can help prevent many diseases including those that affect the eyes. Visit the nutrition page on what foods contain these vitamins and minerals. Certain diseases might be causes by other factors such as obesity, diabetics, exposure to ultraviolet light, smoking, certain medications such as steroids, and many other factors. It is a good idea to ask your eye care doctor for specific steps you can take. People with obesity or who are diabetic should have more routine checkups since they are more likely to develop certain eye diseases.
Those who are outside most of the day should use protective sunglasses or eyeglasses to prevent heavy exposure to ultraviolet light. People who smoke or take certain medications should ask their doctor if they are more likely to have eye diseases because of family background. These little measures can help your eyes healthy and keep all the emotional, physical, and financial problems that come with eye problems away.
domingo, 11 de julho de 2010
In specular reflection, the direction of the reflected ray is determined by the angle the incident ray makes with the surface normal, a line perpendicular to the surface at the point where the ray hits. The incident and reflected rays lie in a single plane, and the angle between the reflected ray and the surface normal is the same as that between the incident ray and the normal. This is known as the Law of Reflection.
For flat mirrors, the law of reflection implies that images of objects are upright and the same distance behind the mirror as the objects are in front of the mirror. The image size is the same as the object size. (The magnification of a flat mirror is unity.) The law also implies that mirror images are parity inverted, which we perceive as a left-right inversion. Images formed from reflection in two (or any even number of) mirrors are not parity inverted. Corner reflectors retroreflect light, producing reflected rays that travel back in the direction from which the incident rays came.
Mirrors with curved surfaces can be modeled by ray-tracing and using the law of reflection at each point on the surface. For mirrors with parabolic surfaces, parallel rays incident on the mirror produce reflected rays that converge at a common focus. Other curved surfaces may also focus light, but with aberrations due to the diverging shape causing the focus to be smeared out in space. In particular, spherical mirrors exhibit spherical aberration. Curved mirrors can form images with magnification greater than or less than one, and the magnification can be negative, indicating that the image is inverted. An upright image formed by reflection in a mirror is always virtual, while an inverted image is real and can be projected onto a screen.
quarta-feira, 7 de julho de 2010
During the Middle Ages, Greek ideas about optics were resurrected and extended by writers in the Muslim world. One of the earliest of these was Al-Kindi (c. 801–73). In 984, the Persian mathematician Ibn Sahl wrote the treatise "On burning mirrors and lenses", correctly describing a law of refraction mathematically equivalent to Snell's law. He used this law to compute optimum shapes for lenses and curved mirrors. In the early 11th century, Alhazen (Ibn al-Haytham) wrote his Book of Optics, which documented the then-current Islamic understanding of optics and revolutionized the field. It included the first descriptions of optical phenomena associated with pinholes and concave lenses, provided the first correct explanation of vision, described various experiments using an early scientific method, and greatly influenced the later development of the modern telescope. In the 13th century, Roger Bacon, inspired by Ibn al-Haytham, used parts of glass spheres as magnifying glasses, and discovered that light reflects from objects rather than being released from them. In Italy, around 1284, Salvino D'Armate invented the first wearable eyeglasses. Kamal al-Din al-Farisi studied reflection and refraction in rain drops, and their connection to the formation of rainbows.
The earliest known telescopes were refracting telescopes, a type which relies entirely on lenses for magnification. The first rudimentary telescopes were developed independently in the 1570s and 1580s by Leonard Digges,Taqi al-Din and Giambattista della Porta. Their development in the Netherlands in 1608 was by three individuals: Hans Lippershey and Zacharias Janssen, who were spectacle makers in Middelburg, and Jacob Metius of Alkmaar. In Italy, Galileo greatly improved upon these designs the following year. In 1668, Isaac Newton constructed the first practical reflecting telescope, which bears his name, the Newtonian reflector. The first microscope was made around 1595, also in Middelburg.Three different eyeglass makers have been given credit for the invention: Lippershey, Janssen, and his father, Hans. The coining of the name "microscope" has been credited to Giovanni Faber, who gave that name to Galileo's compound microscope in 1625.
Optical theory progressed in the mid-17th century with treatises written by philosopher René Descartes, which explained a variety of optical phenomena including reflection and refraction by assuming that light was emitted by objects which produced it. This differed substantively from the ancient Greek emission theory. In the late 1660s and early 1670s, Newton expanded Descartes' ideas into a corpuscle theory of light, famously showing that white light, instead of being a unique color, was really a composite of different colors that can be separated into a spectrum with a prism. In 1690, Christian Huygens proposed a wave theory for light based on suggestions that had been made by Robert Hooke in 1664. Hooke himself publicly criticized Newton's theories of light and the feud between the two lasted until Hooke's death. In 1704, Newton published Opticks and, at the time, partly because of his success in other areas of physics, he was generally considered to be the victor in the debate over the nature of light. Newtonian optics was generally accepted until the early 19th century when Thomas Young and Augustin-Jean Fresnel conducted experiments on the interference of light that firmly established light's wave nature. Young's famous double slit experiment showed that light followed the law of superposition, something normal particles do not follow. This work led to a theory of diffraction for light and opened an entire area of study in physical optics. Wave optics was successfully unified with electromagnetic theory by James Clerk Maxwell in the 1860s.The next development in optical theory came in 1899 when Max Planck correctly modeled blackbody radiation by assuming that the exchange of energy between light and matter only occurred in discrete amounts he called quanta. In 1905, Albert Einstein published the theory of the photoelectric effect that firmly established the quantization of light itself. In 1913, Niels Bohr showed that atoms could only emit discrete amounts of energy, thus explaining the discrete lines seen in emission and absorption spectra. The understanding of the interaction between light and matter, which followed from these developments, not only formed the basis of quantum optics but also was crucial for the development of quantum mechanics as a whole. The ultimate culmination was the theory of quantum electrodynamics, which explains all optics and electromagnetic processes in general as being the result of the exchange of real and virtual photons.Quantum optics gained practical importance with the invention of the maser in 1953 and the laser in 1960. Following the work of Paul Dirac in quantum field theory, George Sudarshan, Roy J. Glauber, and Leonard Mandel applied quantum theory to the electromagnetic field in the 1950s and 1960s to gain a more detailed understanding of photodetection and the statistics of light.
terça-feira, 6 de julho de 2010
Most optical phenomena can be accounted for using the classical electromagnetic description of light. Complete electromagnetic descriptions of light are, however, often difficult to apply in practice. Practical optics is usually done using simplified models. The most common of these, geometric optics, treats light as a collection of rays that travel in straight lines and bend when they pass through or reflect from surfaces. Physical optics is a more comprehensive model of light, which includes wave effects such as diffraction and interference that cannot be accounted for in geometric optics. Historically, the ray-based model of light was developed first, followed by the wave model of light. Progress in electromagnetic theory in the 19th century led to the discovery that light waves were in fact electromagnetic radiation.
Some phenomena depend on the fact that light has both wave-like and particle-like properties. Explanation of these effects requires quantum mechanics. When considering light's particle-like properties, the light is modeled as a collection of particles called "photons". Quantum optics deals with the application of quantum mechanics to optical systems.
Optical science is relevant to and studied in many related disciplines including astronomy, various engineering fields, photography, and medicine (particularly ophthalmology and optometry). Practical applications of optics are found in a variety of technologies and everyday objects, including mirrors, lenses, telescopes, microscopes, lasers, and fiber optics.
segunda-feira, 5 de julho de 2010
The ability to see is dependent on the actions of several structures in and around the eyeball. The graphic below lists many of the essential components of the eye's optical system.
When you look at an object, light rays are reflected from the object to the cornea, which is where the miracle begins. The light rays are bent, refracted and focused by the cornea, lens, and vitreous. The lens' job is to make sure the rays come to a sharp focus on the retina. The resulting image on the retina is upside-down. Here at the retina, the light rays are converted to electrical impulses which are then transmitted through the optic nerve, to the brain, where the image is translated and perceived in an upright position.
Think of the eye as a camera. A camera needs a lens and a film to produce an image. In the same way, the eyeball needs a lens (cornea, crystalline lens, vitreous) to refract, or focus the light and a film (retina) on which to focus the rays. If any one or more of these components is not functioning correctly, the result is a poor picture. The retina represents the film in our camera. It captures the image and sends it to the brain to be developed. The macula is the highly sensitive area of the retina. The macula is responsible for our critical focusing vision. It is the part of the retina most used. We use our macula to read or to stare intently at an object.
The conjunctiva is the thin, transparent tissue that covers the outer surface of the eye. It begins at the outer edge of the cornea, covering the visible part of the sclera, and lining the inside of the eyelids. It is nourished by tiny blood vessels that are nearly invisible to the naked eye.
The conjunctiva also secretes oils and mucous that moisten and lubricate the eye.
The ciliary body lies just behind the iris. Attached to the ciliary body are tiny fiber "guy wires" called zonules. The crystalline lens is suspended inside the eye by the zonular fibers. Nourishment for the ciliary body comes from blood vessels which also supply the iris.
One function of the ciliary body is the production of aqueous humor, the clear fluid that fills the front of the eye. It also controls accommodation by changing the shape of the crystalline lens. When the ciliary body contracts, the zonules relax. This allows the lens to thicken, increasing the eye's ability to focus up close. When looking at a distant object, the ciliary body relaxes, causing the zonules to contract. The lens becomes thinner, adjusting the eye's focus for distance vision.
With age, everyone develops a condition known as presbyopia. This occurs as the ciliary body muscle and lens gradually lose elasticity, causing difficulty reading.
The cornea is the transparent, dome-shaped window covering the front of the eye. It is a powerful refracting surface, providing 2/3 of the eye's focusing power. Like the crystal on a watch, it gives us a clear window to look through.
Because there are no blood vessels in the cornea, it is normally clear and has a shiny surface. The cornea is extremely sensitive - there are more nerve endings in the cornea than anywhere else in the body.
The adult cornea is only about 1/2 millimeter thick and is comprised of 5 layers: epithelium, Bowman's membrane, stroma, Descemet's membrane and the endothelium.
The epithelium is layer of cells that cover the surface of the cornea. It is only about 5-6 cell layers thick and quickly regenerates when the cornea is injured. If the injury penetrates more deeply into the cornea, it may leave a scar. Scars leave opaque areas, causing the corneal to lose its clarity and luster.
Boman's membrane lies just beneath the epithelium. Because this layer is very tough and difficult to penetrate, it protects the cornea from injury.
The stroma is the thickest layer and lies just beneath Bowman's. It is composed of tiny collagen fibrils that run parallel to each other. This special formation of the collagen fibrils gives the cornea its clarity.
Descemet's membrane lies between the stroma and the endothelium. The endothelium is just underneath Descemet's and is only one cell layer thick. This layer pumps water from the cornea, keeping it clear. If damaged or disease, these cells will not regenerate.
Tiny vessels at the outermost edge of the cornea provide nourishment, along with the aqueous and tear film.
The six tiny muscles that surround the eye and control its movements are known as the extraocular muscles (EOMs). The primary function of the four rectus muscles is to control the eye's movements from left to right and up and down. The two oblique muscles move the eye rotate the eyes inward and outward.
All six muscles work in unison to move the eye. As one contracts, the opposing muscle relaxes, creating smooth movements. In addition to the muscles of one eye working together in a coordinated effort, the muscles of both eyes work in unison so that the eyes are always aligned.
The colored part of the eye is called the iris. It controls light levels inside the eye similar to the aperture on a camera. The round opening in the center of the iris is called the pupil. The iris is embedded with tiny muscles that dilate (widen) and constrict (narrow) the pupil size.
The sphincter muscle lies around the very edge of the pupil. In bright light, the sphincter contracts, causing the pupil to constrict. The dilator muscle runs radially through the iris, like spokes on a wheel. This muscle dilates the eye in dim lighting.
The iris is flat and divides the front of the eye (anterior chamber) from the back of the eye (posterior chamber). Its color comes from microscopic pigment cells called melanin. The color, texture, and patterns of each person's iris are as unique as a fingerprint.
The macula is located Troughly in the center of the retina, temporal to the optic nerve. It is a small and highly sensitive part of the retina responsible for detailed central vision. The fovea is the very center of the macula. The macula allows us to appreciate detail and perform tasks that require central vision such reading.
The optic nerve transmits electrical impulses from the retina to the brain. It connects to the back of the eye near the macula. When examining the back of the eye, a portion of the optic nerve called the optic disc can be seen.
The retina's sensory receptor cells of retina are absent from the optic nerve. Because of this, everyone has a normal blind spot. This is not normally noticeable because the vision of both eyes overlaps.
The pupil is the opening in the center of the iris. The size of the pupil determines the amount of light that enters the eye. The pupil size is controlled by the dilator and sphincter muscles of the iris. Doctors often evaluate the reaction of pupils to light to determine a person's neurological function.
The retina is a multi-layered sensory tissue that lines the back of the eye. It contains millions of photoreceptors that capture light rays and convert them into electrical impulses. These impulses travel along the optic nerve to the brain where they are turned into images.
There are two types of photoreceptors in the retina: rods and cones. The retina contains approximately 6 million cones. The cones are contained in the macula, the portion of the retina responsible for central vision. They are most densely packed within the fovea, the very center portion of the macula. Cones function best in bright light and allow us to appreciate color.
There are approximately 125 million rods. They are spread throughout the peripheral retina and function best in dim lighting. The rods are responsible for peripheral and night vision.
This photograph shows a normal retina with blood vessels that branch from the optic nerve, cascading toward the macula.
The sclera is commonly known as "the white of the eye." It is the tough, opaque tissue that serves as the eye's protective outer coat. Six tiny muscles connect to it around the eye and control the eye's movements. The optic nerve is attached to the sclera at the very back of the eye.
In children, the sclera is thinner and more translucent, allowing the underlying tissue to show through and giving it a bluish cast. As we age, the sclera tends to become more yellow.
The vitreous is a thick, transparent substance that fills the center of the eye. It is composed mainly of water and comprises about 2/3 of the eye's volume, giving it form and shape. The viscous properties of the vitreous allow the eye to return to its normal shape if compressed.
In children, the vitreous has a consistency similar to an egg white. With age it gradually thins and becomes more liquid. The vitreous is firmly attached to certain areas of the retina. As the vitreous thins, it separates from the retina, often causing floaters.
The aqueous is the thin, watery fluid that fills the space between the cornea and the iris (anterior chamber). It is continually produced by the ciliary body, the part of the eye that lies just behind the iris. This fluid nourishes the cornea and the lens and gives the front of the eye its form and shape.
It was in Arabian literature that figures illustrating the anatomy of the eye first made their appearance. Arabic manuscripts still exists in which reference is made in the text to figures, themselves missing, though space from them is provided. The earliest drawing as yet available appears in Hunain ibn Is-hâq's Book of the Ten Treatises on the Eye, recently discovered and edited by Meyerhof (frontispiece).
Through lack of illustrations it is difficult to get a clear conception of Greek and Roman knowledge of ocular anatomy, for the descriptions are frequently not only scant, but confused through a multitude of names, which may or may not have had the same meaning.
Pre-Hippocratic anatomy had hardly passed beyond the stage of recognizing a transparent cornea continuous with an opaque sclera, the whole being lined by a layer with a perforation which formed the pupil. These two layers enclosed a fluid substance. This conception of the anatomy of the eye was not based on detailed observation, but on speculation as to the nature of vision. The fluid in the eye was regarded as the principle of vision and a tube leading from the eye to the brain, allowing for the free movement of this visual substance, led Alcamaeon to postulate the pÓroz, poros. This postulated hollow tube is hardly the solid optic nerve of modern anatomy. An advance of these speculations is to be found with Aristotle, who obviously dissected animal eye. (Figure 1). Three layers instead of two are recognized, though knowledge of the retina hardly went beyond the recognition of its existence. Knowledge of the structure of the cavity of the eye was vague. There was no recognition of the anterior chamber; it was held that the three layers of the eye are intimately apposed to each other. The ocular fluid was considered as of uniform consistency, though some differentiation occurred on exposure to air; the lens, as far as it was clearly recognized, was thus regarded as a post-mortem manifestation. The hollow tube of Alcamaeon became three in number, one of which entered the skull and joined with a corresponding structure from the other eye. The recognition of the chiasma and of ocular vessels had therefore been achieved.
The Alexandrian school contributed largely to the knowledge of the anatomy of the eye. Herophilus in particular seems to have devoted much attention to the eye; from a reference in Aetius it is clear that he wrote a special treatise on the subject. As no manuscripts of this period have survived one has to rely on Celsus for information (Figure 2), and Celsus' account is by no means clear for the reason, as Hirschberg puts it, that he did not understand the subject. There is a clear recognition of the existence of the lens, a drop-like body named Krustalloidez, crystalloides. Whilst no anterior chamber is indicated -- the second layer is still contiguous with the first, except in the pupillary area, which is a mere perforation -- it is recognized that the retina does not come up to the cornea; it forms a smaller enclosing structure, and comes to surround the ocular fluid including the lens. This arrangement leaves a large empty space -- locus vacuus -- between the two outer layers and the smaller retina. As this locus vacuus is also spoken of as containing "humor", a near approach to the appreciation of the existence of the anterior chamber may have been made. What exactly Celsus knew of the optic nerve is not clear: he does not speak of any hollow canal, nor does he speak of a continuation of the retina into the nerve. The optic nerve probably appeared to him as a continuation of the fused two outer layers of the eye.
With Rufus a much clearer conception of ocular structure emerges. The conjunctiva is recognized, though of course not distinct from the capsule of Tenon, which indeed was not described till 1806. Under the name of epidermiz, epidermis, it is regarded as a fourth covering layer extending from the junction of the cornea and sclera to the posterior pole. The corneo-scleral junction stefauh, stephane, is regarded as also indicating the site where the retina branches off to line the posterior aspect (no longer the anterior) of the lens. The lens itself is invested with a lining layer, but whether this is a distinct layer or a decomposition product of the lens (?liquefied cortex) is not clear to Rufus. Of significance is Rufus' conception of the internal structure: as Magnus points out, this approaches the modern view. Two spaces are recognized, one lying between the cornea and iris, and another behind the lens. The first space, a mere chink, was filled with a fluid very much like water, whilst the second contains a substance like the white of a raw egg.
Four serious defects mar the description by Rufus. He failed to recognize the existence of the posterior chamber, the greater curvature of the cornea as compared with the sclera, and the inequality in the curvature of the lens surfaces; and his reference to the optic nerve is most scanty. These defects were in a large measure rectified by Galen (Figure 3).
Just how much the description given by Galen is the result of his own observations or that of predecessors is not known. But Galen's account is of significance not only because it marked an advance, but even more because no advance was on it till after Vesalius. If pre-Hippocratic anatomy was speculative, and Alexandrian anatomy truly descriptive, anatomy after Galen became a historical exercise on which commentators were busy for well over a thousand years.
A fairly clear recognition of the ciliary body seems to have been arrived at. The corneo-scleral junction -- one name for which, incidentally, was iris, a designation that persisted till well into the 18th century -- was also the seat of fusion of the choroid and retina, where in addition a layer lining the anterior surface of the lens also terminated. The posterior chamber was clearly recognized, as was also the fact that it contains the same fluid as the anterior chamber. The greater curvature of the posterior surface of the lens was likewise recognize; the lens itself was held to fuse with the choroid by which it was kept in position.
It should be noted that whilst the recognition of the greater curvature of the cornea over the sclera was obviously the result of observation, the recognition of the existence of the posterior chamber was the result of speculation. Galen's writings are not clear on the subject, and as Magnus points out, he could not possibly find a space between the lens and iris in an eye cut open without the modern methods of preliminary fixation; but his theory of vision which postulated dilatation of the pupil by , pneuma, called for a posterior chamber through which the pneuma could diffuse on to the lens.
Speculation also entered into the description of the optic nerve. Whilst Galen recognized its solid structure he had to maintain a central hollow canal, in the sense of Alcmaeon. At the chiasma fusion of the hollow canals of both nerves took place. That Galen drew on animal dissection is clearly seen from his description of extraocular muscles, of which there are seven -- the six of present-day human anatomy with an additional massive ensheathing muscle which arises from where the optic nerve enters the orbit -- obviously the retractor bulbi of comparative anatomy. Furthermore, in describing the lacrimal apparatus he speaks of two glands, one in the upper and one in the lower lid. Galen recognized another source of tears - glands in the conjunctiva of the lids. The conjunctiva itself he held to be derived from the pericranium.
Arabian anatomy was the anatomy of Galen modified not by the evidence of dissection but by conclusions drawn from speculation. Depression of cataract extensively practised; and as the prevailing view was that a corrupted humour in front of the lens was displaced in the process, it was necessary to conceive the lens as being situated further back than in Galen's scheme. This view as to the seat of the lens persisted till the beginning of the 17th century.
With the coming of Vesalius, anatomy turned once more from speculation and commentaries to dispassionate observation. But to ocular anatomy Vesalius contributed nothing (Figure 4). His teaching is distinctly inferior to that of Galen and even of Arabian ophthalmology. The recognition of the greater curvature of the cornea over the sclera, and of the posterior surface of the lens over the anterior, is lost. A central position of the lens is once more in evidence. Even more astounding is Vesalius' acceptance of Galen's retractor bulbi.
Modern anatomy of the eye did not emerge till the physicists had demolished the old conceptions of the nature of vision. It began when it was realized that the lens is not the seat of vision, but part of a refractive system. With Fabricius as a precursor in showing the true position of the lens (A.D. 1600), a host of observers rapidly built up the basis of the anatomical scheme as we know it today. Fallopius rediscovered the greater curvature of the cornea and stressed the difference in structure as between the cornea and sclera. A clearer view of the capsule of the lens and a description of the hyaloid membrane likewise came from him. He differed from Vesalius in regarding the ciliary body as a membrane, and held it to be a ligament binding the lens to the choroid. Incidentally, he also disproved the existence of the retractor bulbi in man. Ruysch, who studies the vascular structure of the choroid, is also responsible for showing the existence of circular muscle fibres in the iris. Briggs, who is remembered for his demonstration of the existence of the optic papilla (regarded by him as a projection, as its name implies), showed that the retina extended up to the ciliary "ligament." What the 16th century began falteringly was well done in the 17th. A comparison of two reproduction showing the state of anatomical knowledge towards the beginning and the end of the 17th century is of interest (Figure 5 and 6).
The finer methods of anatomical study were first used in that century; Ruysch employed injected preparations for the study of the vascular system of the eye; Malpighi used the hand-lens and Leeuwenhoek made the first observations with the microscope; but it was left for the succeeding century to introduce the study of the frozen eye, an innovation due to Petit. The combination of these methods led to the rise of a detailed anatomy, for the bold outlines were by now firmly established. Petit was the first to attempt measurements of the components of the eye. Priority in the description of Descemet's membrane was the subject of a word dispute between Demours and Descemet, but its first indication is really to be found in Duddell.
In studying the constitution of the lens, Morgagni found fluid between the capsule and the lens fibres. This fluid was held to nourish the lens - a mistaken notion but one which, at any rate, was an advance on the belief that the lens and cornea contained vasa serosa, which possessed the property of impermeability to red blood cells. To the anatomy of this period belongs the description of the spaces of Fontana, as also the discovery by Demours of the canal of Petit, so named by him, the Zonula of Zinn commemorates the name of an observer who also contributed studies on the blood-vessels around the entry of the optic nerve (circulus arteriosus of Zinn) and on the action of the ciliary body.
The presence of muscle fibres in the ciliary body was a matter of much discussion; some held with Morgagni that they existed and affected accommodation, others with Zinn, that they were non-existent. Similarly contraction and dilatation of the pupil were explained on the conflicting view that different degrees of congestion of the vessels of the iris produced changes in the size of the pupil.
It is noteworthy that even at this late stage some gross points were still unsettled. Though Petit in 1728 had clearly demonstrated the posterior chamber, its existence was being questioned down to 1855 and it was not until the work of Helmholtz, Henle and Arlt that this question was finally settled.
Whilst by the end of the 18th century the uveal tract had been fairly well described, the retina was barely recognized, for the day of cellular anatomy had not yet come. At the turn of the century Buzzi, Sömmering and Reil described the macula lutea. The additions to our knowledge of the anatomy of the eye during the 19th century are largely the history of the consequences of the introduction of the compound microscope and the rise of the cellular theory.
The advances recorded during the earlier part of the 19th century, before the introduction of the microscope, are typified by the description of Jacob's membrane. Jacob described a serious layer in the eye, lying between the retina and the choroid; this ultimately came to be regarded as a constituent part of the retina, which was held to consist of three layers, a limiting layer, a nervous layer -- the retina proper -- and Jacob's membrane. Jacob's membrane is indeed nothing else than the rods and cones of modern histology. To this period belongs also the discovery of the canal of Schlemm.
The compound microscope opened a new realm of observation, and the realization of the significance of the new facts which were rapidly gathered, culminated in Schwann's theory that all living matter consists of cells. As early as 1722 Leeuwenhoek had noted the rods and cones of the retina, but their existence had to be rediscovered in 1834 by Treviranus. And just as the retina was gradually being recognized, so other tissues were studies by the new microscopic methods. In a few brilliant years of intense work.