Typically, something moves at the periphery of our field of vision. The eyes shift to bring whatever it is into detailed vision by projecting its image on to the centres of the retinae. The retinae then provide the data that the brain uses to decide whether the moving object is threatening, edible, sexy, inconsequential, or, if you are playing cricket, catchable.
By contracting the pupil the iris assists vision in three ways. First, it stops light from the sides of the cornea forming blurred images on the retina. Secondly, it prevents too much light from entering the eye. Thirdly, it enables depth of focus. Read the rest of this entry »
The colour of your eyes is inherited. The colour and pattern of the iris are as individuated as fingerprints. It is coated on its back with brown-black pigment. This prevents light penetrating to the back of the eye except through its centre, or pupil. This hole can change its size, dilating or contracting as the level of light requires. In bright light, or when we wish to scrutinize a near object, it contracts. In dull light, or when we wish to relax our eyes and stare in the distance, it dilates. It also becomes bigger if we are frightened or excited, and it dilates in death. Read the rest of this entry »
The spectacle lens that corrects short sight or myopia is a negative power: light rays from distant objects are bent in such a way that they become more, not less, divergent. The degree of divergence will depend on the power of the lens. When the degree of (artificial) divergence is equal to the degree of short sight, the eye will form a clear image on the retina. A usual degree of short sight is up to —8.00 dioptres. But in fact it is how healthy the back of the eye remains, and how thin the outer coats of the eye become, that determine the future of the shortsighted person. Read the rest of this entry »
When light rays from a distant object pass through the cornea only the central rays are likely to form an image on the central and most sensitive part of the retina. Only the central part of the cornea (an inner diameter of between 3 and 5 mm) is sufficiently curved to bend the light-rays regularly. The light entering the more peripheral parts of the cornea only stimulate the more peripheral parts of the retina. These are bent irregularly and do not form a clear central retinal image. This ‘peripheral vision‘ is most useful for locating objects in space, and, by a reflex nerve stimulation, regulating the size of the pupil. Read the rest of this entry »
The eye, we have seen, is a ball with a stalk behind that conveys data to the brain. The inside of the eye has been described as consisting of the light-sensitive retinal film in the back half, and a lens called the cornea and a pupil (entry hole) in the front half. The iris or coloured part has a black pigment behind it so that the whole back part of the eye globe is in darkness. Between the pupil and the retina, suspended by fine fibres or ligaments, is the inner lens of the eye, which is made of transparent layers of cells. Read the rest of this entry »
Basically, good and bad characteristics, healthy and unhealthy trends, are divided into dominant and recessive. Those that are dominant are handed down and show in the offspring; those that are recessive are handed down but do not show. Further, dominance and recession are often linked to the sex of the individual. Thus we talk about traits that are ‘dominant in females’, ‘dominant in males’, ‘recessive in females’, and ‘recessive in males’.
Suppose, for example, that both parents had a recessive trait `A’. If they had nine children the chances are that only three of them would be marked by ‘A’. But if they had only two children, the chances of either of their offspring showing trait ‘A’ are small indeed, whether the trait is for weak legs or musical genius. But if both parents had dominants of a certain trait, this would almost certainly come out in the offspring. Read the rest of this entry »
The retina, then, is a nerve pad dependent upon a complex network of support systems. One of these support systems is a supply of blood, for it is in the blood that many of the essential nutrients are carried from one part of the body to another. Therefore the eye, and more particularly the retina, is or can be affected by deficiencies in other organs. The retina is in fact fed by blood systems both in front and behind, and many blood diseases can have a deleterious effect on retinal functions; and both systems are sufficiently intricate and fine to be hypersensitive to abnormalities in the blood. However, since the variations and permutations of retinal degeneracy are enormous it is sometimes very difficult to diagnose its cause. It is always possible to describe what one sees as being wrong with a decayed retina, but even after many specialized tests it can still be impossible to say with any accuracy what is the precise cause. This has seriously hampered the treatment of some retinal conditions. Where the cause of a disease is known it is always much easier to prescribe a cure, or at least medication that will halt or slow down what is usually a progressive condition. Read the rest of this entry »
The retinae are the sensory parts of the eyes, and to all intents and purposes may be regarded as a part of the brain. At the embryonic stage the developing brain very early on grows two buds which will become the eyes. This brain tissue gradually transforms into the light-sensitive retinae, or receptor organs. In the young foetus the buds lie just below the skin surface. The forward part of the eye tissue then invaginates to form the crystal lens of the eye, while the skin forms the cornea, or front lens of the eye. Slowly each bud is extended away from the brain, until it is only attached to the brain by a stalk, or optic nerve. Read the rest of this entry »
As we get older, all our blood vessels acquire harder walls, making it more difficult for the heart to pump blood through the whole system, especially the smaller vessels. Resistance to the flow of blood results in increased, or ‘higher’, blood pressure. The heart simply has to work harder in order to meet and overcome the developing resistance. The small vessels of the retina, or arterioles, can be examined minutely with an ophthalmoscope and changes of hardness in their walls easily seen. Because they are so small and fine they are sometimes the first to break down, so that if the blood pressure becomes very high they may leak blood and serum. When this happens the results can easily be seen on the surface of the retina between the vessels. Read the rest of this entry »
The nerve layer of the retina is adherent to the pigment layer behind or outside it, and the pigment layer in turn is connected to the choroid blood vessel membrane. When, as may sometimes happen, the nerve layer separates off from the pigment layer the result is retinal detachment. Next to natural senile atrophy of the retinae, detachment is probably the single most common condition affecting the back of the eye. At big eye hospitals whole wards and departments are set aside to deal with this problem. Read the rest of this entry »
