The Features of the Visual System in a Child
The normal functioning of the child’s visual system is an essential condition not only for the perception itself, but also for the development of all organs and systems. The eye is a sight organ, but also a “consumer” of light energy. Due to the stimulating action of the light, the endocrine glands (pituitary, adrenal, thyroid, gonads) synthesize hormones. Faster adaptation of the new-born to the environment, the growth and development of the child depends largely on the proper functioning of the visual system, which is why the visual analyser of the child is formed very quickly. The growth and development of the eye in a child ends mainly by the age of 2-3 years, in the next 15-20 years there are much less changes than in the first years.
- New-born’s eyeAs a rule, the eye of a new-born and of a young child is with hypermetropia. As the eye grows, the refraction becomes normal. During the first 3 years of life the important increase of the eyeball occurs, as well as the flattening of the cornea and especially of the lens.
- In the first few months of life, due to the incomplete functional development of the cranial nerves, the cornea is not sensitive. This period is particularly dangerous, because when foreign bodies enter the eye, there is no eye irritation; the child does not experience pain and anxiety and serious damage could happen to the cornea, even until it is destroyed. Subsequently, the sensitivity of cornea increases and in a one-year-old child it is almost the same as in an adult.
- The iris is the anterior part of the choroid and represents a vertical diaphragm with a hole in the middle – pupil – for adjusting the light influx into the eye, depending on external conditions. The colour of the iris may be different – from light blue to black, depending on the melanin content: the higher the amount of pigment, the darker is the iris. Children’s iris contains a few pigments, so babies and infants have blue-grey eyes. The definite colour of the iris is established at the age of 10 to 12 years. In infants, muscle fibres that dilate pupil are poorly developed, therefore the pupil is narrow (2 – 2.5 mm). By the age of 1-3 years, pupil size becomes the same as in an adult (3 – 3.5 mm).
- It is the second major optical system, which accounts for approximately one-third of the eye’s refractive power (up to 20.0 D). The lens has the ability to change the curvature of the anterior surface and to contribute to the clear vision at different distances (accommodation function). The shape and size of the lens varies considerably depending on age. In new-borns, the shape of the lens is almost spherical, its thickness is about 4 mm, the diameter is 6 mm, and the curvature of the anterior surface is 5.5 mm. In young adults and elderly, the thickness of the lens reaches 4.6 mm and the diameter increases to 10 mm; at the same time, the curvature radius of the anterior surface increases to 10 mm, and of the posterior – to 9 mm. subsequently, the refractive power of the lens changes: it is 43.0 D in children and decreases to 20.0 D in adult.
- Retina is the most important part of the visual analyser, its peripheral unit. The complex structure of the retina allows it to perceive light and convert light energy into nerve impulses, which are then transmitted through a neuronal chain of to the visual cortex where visual information is perceived and processed. Retina is the inner layer of the eyeball; the most important place of retina is the macula with its central area of 0.075 mm (fovea centralis). This area has the best perception of visual stimuli.
- The anterior chamber of the eye
- The anterior chamber of the eye is delineated in front by the posterior surface of cornea, at the periphery (in the corners) – by the iris radix and the ciliary body, in the back – by the anterior surface of the iris, and in the pupil area – by the anterior capsule of the lens. At birth, the anterior chamber of the eye is already formed, but it’s the shape and size differs greatly from the anterior chamber of the adult eye. This is due to the short antero-posterior axis of the eye, the shape peculiarities of the iris and spherical shape of the lens.
- About 5% of children are born with closed orifice of the nasal tear duct but, under the action of tear fluid, this orifice opens during the first days of life, and the tear fluid is drained normally. Otherwise, congestion of the tear fluid leads to dacryocystitis of the new-born.
- Orbit is the bone structure for eye protection. In new-borns the horizontal diameter of the orbit is bigger than the vertical one, the orbit has a small depth and a shape of a triangular pyramid whose axis is facing forward, which sometimes can give an impression of convergent strabismus. Only the superior wall of the orbit is well developed. At the age of 8-10 years, the shape and size of the orbit are almost the same as in an adult.