Nerve06/eyes and ears/focus adjustment
From a certain point, light spreads to all directions (Fig 2A). This light reaching the eye, some distance away, will have a certain spreading angle at the lens (B). Since the lens can refract (bend) the path of the light, it can converge this spreading angle. When the paths converge to one point at the retina, the point A will not be blurred and can be seen sharply. This is called 'in focus'. Secondly, if point A was in a far distance, then the path of the light reaching the would be parallel (C). In a normal eye, when the lens is thinnest (with least refractive bending power), the light path from a far distance converges to one point. However, if the light originated closer to the eye, the light path will converge to one point behind the retina if the lens remains thin (D). Thus, at this circumstance, the refractive power (thickness) of the lens need to be increased for focusing.
The ciliary body surrounds the lens like the rings of the saturn, pulling the lens outwards, which makes the lens thin. Also, when the the muscles in the ciliary body, which are in the circular direction, contract, the diameter of the ciliary body decreases, and the lens, being pulled less outwards, becomes thicker. So, when the ciliary muscle are dilated, the diameter is increased, pulling the lens outwards, decreasing the thickness and the refractive (bending) power of the lens (Fig 2C). When the point is near, the ciliary muscle contracts, the diameter is decreased, pulling the lens less outwards, increasing the thickness and the refractive (bending) power of the lens (Fig 2B). This is done by the parasympathetic nerve activity of the oculomotor (III cranial) nerve.
Challenge Quiz
近点に焦点を合わせるとき、毛様体が 収縮. 弛緩 し、水晶体が 厚く. 薄く なる。