「Nerve06/eyes and ears/focus adjustment」の版間の差分

提供：一歩一歩
ナビゲーションに移動 検索に移動
編集の要約なし
編集の要約なし
1行目: 1行目:
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 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 blur 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 optic 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 optic power (thickness) of the lens need to be increased for focusing.   
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.   


[[ファイル:00739.jpg|500px]]
[[ファイル:00739.jpg|500px]]


毛様体は土星の輪のように水晶体を取り囲んでいる。水晶体は毛様体に引っ張られると薄くなる。また、毛様体の中に輪状にある毛様体筋が収縮すると、毛様体は半径が小さくなり、水晶体は厚くなる。すなわち、毛様体筋が弛緩していると毛様体の半径は大きくなり中心から離れ、水晶体を引っ張るため、水晶体は薄く屈折率は小さい。図２Ｃがそのような状態である。近くの点を見ようとすると、動眼神経(III)に含まれる副交感神経活動が亢進し、それに支配される毛様体筋が収縮すると毛様体は中心に向い、水晶体を引っ張る力は弱まり、水晶体は厚くなり、屈折率は大きくなる(図２Ｂ)。この調節は自律神経系により不随意的におこなわれている。
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. 
 
　毛様体が弛緩しているときに平行線が網膜より後に結像してしまう状態を遠視(図２Ｅ)、手前に結像してしまう場合を近視(図２Ｆ)という。遠視は水晶体の屈折率が小さいのであるから水晶体と同じ凸レンズで補正される。逆に近視では、水晶体の屈折率が相対的に大きすぎるので、水晶体とは逆の凹レンズで補正される。
 
 
 


{{QuizTitle}}
{{QuizTitle}}

2012年9月17日 (月) 16:49時点における版

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.

00739.jpg

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

1.

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。

「https://life-science-edu.net/wiki/index.php?title=Nerve06/eyes_and_ears/focus_adjustment&oldid=2764」から取得