📚 The Eye as a Receptor: Structure and Accommodation | 作为感受器的眼睛：结构与调节
The human eye is a remarkable receptor organ that converts light energy into electrical impulses, enabling vision. For Edexcel IGCSE Biology, you need to describe how the retina detects light and how the lens changes shape during accommodation to focus on near and distant objects.
人眼是一个非凡的感受器官，能将光能转换为电脉冲，从而产生视觉。对于Edexcel IGCSE生物学，你需要描述视网膜如何探测光线，以及晶状体在调节过程中如何改变形状以聚焦近处和远处的物体。
1. The Eye as a Receptor Organ | 作为感受器官的眼睛
The eye is a sensory organ containing receptor cells that are sensitive to light (photoreceptors). These receptors are located in the retina, a layer at the back of the eye. When light hits the retina, the photoreceptors generate nerve impulses that travel via the optic nerve to the brain, where an image is perceived.
眼睛是一个感觉器官，包含对光敏感的感受器细胞（光感受器）。这些感受器位于眼球后部的视网膜层。当光线照射到视网膜时，光感受器产生神经冲动，通过视神经传递到大脑，形成视觉。
2. Major Structures of the Eye | 眼球的主要结构
Understanding the function of the retina and accommodation requires familiarity with key parts of the eye. The table below summarises the main structures and their roles.
理解视网膜的功能和调节作用需要熟悉眼球的关键部位。下表总结了主要结构及其功能。
| Structure | 中文名称 | Function | 功能 |
|---|---|---|---|
| Cornea | 角膜 | Transparent outer layer that refracts (bends) light entering the eye | 透明的外层，折射进入眼睛的光线 |
| Iris | 虹膜 | Coloured part that controls the size of the pupil and regulates light entry | 有颜色的部分，控制瞳孔大小，调节进光量 |
| Pupil | 瞳孔 | Hole in the centre of the iris that allows light to pass through | 虹膜中央的孔，让光线通过 |
| Lens | 晶状体 | Transparent, flexible biconvex structure that focuses light on the retina by changing shape | 透明、有弹性的双凸结构，通过改变形状将光线聚焦在视网膜上 |
| Retina | 视网膜 | Light-sensitive layer containing photoreceptor cells (rods and cones) | 包含光感受器细胞（视杆细胞和视锥细胞）的感光层 |
| Optic nerve | 视神经 | Carries nerve impulses from the retina to the brain | 将神经冲动从视网膜传递到大脑 |
| Ciliary muscle | 睫状肌 | Ring of muscle that changes the shape of the lens during accommodation | 环形肌肉，在调节过程中改变晶状体的形状 |
| Suspensory ligaments | 悬韧带 | Fibres that connect the ciliary muscle to the lens; transmit tension to alter lens shape | 连接睫状肌和晶状体的纤维；传递张力以改变晶状体形状 |
These structures work together to refract light accurately onto the retina. The lens, ciliary muscles and suspensory ligaments are particularly important for accommodation.
这些结构协同作用，将光线精确地折射到视网膜上。晶状体、睫状肌和悬韧带对于调节作用尤为重要。
3. The Retina: The Light-Sensitive Layer | 视网膜：感光层
The retina contains millions of photoreceptor cells called rods and cones. These are the actual receptors that convert light energy into electrical impulses. The retina is therefore the site where light is detected and the visual signal begins.
视网膜中含有数百万个称为视杆细胞和视锥细胞的光感受器细胞。这些才是真正将光能转换为电脉冲的感受器。因此，视网膜是探测光线并产生视觉信号的部位。
In the IGCSE specification, you are expected to state that the retina is sensitive to light. This means that the retina is the layer that responds to light stimulus, and the subsequent nerve impulses travel to the brain through the optic nerve.
在IGCSE考试大纲中，要求说明视网膜对光线敏感。这意味着视网膜是响应光刺激的层面，随后神经冲动通过视神经传到大脑。
4. Rods and Cones: Two Types of Photoreceptors | 视杆细胞与视锥细胞：两种光感受器
There are two main types of photoreceptor cells in the retina: rods and cones. Rods are more numerous and are highly sensitive to low light levels; they allow us to see in dim light but do not detect colour. Cones are concentrated in the fovea (central retina) and are responsible for colour vision and detailed vision in bright light.
视网膜中有两种主要的光感受器细胞：视杆细胞
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