・「網膜周辺部では錐状体よりはるかに多いが，黄斑に近づくにつれ，その関係は逆になって次第に少くなる。
」（ブリタニカ国際大百科事典）
・「On average, there are approximately 92 million rod cells (vs ~6 million cones) in the human retina.」（Wikipedia）
・「Rods are a little longer and leaner than cones but have the same basic structure. 」（Wikipedia）
・「A human rod cell is about 2 microns in diameter and 100 microns long.」（Wikipedia）
以下は「Wikipedia」の解説文となる。
Rod cells are photoreceptor cells in the retina of the eye that can function in lower light better than the other type of visual photoreceptor, cone cells. Rods are usually found concentrated at the outer edges of the retina and are used in peripheral vision. On average, there are approximately 92 million rod cells (vs ~6 million cones) in the human retina. Rod cells are more sensitive than cone cells and are almost entirely responsible for night vision. However, rods have little role in color vision, which is the main reason why colors are much less apparent in dim light.
【 語 句 】
・photoreceptor cell：光受容細胞、視細胞 ・retina：網膜 ・cone cell：錐体細胞 ・concentrated：集中した ・peripheral vision：周辺市や ・apparent：はっきりした
【Structure】
Rods are a little longer and leaner than cones but have the same basic structure. Opsin-containing disks lie at the end of the cell adjacent to the retinal pigment epithelium, which in turn is attached to the inside of the eye. The stacked-disc structure of the detector portion of the cell allows for very high efficiency. Rods are much more common than cones, with about 120 million rod cells compared to 6 to 7 million cone cells.
【 語 句 】
・opsin：オプシン（レチネンと結合し色素ロドプシンを合成するタンパク質） ・adjacent to ～：～に隣接する ・retinal pigment epithelium：網膜色素上皮 ・stacked：積層の？ ・detector portion：探知部？ ・efficiency：効率
Like cones, rod cells have a synaptic terminal, an inner segment, and an outer segment. The synaptic terminal forms a synapse with another neuron, usually a bipolar cell or a horizontal cell. The inner and outer segments are connected by a cilium, which lines the distal segment. The inner segment contains organelles and the cell's nucleus, while the rod outer segment (abbreviated to ROS), which is pointed toward the back of the eye, contains the light-absorbing materials.
A human rod cell is about 2 microns in diameter and 100 microns long. Rods are not all morphologically the same; in mice, rods close to the outer plexiform synaptic layer display a reduced length due to a shortened synaptic terminal.
【 語 句 】
・synaptic terminal：シナプス終末 ・bipolar cell：双極細胞 ・horizontal cell：水平細胞 ・cilium：微笑突起、絨毛 ・organelle：細胞小器官 ・morphologically：形態的に ・plexiform：網状の
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