[展示] 【全网独家·英汉双语详细索引】不列颠百科全书（国际中文版）（修订版·共20卷）

in anatomy, the pigmented muscular curtain near the front of the eye, between the cornea and the lens, that is perforated by an opening called the pupil. The iris consists of two sheets of smooth muscle with contrary actions. These muscles control the size of the pupil and thus determine how much light reaches the sensory tissue of the retina. The sphincter muscle of the iris is a circular muscle that constricts the pupil in bright light, whereas the dilator muscle of the iris expands the opening when it contracts. The amount of pigment contained in the iris determines eye colour. When there is very little pigment, the eye appears blue. With increased pigment, the shade becomes deep brown to black.

chigre3 发表于 2016-2-15 04:34
只是为了让更多的人看到，所以回复顶上。