The doctor observed that his pupils became very small under bright light.

pupils 在这里指"瞳孔"，是眼睛中央的圆形开口，可以调节进入眼睛的光线量。这个词来自拉丁语 pupilla（小娃娃），因为瞳孔中会映出观察者的微小倒影，看起来像个小人。