When the pupil diameter is less than 2mm, it is called pupillary constriction. One side of the pupil narrowing can be seen in the actinic nerve stimulation, cervical sympathetic nerve destruction, corneal intraocular foreign body, etc.. Bilateral pupil narrowing can be seen in the elderly, infants, syphilis, cerebral bridge lesions, cerebrovascular disease, drug poisoning (morphine poisoning), organophosphorus poisoning. Changes in the diameter of the pupil can reflect certain lesions in the body, and some lesions of the nervous system can be reacted according to the changes in the pupil to make a localized diagnosis.