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Mydriasis is the dilation of the pupil, usually defined as when having a non-physiological cause,〔(Mydriasis ) in Farlex medical dictionary. In turn citing: *The American Heritage Medical Dictionary (2007) *Mosby's Dental Dictionary, 2nd edition.〕 but sometimes defined as potentially being a physiological pupillary response.〔(Mydriasis ) in Farlex medical dictionary. In turn citing: Mosby's Medical Dictionary, 8th edition.〕 Non-physiological causes of mydriasis include disease, trauma, or the use of drugs. Normally, as part of the pupillary light reflex, the pupil dilates in the dark and constricts in the light to respectively improve vividity at night and to protect the retina from sunlight damage during the day. A ''mydriatic'' pupil will remain excessively large even in a bright environment. The excitation of the radial fibres of the iris which increases the pupillary aperture is referred to as a mydriasis. More generally, mydriasis also refers to the natural dilation of pupils, for instance in low light conditions or under sympathetic stimulation. An informal term for mydriasis is blown pupil,〔(【引用サイトリンク】url=http://www.aans.org/Patient%20Information/Conditions%20and%20Treatments/Traumatic%20Brain%20Injury.aspx )〕 and is used by medical providers. It is usually used to refer to a fixed, unilateral mydriasis, which could be a symptom of raised intracranial pressure. The opposite, constriction of the pupil, is referred to as miosis. Both mydriasis and miosis can be physiological. Anisocoria is the condition of one pupil being more dilated than the other. ==Mechanism== There are two types of muscle that control the size of the iris: the iris sphincter, composed of circularly arranged muscle fibers, and the iris dilator, composed of radially arranged muscle fibers. The sphincter is innervated by the parasympathetic nervous system; the dilator by the sympathetic nervous system. Sympathetic stimulation of the adrenergic receptors causes the contraction of the radial muscle and subsequent dilation of the pupil. Conversely, parasympathetic stimulation causes contraction of the circular muscle and constriction of the pupil. The mechanism of mydriasis depends on the agent being used. It usually involves either a disruption of the parasympathetic nerve supply to the eye (which normally constricts the pupil) or overactivity of the sympathetic nervous system (SNS). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Mydriasis」の詳細全文を読む スポンサード リンク
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