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The eyes are the organs through which the essence of the five Zang organs manifests, and are the greatest treasures of a lifetime, just as the sun and moon exist in the sky. How can they not be protected? However, the essence of bone gathers in the pupil (Tongzi), belonging to the Kidney; the essence of sinews gathers in the black eye (Heiyan, referring to the iris/cornea area), belonging to the Liver; the essence of blood gathers in the collaterals and around the pupil (Luoguo), belonging to the Heart; the essence of Qi gathers in the white eye (Baiyan, referring to the sclera), belonging to the Lung; the essence of flesh gathers in the eyelid constraints (Yueshu), belonging to the Spleen. These gather and extract the essence of sinews, bones, blood, and Qi, merging with the meridians to form the Eye System (Muxi). The system connects upward to the brain and emerges posteriorly at the nape. Therefore, pathogenic factors of the Six Excesses (Liuyin, i.e., Wind, Cold, Summerheat, Dampness, Dryness, Fire) invading from outside, internal stagnation of the Five Zang organs, improper diet or sexual activity, prolonged visual strain, sorrow and weeping, copying, carving, embroidery, gambling, exposure to smoke and dust, and inappropriate bloodletting or sweating therapies can all cause eye diseases. Thus, theories regarding the Five Wheels (Wulun) and Eight Palaces (Baguo), as well as internal and external cataracts/opacities, vary greatly. One must specifically distinguish the etiology and the involved Zang-Fu Yin-Yang attributes; they cannot be confused or treated indiscriminately. For instance, the outer canthus (Duiwei) belongs to the Shaoyang channel; the area near the nose above is the lateral canthus (Waibi), belonging to the Taiyang channel; the lower area is the medial canthus (Neibi), belonging to the Yangming channel. Red vessels extending from top to bottom indicate a Taiyang disease; those extending from bottom to top indicate a Yangming disease; those moving from outside to inside indicate a Shaoyang disease. These three Yang channel diseases must not be confused. A red eyeball color indicates disease in the Heart; white indicates the Lung; green/blue indicates the Liver; black indicates the Kidney; yellow indicates the Spleen; an unnamable color indicates disease in the Stomach. These Five Zang and Three Yang diseases must not be confused. The Three Causes (Sanyin) are described below.

If patients lack moderation in joy and anger, and combine worry with excessive thinking, it leads to imbalance of Zang Qi. Stagnation generates phlegm-saliva (Xian), which rises with Qi. Encountering deficiency in the Brain, it invades and infiltrates the Eye System, shading the eyes. Mild cases result in blurred vision and dryness/astringency; severe cases result in opacities (Yiyi), excessive discharge (Chilei), pterygium (Nurou), and white membranes covering the cornea. All these are due to Internal Causes (Neiyin). Alternatively, frequent exposure to Wind-Cold, failure to avoid summer heat and dampness, pathogenic invasion of the neck, entering the brain via the Eye System due to deficiency, generate external opacities. As discussed in the Opacity Theory, conditions such as Qingfeng (Green Wind), Lüfeng (Green Wind), Zifeng (Purple Wind), Heifeng (Black Wind), Chifeng (Red Wind), Baifeng (White Wind), Baiyi (White Opacity), and Huangyi (Yellow Opacity) arise from the influence of the Eight Winds, transforming into various syndromes. These are External Causes (Waiyin). Or, if desires are immoderate, drinking is unrestrained, raw pungent foods (Five Pungents) are consumed, roasted/fried foods are eaten excessively, horse riding and hunting occur, exposure to smoke and dust happens, and external essence is exhausted, leading to the root of blindness—indulging in temporary leisure and causing permanent health deficits for a century—these belong to Neither-Internal-Nor-External Causes (Bu Nei Wai Yin). Each has its respective treatment formulas.