筋，为联系形骸之物，故先易筋，筋易而无处不易矣。
髓，为出入灵明之区，故必洗髓，髓洗而无微不洗矣。
筋何贵乎易？按秘书说∶日月为易，象阴阳也。易系辞∶生生之谓易。注∶阴阳转易，以成化生。
是易为变化之总名，改换之殊称。筋一易而乾坤合，坎离交矣。至引伸为交易、变易、互易、移易、反易、
对易、辟易、改易、平易、和易之义。又何难使剥复运，否泰转耶？！所以一使之易，寒可易为暑，
暑可易为寒，昼可易为夜，夜可易为昼，老可易为少，少可易为老，阳可易为阴，阴可易为阳，变化
无端，皆易之道。
髓又何言乎洗？按洗广韵，先礼切，正韵，想礼切，并音 ，与洒同，涤也。说文“洒足也”。史
记高帝纪有∶使两女子洗之文书酒诰，自洗腆改用酒。洗，又洁也。是洗之义，自上洒下，又由下升上而洁
上也。他如钟名姑洗，律名姑洗，官名洗马，承水器曰洗，石名有洗石，皆取义于易系辞。圣人以此洗
心，退藏于密之洗，所谓洒涤、洒洁也。所以一为之洗，浊积者能使之清，黑暗者能使之明，凝重者能
使之轻，尘蔽者能使之净，洗髓功深能自放光明。
昔汉武帝时，客有问于东方朔，曰∶“先生有养生诀乎？”答曰∶“无他术，吾能三千年一洗髓，三
千年一伐毛。吾已三洗髓，三伐毛矣。”客不识道，以为滑稽戏语。后考据家及注家亦多引之作喻言，
不知为实语也。
行功者能晋进不退，恒久有常，困而升，革而鼎，蹇而解，贲临中节，屯蒙、悉涣、中孚、独履，
由小过而大过，小蓄而大蓄，颐养与井养兼施，未济既济，有震无兑，有益无损，噬嗑需讼，师遁 ，
比随而谦，谦而豫蛊，明夷，渐旅、渐异、渐丰、渐萃、渐艮、渐归妹家人，不暌同人，大壮，咸观、
大有。一易筋即洗髓，一洗髓愈易筋，阳阴造化，往来不穷，无方无体，无际无涯与天地参，非其能事
乎？
Jin (Tendons), as the substance connecting the physical form, is thus addressed first by Yi Jin (Transforming Tendons). Once the tendons are transformed, there is no part of the body that does not undergo change.
Sui (Marrow), as the realm where spiritual clarity enters and exits, must therefore be washed by Xi Sui (Washing Marrow). Once the marrow is cleansed, every subtle aspect becomes pure.
Why is Jin valued for its transformation (Yi)? According to secret texts: "Sun" and "Moon" constitute the character "Yi," symbolizing Yin and Yang. The *Xi Ci* (Commentary on the Appended Phrases) of the *I Ching* states: "Generating life continuously is called Yi." The commentary notes: "The transformation of Yin and Yang leads to generation and creation."
Thus, "Yi" is the general term for change and a specific designation for alteration. When the tendons are transformed, Qian and Kun (Heaven and Earth / Yin and Yang) harmonize, and Kan and Li (Water and Fire / Heart and Kidney) intersect. This extends to meanings such as exchange, mutation, mutual change, shifting, reversal, opposition, avoidance, modification, ease, and harmony. What difficulty could prevent the operation of Bo and Fu (Decline and Return), or the transition from Pi and Tai (Stagnation and Peace)?! Therefore, once transformation occurs, cold can become heat, heat can become cold, day can become night, night can become day, old age can become youth, youth can become old age, Yang can become Yin, and Yin can become Yang. Changes are endless; this is the Way of Yi.
Why speak of washing (Xi) regarding the marrow? According to the *Guang Yun*, "Xi" is pronounced with the initial Xianli cut, and in the *Zheng Yun*, with the initial Xiangli cut, sounding like "Sa" (sprinkle), identical to "Sa," meaning to cleanse. The *Shuo Wen Jie Zi* states: "To sprinkle the feet." In the *Records of the Grand Historian: Annals of Gaozu*, it mentions: "Having two women wash the documents and wine admonitions, personally cleansing and reverently using wine." "Xi" also means cleanliness. Thus, the meaning of "Xi" involves sprinkling from above downwards, and rising from below upwards to purify. Other examples include the bell named Gu Xi, the musical pitch named Gu Xi, the official title Xi Ma (Groom), the water receptacle called Xi, and the stone named Xi Shi, all deriving their meaning from the *Xi Ci* of the *I Ching*. The sage uses this to "wash the heart," retreating into secrecy, referred to as sprinkling and cleansing. Therefore, once washed, accumulated turbidity becomes clear, darkness becomes bright, heaviness becomes light, dust-covered things become clean, and deep practice of Xi Sui naturally allows one to emit light.
In the time of Emperor Wu of Han, a guest asked Dongfang Shuo: "Do you have any secrets for preserving life?" He replied: "No other art; I can wash my marrow once every three thousand years and shave my hair once every three thousand years. I have already washed my marrow three times and shaved my hair three times." The guest, unfamiliar with the Dao, considered it a jest. Later, researchers and commentators often cited it as a metaphor, unaware that it was stated as fact.
Practitioners who advance without retreat, maintaining constancy over time, rising from adversity, transforming like the Ding (Cauldron) hexagram, resolving difficulties like the Jie (Deliverance) hexagram, observing propriety in Bi (Adornment) and Lin (Approach), navigating Tun (Difficulty at the Beginning) and Meng (Youthful Folly), Huan (Dispersion), Zhong Fu (Inner Truth), and Du Lü (Walking Alone), progressing from Xiao Guo (Small Exceeding) to Da Guo (Great Exceeding), Xiao Xu (Small Taming Power) to Da Xu (Great Taming Power), combining Yi (Nourishment) and Jing (Well) nourishment, moving between Wei Ji (Before Completion) and Ji Ji (After Completion), possessing Zhen (Arousing) without Dui (Joy), having benefit without loss, engaging in Shi He (Biting Through), Xu (Waiting), and Song (Conflict), Shi (Army) and Dun (Retreat), Bi (Holding Together) and Sui (Following) leading to Qian (Modesty), Qian leading to Yu (Enthusiasm) and Gu (Work on What Has Been Spoiled), Ming Yi (Darkening of the Light), Jian (Gradual Progress), Lv (The Wanderer), Yi (Injury), Feng (Abundance), Cui (Gathering Together), Gen (Keeping Still), Gui Mei (Marriage Maiden), Jia Ren (Family), Bu Kui (Opposition), Tong Ren (Fellowship), Da Zhuang (Great Strength), Xian (Influence), Guan (Contemplation), and Da You (Great Possession)—once one transforms the tendons, one washes the marrow; once one washes the marrow, one transforms the tendons more effectively. The creation of Yin and Yang flows endlessly, without fixed direction or form, without boundary or limit, participating with Heaven and Earth. Is this not within his capability?