（《灵枢·邪客篇》）
黄帝问于岐伯曰∶人有八虚，各何以候？（八虚，即《五脏生成篇》所谓八溪也，是皆筋骨之隙，气血之所流注者，
故曰八虚。）岐伯答曰∶以候五脏。（谓可因八虚以察五脏之病。）黄帝曰∶候之奈何？岐伯曰∶肺心有邪，其气留于两肘。
（人之五脏，惟肺与心居于膈上，其经属手，脾肝肾俱在膈下，其经属足，故肺心有邪，乘虚而聚，其气必留于两肘，
在肺则尺泽，在心则少海之次。）肝有邪，其气流于两腋。（肝与胆合，其经自足而上，皆行胁腋之间，故肝邪乘虚而聚
者，其气当流于两腋，即期门、渊腋等穴之次。）脾有邪，其气留于两髀。（脾与胃合，其脉皆自胫股上出冲门、气冲之
间，故邪气留于髀跨间者，知为脾经之病。髀，并米切，又音比。）肾有邪，其气留于两 。（肾与膀胱为表里，其经皆
出膝后阴谷、委中之间，故邪气留于两 者，知为肾经之病。 音国。）凡此八虚者，皆机关之室，真气之所过，血络之
所游，邪气恶血固不得住留，住留则伤经络骨节机关，不得屈伸，故 挛也。（机，枢机也。关，要会处也。室，犹房室
也。凡此八者，皆气血之所由行也，正气居之则为用，邪气居之则伤经络机关，而屈伸为之不利，此八虚可候五脏也。
音拘。）
(From *Ling Shu: Evil Guest Chapter*)
The Yellow Emperor asked Qibo: The human body has eight "deficient" (Xu) areas. How can each be used for diagnosis? (The so-called "eight Xu" refers to the "eight streams" mentioned in the *Chapter on the Generation of the Five Zang Organs*. These are all gaps between sinews and bones, where Qi and Blood flow and pour through; hence they are called the "eight Xu".)
Qibo replied: They are used to diagnose the Five Zang organs. (This means that diseases of the Five Zang organs can be observed through these eight deficient areas.)
The Yellow Emperor asked: How is this diagnosis performed?
Qibo said: When there is evil Qi in the Lung and Heart, its pathogenic Qi remains in the two elbows. (Among the five Zang organs, only the Lung and Heart are located above the diaphragm, and their meridians belong to the hand; the Spleen, Liver, and Kidney are below the diaphragm, and their meridians belong to the foot. Therefore, when evil Qi affects the Lung and Heart, it gathers in the deficient areas, and its pathogenic Qi must remain in the two elbows. In the Lung meridian, this corresponds to the vicinity of Chize [LU-5]; in the Heart meridian, it corresponds to the vicinity of Shaohai [HT-3].)
When there is evil Qi in the Liver, its pathogenic Qi flows into the two axillae. (The Liver and Gallbladder are interior-exteriorly related. Their meridians ascend from the foot, passing through the hypochondrium and axilla regions. Thus, when Liver evil Qi gathers in the deficient areas, its pathogenic Qi flows into the two axillae, corresponding to the vicinity of acupoints such as Qimen [LR-14] and Yuanxi [GB-9].)
When there is evil Qi in the Spleen, its pathogenic Qi remains in the two thighs/groin areas (Bi). (The Spleen and Stomach are interior-exteriorly related. Their pulse Qi ascends from the lower leg and thigh, emerging between the Chongmen [SP-12] and Qichong [ST-30] regions. Therefore, if evil Qi remains in the groin/thigh area, it indicates a disease of the Spleen meridian. "Bi" is pronounced bǐ, or alternatively bì.)
When there is evil Qi in the Kidney, its pathogenic Qi remains in the two popliteal fossae (Guo). (The Kidney and Bladder are interior-exteriorly related. Their meridians emerge between the Yinvalley [BL-39] and Weizhong [BL-40] points behind the knee. Therefore, if evil Qi remains in the two popliteal fossae, it indicates a disease of the Kidney meridian. "Guo" is pronounced guó.)
Generally speaking, these eight deficient areas are chambers of joint mechanisms, places where True Qi passes, and routes where blood collaterals travel. Evil Qi and foul blood cannot naturally reside here. If they do reside, they damage the meridians, bones, joints, and mechanisms, leading to an inability to flex and extend, resulting in spasms or contractures. ("Ji" refers to the pivot/mechanism; "Guan" refers to key intersections; "Shi" is like a room. These eight areas are where Qi and Blood circulate. When Righteous Qi occupies them, they function normally; when Evil Qi occupies them, it damages the meridians and mechanisms, causing difficulty in flexion and extension. This is why the eight Xu areas can be used to diagnose the Five Zang organs. "Ju" is pronounced jū.)