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Lao pulse (Deep-Firm Pulse) represents the extreme manifestation of Qi and Blood mutually constrained, and deep-seated, entrenched Cold-Dampness. At the initial stage of the pulse and symptom presentation, when exhaustion has not yet appeared, people often overlook it, unaware that the pathogenesis is extremely critical and the progression is extremely rapid.

Wherever Cold resides, the pulse tends to be Xian (Wiry). Now, the pulse is firm, large, and striking, appearing at an extremely deep position; thus, interior Cold can be known. However, patients often do not perceive interior Cold but instead suffer from interior Heat. This is because the Cold is not in the Zang-Fu organs but in the Yin Collaterals (collateral vessels distributed between muscles, bones, and internal membranes). The Yin Collaterals are located between muscles and bones, belonging to the region of internal membranes. Within the gaps of the chest and ribs, there is a hidden sensation of tension and discomfort; during flexion, extension, shaking, or twisting, there is a hidden sensation of pulling and disharmony. Consequently, the interior and exterior of the body shell are blocked and cannot communicate. Exterior Qi scatters on the exterior and cannot interact with the interior; therefore, the four limbs lack strength, the skin swells slightly, and scratching feels as if obstructed. Above the waist, sweating continues incessantly; Interior Qi knots within the interior and drains internally, unable to reach the exterior; therefore, the chest and diaphragm feel distended and full, panting occurs with slight movement, and bowel movements vary between loose and constipated without regularity. These conditions are mostly due to Wind-Dampness invading internally, infiltrating deeply over days. Generally, this stems from prior exertion and sweating followed by exposure to wind and pathogens, then again exertion and sweating while pathogens are attempting to exit but have not fully done so, followed by further exposure. Such alternating recurrence causes pathogens to be stuck between advance and retreat, unable to enter the interior nor close off at the exterior, remaining solely stagnant in the fascia of the interstices. Thus, this pulse and these symptoms are most common in individuals who expose themselves to wind and dew, sleep or reside in low-lying damp places, and exhaust both mind and body, leading to depletion of both exterior and interior.

In Zhang Zhongjing's "Chapter on Differentiating Pulses," it states: "The Cun Kou pulse shows Yin and Yang both tight." Regarding the section where Clear Pathogens attack the lower part and Turbid Pathogens attack the lower part, the symptoms described before and after completely reflect the separation of Ying and Wei (Nutritive and Defensive Qi), Qi constraint, and Blood stagnation. Nothing does not align with the diseases governed by the Lao pulse. The treatment method should deeply understand the pivot of Yin-Yang opening and closing. Initially, prioritize moving Qi and invigorating Blood, relaxing sinews, and activating collaterals. Initially, one may observe a voice sounding like it comes from a jar, clear nasal discharge, and symptoms resembling a Wind-Cold injury; this indicates Interior Yang stirring. Subsequently, sudden onset of chills and fever resembling malaria indicates Yang reaching the exterior. Subsequently, profuse sweating during sleep, with sweat cool as water, indicates pathogens gradually exiting. Subsequently, slippery, urgent defecation slightly resembling dysentery indicates warming and soothing of internal membranes, with condensed Phlegm-Turbidity draining downward. At this point, urgently use formulas that strengthen sinews and consolidate Qi for regulation. Essentially, this involves first pushing pathogens in the fascia to the exterior, then gathering righteous Qi of the Zang-Fu back into the interior. Acting alternately as sovereign and minister herbs, this process requires months or years to achieve efficacy. If delayed for a long time, the hundred vessels become bi (obstructed), and the disease becomes acute; how can one wait? Mutual attack of Cold and Heat leads to mutual ruin of Qi and Blood; nothing is more severe than this pulse and condition. What Zhang Zhongjing called "drilling pain in the umbilicus, shallow and painful, with stool like sweat-mud" refers to the result of neglected treatment, leading to flaccid sinews and溃散 (scattering/rupture) of blood.