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Headache caused by internal injury occurs intermittently, sometimes appearing and sometimes ceasing; headache caused by external contraction is persistent, continuing only until the pathogen transmits to the interior excess stage. This illustrates the difference between internal and external conditions.

In diseases of internal injury, the Qi of the Heart and Lung has already exhausted at the exterior, inevitably leading to lethargy and a desire to lie down, along with heaviness and weakness in the four limbs that cannot be sustained. This is due to Heat damaging the Yuan Qi (Original Qi). The Spleen governs the four limbs; once attacked by Heat, there is no Qi to move them. The Classics state: "Heat damages Qi." It also states: "Heat causes bone emaciation and tendon laxity." This refers to such cases. If it is an external contraction of Wind-Cold, then the Qi of the Kidney and Liver has already exhausted within. The Kidney governs bones and is affected by Cold; the Liver governs tendons and is affected by Wind. Since ancient times, diseases of the Kidney and Liver have been treated similarly because they mutually support each other; thus, the Classics mention that the Gallbladder governs tendons and the Bladder governs bones for this reason. Whether it is Wind-Stroke or Cold Damage, on the day of onset, the patient is bedridden, unable to rise without assistance, suffering from pain in the sinews and bones, and unable to move. This is damage to the physical structure. The Classics state: "Cold damages the form." It also states: "Cold causes sinew spasm and bone pain." This refers to such cases.

To distinguish external contraction from non-aversion to food: if it is due to overexertion, irregular diet, or inappropriate temperature regulation, all three lead to aversion to food. Zhang Zhongjing's *Treatise on Cold Damage* states that patients with Wind-Strike can eat, while those with Cold Damage cannot, but in both cases, the mouth feels harmonious and there is no aversion to food. However, in cases of injury from overexertion, irregular diet, or inappropriate temperature regulation, there is aversion to food, the mouth loses its sense of the five flavors, and one does not perceive the taste of grains. This single distinction is sufficient to differentiate between the two syndromes of internal deficiency and external excess. Although patients with Cold Damage may not be able to eat, they do not have an aversion to food; their mouths feel harmonious, they know the five flavors, and they perceive the taste of grains. This is because, without internal injury, the Heart Qi is peaceful and the Spleen Qi is unobstructed, allowing perception of the taste of grains.

When external Wind-Cold pathogens persist beyond three days, after grain digestion and water metabolism normalize, thirst begins only when the pathogen transmits to the interior. Patients with internal injury from irregular diet, chronic overexertion, or prolonged illness will definitely not experience thirst, as this indicates excess pathogen in the blood vessels. In initial stages of physical overexertion and dietary irregularity, if the injury is severe, thirst will definitely occur because Heart Fire becomes blazing and attacks Metal of the Lung upward, causing thirst. This should also be used for differentiation. Even if the patient desires cold water, it should be given slowly and in small amounts. One must not allow unrestricted drinking, lest excessive water cause rapid downward purging, further weakening Stomach Qi. Mildly, this leads to abdominal distension; severely, it transforms into various diseases, causing recurrent restlessness and agitation, discomfort in the hundred vessels, worsening symptoms at night, inability to sleep peacefully, and these consequences must be anticipated in advance.