李杲曰。古人以脉上辨内外伤于人迎气口。人迎脉大于气口为外伤。气口脉大于人迎为内伤。
此辨固是。犹有未尽耳。外感风寒。皆有余之症。是从前客邪来也。其病必见于左手。左手主表。
乃行阳二十五度。内伤饮食。及饮食不节。劳役不节。皆不足之病也。必见于右手。右手主里。乃
行阴二十五度。故外感寒邪。则独左寸人迎脉浮紧。按之洪
大。紧者。急甚于弦。是足太阳寒水之脉。按之洪大而有力。见乎少阴心火之脉。丁与壬合。内
显洪大。乃伤寒脉也。若外感风邪。则人迎脉缓而大于气口一倍。或两倍。或三倍。内伤饮食。则
右手气口脉大于人迎一倍。伤之重者。过在少阴及两太阴。则三倍。此内伤饮食之脉。若饮食不节。
劳役过甚。则心脉变见于气口。是心火刑肺。其肝木挟心火之势亦来薄肺。经云。侮所不胜。寡
于畏者是也。故气口脉急大而涩数。时一代而涩也。涩者。肺之本脉。代者。元气不相接。脾胃不
及之脉。洪大而数者。心脉刑肺也。急者。肝木挟心火而反克肺金也。若不甚劳役。惟右关脾脉大
而数。谓独大于五脉。数中显缓。时一代也。若饮食不节。寒温失所。则必右关胃脉损弱。甚则隐
而不见。惟内显脾脉之大数微缓时一代也。宿食不消。则独右关脉沉而滑。经云。脉滑者。有宿食
也。以此辨也。岂不明白易见乎。
李杲说：古人通过比较人迎脉和气口脉来辨别内伤和外感。人迎脉比气口脉大，属于外感病；气口脉比人迎脉大，属于内伤病。
这种辨析固然正确，但还有不够详尽的地方。外感风寒，都属于有余（实证）的病证，这是由外来邪气侵袭所致。其病症必然表现在左手。左手主表，对应阳气运行二十五度。内伤于饮食，以及饮食不节制、劳役过度，都属于不足（虚证）的病证。其病症必然表现在右手。右手主里，对应阴气运行二十五度。
因此，外感寒邪时，唯独左寸人迎脉浮紧，重按之下洪大。紧脉，是比弦脉更急迫的脉象，这是足太阳膀胱经寒水之脉的表现。重按感觉洪大且有力，显示出少阴心火之脉的特征。丁（心火）与壬（膀胱寒水）相合，内部显现出洪大的脉象，这就是伤寒的脉象。
如果外感风邪，则人迎脉缓而比气口脉大一倍、两倍或三倍。如果是内伤饮食，则右手气口脉比人迎脉大一倍。伤势较重者，病位在少阴及太阴经，则大三倍。这就是内伤饮食的脉象。
如果是饮食不节，劳役过度，则心脉的变化会反映在气口上。这是心火克制肺金（火刑金）。肝木挟带心火的势头也来侵犯肺金。医经说：“侮所不胜，寡于畏者”，指的就是这种情况。所以气口脉急大而涩数，偶尔出现代脉且带有涩象。涩脉，是肺的本脉；代脉，是元气不相接续、脾胃之气不及的脉象；洪大而数，是心脉克制肺金的表现；急脉，是肝木挟带心火反过来克制肺金的表现。
如果劳役不是很严重，只有右关脾脉大而数，谓其单独大于其他五部脉，在数脉中显现出缓象，偶尔出现代脉。如果是饮食不节，寒温失调，则必然导致右关胃脉损弱，严重时隐伏不见，只在内部显现出脾脉的大数微缓并偶尔出现代脉。如果有宿食不消化，则唯独右关脉沉而滑。医经说：“脉滑者，有宿食也”。以此来判断，难道不是明白易见吗？
Li Gao stated: The ancients distinguished between external injuries (exogenous diseases) and internal injuries by comparing the Renying pulse (pulse at the carotid artery, representing Yang/exterior) and the Qikou pulse (pulse at the radial artery, representing Yin/interior). When the Renying pulse is larger than the Qikou pulse, it indicates an external injury. When the Qikou pulse is larger than the Renying pulse, it indicates an internal injury.
This differentiation is indeed correct, yet it remains incomplete. External contraction of Wind-Cold (Wind-Cold pathogenic factors) constitutes a syndrome of excess (repletion), originating from invading exogenous pathogens. Such pathology necessarily manifests in the left hand. The left hand governs the exterior (Biao), corresponding to the circulation of Yang Qi for twenty-five degrees. Internal injury due to diet, as well as irregular diet and overexertion/labor, constitute syndromes of deficiency (deficiency). Such pathology necessarily manifests in the right hand. The right hand governs the interior (Li), corresponding to the circulation of Yin Qi for twenty-five degrees.
Therefore, upon external contraction of Cold pathogens, only the left Cun position of the Renying pulse appears floating and tight (Jin), and when pressed deeply, it feels洪大 (Hongda - broad and large). A tight pulse is more urgent than a wiry (Xian) pulse; this is the pulse of the Foot-Taiyang Bladder channel (Cold Water). When pressed, it feels Hongda and forceful, revealing the characteristics of the Shaoyin Heart Fire pulse. Ding (Heart Fire) combines with Ren (Bladder Cold Water); internally manifesting a Hongda quality, this is the pulse of Shanghan (Cold Damage).
If there is external contraction of Wind pathogens, the Renying pulse will be slow (Huan) and one, two, or three times larger than the Qikou pulse. In cases of internal dietary injury, the right-hand Qikou pulse will be one time larger than the Renying pulse. In severe cases, involving the Shaoyin and both Taiyin channels, it will be three times larger. This is the pulse pattern of internal dietary injury.
If there is irregular diet or excessive overexertion, the Heart pulse changes will manifest at the Qikou. This represents Heart Fire punishing Lung Metal (Fire insulting Metal). Liver Wood, carrying the momentum of Heart Fire, also invades Lung Metal. As the classics state: "Insulting what it cannot overcome, lacking fear," refers to this situation. Thus, the Qikou pulse becomes urgent, large, choppy (Se), and rapid (Shu), occasionally presenting a intermittent pulse (Dai) with a choppy quality. A choppy pulse is the intrinsic pulse of the Lung; an intermittent pulse indicates a lack of connection among Yuan Qi (Original Qi) and insufficiency of Spleen-Stomach Qi; Hongda and rapid qualities indicate Heart Fire punishing Lung Metal; an urgent quality indicates Liver Wood carrying Heart Fire to counter-attack Lung Metal.
If the overexertion is not severe, only the Pi (Spleen) pulse at the right Guan position will be large and rapid, being uniquely larger than the other five positions, appearing slow within the rapidness, occasionally presenting an intermittent pulse. If there is irregular diet and improper regulation of cold and warmth, the Wei (Stomach) pulse at the right Guan position will necessarily become damaged and weak, and in severe cases, may vanish entirely, leaving only the internal manifestation of a large, rapid, slightly slow Spleen pulse with occasional intermittent beats. If food stagnation fails to digest, only the right Guan pulse will be deep (Chen) and slippery (Hua). The classics state: "A slippery pulse indicates food stagnation." By using this method of differentiation, is it not clear and easy to observe?