《史记·七十列传·扁鹊仓公列传》
到夏天而死的原因，依照病理 病情严重，而脉象正常的是内关病。 内关病，病人不会感到疼痛，好像没有一点儿痛苦，如果再添任何一种病，就会死在仲春的二月；如果能精神愉快顺天养性，能够拖延一季度。
所以至夏死者，脉法曰 病重而脉顺清者曰内关 ，内关之病，人不知其所痛，心急然无苦。若加以一病，死中春；一愈顺，及一时。
The reason for death in the summer is that according to pathology, the disease is serious, and the pulse is normal, it is an internal disease. Internal disease, the patient will not feel pain, as if there is no pain at all, if any other disease is added, he will die in the second month of mid-spring; if he can be happy and follow nature, he can delay for a quarter.