柳树皮故名思义就是柳树的树皮。古代中国和西方国家，都有把柳树皮入药的记录。柳树皮含有天然的水杨苷，吸收后经酵素转化成水杨酸，更是一种天然的止痛药。 在西方现代制药技术未普及之前，“柳树皮提取液”（Willow Bark Extract）就是当时唯一的止痛药水。
历史上一直有人提到柳树皮的止痛用途。
Throughout history, there have been mentions of the pain-killing uses of willow bark.
有一位剥下了柳树皮，做成一个戒指，丢在我桌上。
One has peeled a willow wand, woven it into a ring, and dropped it on my table.
它来自早在公元前五世纪就被用于民间配方的柳树皮，是这种树皮的一种有效成分。
The active component, willow bark, was used as a folk remedy as long ago as the 5th century BC.
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