Analysis of information sources in references of the Wikipedia article "Donkey-hide gelatin" in English language version.
但今牛皮膠製作不甚精,只可膠物,故不堪入藥也。[...]大抵古方所用多是牛皮,後世乃貴驢皮。[Yet today's beef-gelatin is not made carefully and can only be used to glue things, hence it's not good for medicine. [...] Most of the ancient formulas probably use beef-skin and prizing donkey-skin came later.]