Bencao Yanyi, in twenty volumes, was written by Kou Zong-shi during the Song Dynasty. This book, also known as "The Expanded Materia Medica," was completed in the sixth year of the Zhenghe era (1116) and published in the first year of the Xuanhe era (1119). It aims to elaborate on the meanings not fully explored in Jiayou Bencao and Bencao Tujing, hence the title "Yanyi" (Expanded Meanings). The first three volumes serve as an introduction, discussing medical theories and proposing eight essentials for treating diseases (deficiency, excess, cold, heat, evil, proper, internal, and external), revising the traditional "four properties" (cold, hot, warm, cool) to "four properties," and offering new insights into diagnosis, health maintenance, and preparation of medicines. Volumes four to twenty are dedicated to "specific discussions," with classifications and arrangements similar to Jiayou Bencao, but excluding drugs that are "named but not used." The book covers 470 types of drugs, omitting those already thoroughly discussed in Jiayou Bencao with no further elaboration needed. Written in a note-like format, it extensively covers topics such as the origin, morphology, harvesting, identification, processing, preparation, properties, efficacy, indications, and contraindications of drugs. Its identification of Chinese medicinals and exploration of pharmacology are particularly noteworthy, correcting many previous errors and significantly influencing the development of pharmacology in later generations. Li Shi-zhen of the Ming Dynasty commented, "It references facts, verifies their properties and principles, and extensively introduces dialectical methods, making numerous valuable contributions."