漢語音韻的歷史發展從宋代開始以至於明代，是一個關鍵性的階段，現代方言的很多重要音韻現象，都可以往前追溯至宋代，更進一步還能聯繫到明代的韻書和韻圖。其中，「介音的演化」可說是近代語音發展中，牽動漢語聲母、韻母甚至是影響整個音節結構的重要動力，因此，若能釐清漢語介音的發展脈絡，勢必有助於了解從中古後期的宋代到近代漢語語音的變遷。中古洪音到現代漢語變成細音的，大致可以歸納出一個比較嚴格的規律：凡是中古二等開口舌根（牙喉音）字，現代漢語在主要元音和聲母中間增添了一個i 音位，例如：江kɔŋ>ʨiaŋ、巷 ɣɔŋ>ɕiaŋ。早在元代《中原音韻》裡就把「江」字與三等「姜」並列，把「庚、更、羹、耕」與三等「京、驚、荊、兢」及四等「徑、涇」等字混列；此外，江陽韻中二等的「講、港」也與三等的「鏹」並列，可見中古二等開口舌根（牙喉音）字，由洪變細的現象於元代早已發生。故本文研究材料，起自北宋的《四聲等子》，擴及近代官話先驅《中原音韻》（西元1324年），乃至明代中葉《韻略易通》（西元1442 年）與明末葉（西元1642 年）《韻略匯通》，從這四本韻書對於二等開口牙喉音字的歸類，探討這類字在宋代以至於明代末葉產生了怎樣的變化。這個問題不僅反映了共同語中i 介音增生與發展的脈絡，同時在漢語音韻史上也占有重要的地位。

The historical period from Song Dynasty to Ming Dynasty is critical for the development of Chinese phonology. Many phonological phenomena in modern dialects can date back to Song Dynasty. During that time, diachronic sound changers across dialects can be observed, and found in the rhyme books and rhyme tables in Ming Dynasty. This paper aims to examine the special medial [i] which involves composition of onsets, rhymes and syllables. I will mainly focus on the diachronic alternations of the medial [i] so as to tease out the historical phonological changes since Song Dynasty. The medial [i] which is absent in Middle Chinese may appear in Modern Chinese, when the onset is the second grade (i.e. category) of velars, as shown in example (1) and (2): (1) kɔŋ ＞ ʨiaŋ jiang (江) 'big river' (2) ɣɔŋ ＞ ɕiaŋ xiang (巷) 'alley' Other examples include words such as jie (皆) 'all,' jie (佳) 'good,' jian (間) 'room' and jan (眼) 'eyes'. One of the Chinese rhyme books in Yuan Dynasty, Zhong Yuan Yin Yun, made no difference among the second grade of velars (e.g. jiang 江 'big river', jiang 講 'speak', kang 港 'harbor') and the third grade (e.g. jing 京 'capital', jing 驚 'surprise', jiang 姜 'person's name') and the fourth grade (e.g. jing 徑 'route', jing 涇 'name of a river'). Other two rhyme books (c.a. 1442-1642 A.D) in Ming Dynasty also report a similar pattern of the medial [i]. Taking examples from Four-Sound Deng Zi in Song Dynasty, Zhong Yuan Yin Yun (1324 A.D) in Yuan Dynasty, Yun Lue Yi Tong (1442 A.D) and Yun Lue Hui Tong (1642 A.D) in Ming Dynasty, this paper aims to examine the development of the medial [i] and its interplay with structures of onsets, rhymes and syllables.