I've always wondered why「ん」is the only kana without having a vowel sound attached to it. The only thing I've ever heard is that 「ん」 is a special kana that can be the last kana in some words.
I've also heard something about a discrepancy between ん and む regarding 変体仮名. Does this factor into the reason at all?
Japanese only allows gemination and nasal as the coda of a syllable. The two kanas っ and ん correspond to them, and are the kanas without a vowel. Also, when a kana is followed by a glide such as in ゃ, ゅ, or ょ, it loses the vowel. For example, In きゃ, き only represents k, not ki.
In general, languages disfavor coda, and nasal is among the sounds that can easily become a coda because the mouth is closed. For example, Chinese and Korean also allow a limited variety of coda, among which are nasal sounds.
In Classic Japanese, the vowel u in mu often dropped.
風吹かむとす → 風吹かんとす
I think Sawa nailed it, I just want to add a few things that I think are interesting. There is apparently controversy in the syllable-based model of Japanese prosody. This paper Questioning the universality of the syllable : evidence from Japanese* (Labrune, 2012) where, just taking from the abstract:
whereas the mora and the foot are indisputably present and active, the evidence for the syllable is inconspicuous and disputable. Building on this observation, I claim that Tokyo Japanese makes no use of the syllable. Instead, two types of mora are distinguished: regular CV moras and weak (deficient) moras. Weak moras include the moraic nasal, the first part of a geminate and the second part of a long vowel, as well as moras containing an onsetless vowel, a devoiced vowel or an epenthetic vowel.
Then taking from her paper:
The linguistic tradition among Japanese has never had the need to refer to a unit such as the syllable in opposition to the mora in accounts of Tokyo Japanese. A number of modern Japanese phonologists belonging to the traditional mora-model school of Japanese linguistic analysis continue this tradition. Some of them (for instance 現代日本語方言辞典(Hirayama, 1992) and 現代日本語の発音分布(Sato, 2002)) even operate with an explicit distinction between mora-based and syllable-based dialects. Dialects such as Aomori and Akita (North Honshu), and Kagoshima (South Kyushu), are held to be indisputably syllabic, while others, for instance the Osaka dialect and the Izu dialect (south of Tokyo) are clearly moraic, and are considered to be such even by some advocates of a syllabic analysis of Tokyo Japanese.
It is actually Tokyo Japanese (Standard Japanese), the most extensively studied dialect, which poses a problem with regard to classification as a mora or syllable dialect, as it has been analysed both as only moraic and as syllabic with moras.
And I think most importantly:
Moreover the linguistic facts that support the relevance of the syllable in Tokyo Japanese can always receive an alternative, syllable-free account, which works at least as well as the syllabic analysis.
I haven't really read into this topic, so I don't have much opinion on the matter. Just thought it would be interesting to point out.
Also, the geminate /Q/ and nasal /N/ consonants are not the only vowelless mora-long segments in Japanese. [だめっ]! has the transcription /dameQ/ where /Q/→[ʔ] the glottal stop consonant. /Q/ is a phoneme and has mora-long duration, but [ʔ] is not distinctive.
