The term middle voice is one that you don't hear of anymore, at least not in the way in which I am addressing here. To better understand it, one has to think of how the voice was perceived, that is, heard, before Manuel Garcia discovered that there were two distinct mechanisms in the larynx, one that presses the vocal folds together and shortens them, while the other lengthens and thins them. It is not uncommon for singers and voice teachers to associate these two mechanisms with registration, that is, the aforementioned head and chest voice. However, two hundred years ago, it wasn't so clear what was happening in the singer's throat and voice teachers judged matters accordingly. What did these bel canto trained voice teachers hear? That a clear /a/ vowel resonated in the chest, while a clear /u/ resonated in the head. And a clear /i/ vowel? It was heard as resonating on the hard palate and in the face, that is, forward, via a full open throat (this pre-supposes Italianate vowels, clear and vibrant sounds, and not guttural, veiled or nasal ones). Empirically speaking, this tonal quality - chiaroscuro par excellence - was thought of as the middle voice because it was between the chest and head. Its resonance seat was that of the larynx.