[…] there is a third product of the same three positions of mute-closure. By dropping, namely, the veil of the palate, which in ordinary utterance closes the passage from the pharynx into the nose, the intonated current of ‘b’, ‘d’, ‘g’ is allowed entrance to the nose and exit there: and the result is the class of nasals ( or “resonants”), ‘m’, ‘n’, and ‘ng’ (as in ‘singing’). Here, though there is closure of the mouth-organs, the tone is so sonorous and continuable that the breach of contact, or explosion, is reduced to a very subordinate value, and the class belongs high up in the alphabet, toward the vowels.
- Whitney (1875), a pag.63

As a general rule (exceptions to it are not common), any language that has either of these three products [surd; sonant; nasal] of a given mute-closure will have also the other two: thus, the presence of a ‘p’ in the alphabet implies also that of a ‘b’ and an ‘m’; and so on.
- Whitney (1875), a pag.63-64

The central classes, of nasals and semivowels, which are least exposed to this general movement [from extremities toward the middle of the alphabetic scheme], are also, on the whole, the least convertible, of the alphabetic sounds.
- Whitney (1875), a pag.70