The following comes from Liza Lehmann regarding Head voice:
One of the ‘secrets’ of successful placement and preservation of the voice is, I am yearly more and more convinced (especially in the case of soprano and mezzo-soprano voices), the proper use of the “Head voice,” and the capacity to use it throughout the whole compass.
I had the great privilege, when a young girl, of being allowed to attend the lessons given at her own house by the great Jenny Lind to her pupils from the Royal College of Music. She had then long retired from her absolutely unique career, and although a woman of advanced years, she would illustrate with her own still phenomenal voice many marvellous vocal effects, and one which I particularly remember was the power of using “Head voice” throughout her entire compass, even to its lowest notes, when she DESIRED to do so.
It is a well-known fact that “Head voice” requires less breath than any other; and though it is thinner than “Chest voice” and so-called “Medium voice” it is actually more carrying.
Jenny Lind used it sometimes for pianissimo effects in the lower registers with the most thrilling results – a pianissimo that seemed to creep up to you, and could be drawn out to an almost incredible length.
…At the risk of repetition I contend, in the light of accumulated experience, that this capacity to use Head tones everywhere brings health to the voice and preserves it…
I have proved the helpfulness of this again and again by passing on the hint to singers who have come to rehearse with me, and who have benefited in a most marked manner.
In that most scientific book “How To Sing,” by my celebrated namesake, Madame Lilli Lehmann, the great exponent of Wagner roles, I was recently very delighted that she shares my conviction on this subject [the usage of the head voice].