The Choirmaster's Manual/Chapter 2
CHAPTER II
ON BREATHING
The basis of true vocal tone is the management of the breath. It is not the amount of breath taken, but the amount controlled, that tells; but develop the lungs by every means.
Don't spend too much time learning to control the breath alone; take vocal exercises in conjunction with breathing-exercises.
The act of breathing is passive rather than active.
The system known as "lateral-costal" is the method used with the greatest success, and the exercises in this book are based on that method.
All breathing should be deep (abdominal), and while there should be absolutely no rigidity, the chest should be raised and remain so throughout the musical phrase; the shrinking should take place at the waist-line.
The lateral-costal method may be described as the system of "fixation of the diaphragm, and expansion of the sidewalls of the chest." 
In explanation of this it may be useful to remark, that many teachers speak of the "inversion" or "lowering" of the diaphragm when inhaling breath. The diaphragm is a muscular shield extending across the body, separating the breathing apparatus from the digestive organs, and forming the floor of the lungs. A. Lungs. B. Diaphragm.
In other words, the expansion of the lungs when full, according to this method, forces the diaphragm from a convex to a concave position, thereby allowing room for lung-expansion, at the same time pressing down the viscera. Dr. Hulbert, in his able treatise on breathing, says, "It is a physical impossibility for the diaphragm to be lowered and the ribs expanded sideways at one and the same time." Therefore, as the system of lowering the diaphragm makes it necessary, as the breath is expended, for it to resume its original position, and as there is no direct way to govern the even movement of this muscular shield, it often trembles, and, communicating this trembling to the lungs (which are attached to it ), it produces that nervous, bleating vibrato, so distressing to hear. It is obvious, then, that in fixing the diaphragm we gain doubly; first, by obtaining a greater "side-rib" expansion; secondly, by avoiding the tremolo.
Exercise 1.
Stand steady on both feet, with chest expanded; slightly draw in front wall of abdomen (this fixes the diaphragm), and without wriggling or sniffing slowly draw breath through the nose until lungs are well expanded. Exhale slowly through mouth. (In drawing breath through the nose the air is both warmed and filtered.)
Exercise 2.
Same as No. 1, but place hands on side-ribs and concentrate thought on their expansion when inhaling. (Choirmaster should watch the expansion, and measure boys with a piece of string or tape, and note increase after a week or so of practising.) When lungs are full, exhale explosively through mouth; boys will easily feel the sudden collapse of their expanded ribs and realize the extra space occupied by the lungs when full.
See to it that the shoulders do not rise. If they do, it is a sign of clavicular breathing, which is entirely wrong and injurious.
Exercise 3.
If shoulders do rise, practise Exercise 2, but instead of placing hands on ribs, sit in chair and firmly grasp the underside of seat with both hands; the shoulders are fixed by this means, and cannot rise.
N.B. Insist incessantly that the preliminary to all breathing is the slight indrawing of the front wall of the abdomen, and be careful that too much effort is not expended on it.