The sinking of the base of the cavity is accomplished by the contraction of the diaphragm. This muscle is roughly circular in outline. In its relaxed state it rises to a dome in the middle and might be compared in shape to an inverted bowl. As it contracts the diaphragm flattens out and increases the length of the vertical axis of the chest cavity. In its descent it pushes the viscera downward and forces the abdomen to protrude slightly. This manner of breathing is commonly called abdominal, although the muscles of the abdomen are not directly concerned in the filling of the lungs. The expansion of the chest cavity may be accomplished by the raising of the ribs. Owing to their curved shape and sloping position, a slight elevation of the ribs causes them to rotate outward from the central axis of the chest cavity. This increases the horizontal diameter of the cavity in every direction--forward, backward, and sidewise. In the judgment of the most competent investigators the best form of breathing combines the two muscular actions just described. At the start of the inspiration the diaphragm descends, but the protruding of the abdomen is checked almost instantly by a contraction of the abdominal muscles. The inspiration is then completed by the lateral expansion of the chest. In this manner the lungs are filled to their greatest capacity in the least possible time. Another great advantage of this type of breathing is that it imparts a peculiarly erect and graceful carriage, a matter of much importance to the singer and public speaker. In the action of expiration following the taking of a full breath in the manner just described two sets of muscles are involved. These are: first, the abdominal muscles, which push the diaphragm back to its original position; second, the muscles which lower the ribs (the internal intercostals and some of the external abdominal muscles). The actions of both these sets of muscles can easily be observed. When correctly performed the two actions are simultaneous, the whole chest cavity being gradually and uniformly contracted. A highly important feature of correct breathing, in the opinion of most authorities, is the control of the expiration. In the first place, all the expired air must be converted into tone and none of it allowed to escape unused. Further, the vocal cords must not be exposed to the full force of a powerful expiratory blast, as they are too small and weak to withstand this pressure without strain and injury. The air must be fed to the vocal cords in just sufficient quantity to serve the purposes of tone production.