3. The ribs pass from a more oblique to a less oblique position, and may become almost horizontal; their upper edges are also turned out slightly, though this is not so easy to observe.
4. Again, in the case of a very deep and sudden inspiration, the abdomen and the lower ribs also are drawn inward.
The changes above referred to are brought about in this way:
1. The total enlargement is due to the action of many muscles which function in harmony with each other.
2. The chief changes are brought about by those muscles attached between the ribs (intercostales); but these act more efficiently owing to the coöperation of other muscles which steady the ribs and chest generally, such as those attached to the shoulder-bones and the upper ribs; indeed, the most powerful inspiration possible can only be effected when most of the other muscles of the body are brought into action. One may observe that even the arms and legs are called into requisition when a tenor sings his highest tone as forcibly as possible, though this is often overdone in a way to be condemned. Art should not be reduced to a gymnastic feat.
The most important muscle of inspiration is the diaphragm, or midriff, because it produces a greater change in the size of the chest than any other single muscle. Some animals can get the oxygen they require to maintain life by the action of this large muscle alone, when all other respiratory muscles are paralyzed. As it is so important, and above all to the voice-user, it merits special consideration.
In studying the action of a muscle it is necessary to note its points of attachment to harder structures, either bone or cartilage. Nearly always one such point is more fixed than the other, and from this the muscle pulls when it contracts.
The diaphragm is peculiar in that it is somewhat circular in shape and is more or less tendinous or sinew-like in the middle. Being attached to the spinal column behind and to the lower six or seven ribs, when the muscle contracts it becomes less domed in shape—less convex upward—and of course descends to a variable degree depending on the extent of the muscular contraction. As to whether the ribs, and with them the abdominal muscles, are drawn in or the reverse, is determined wholly by the degree of force with which the contraction takes place and the extent to which it is resisted. Throughout the body muscles are arranged in sets which may either coöperate with or antagonize each other, as required. The forcible bending of one's arm by another person may be resisted by one through the use of certain muscles. In this the action of the muscles which bend the arm is imitated by the agent seeking to perform this movement for us. The muscles acting in opposition to certain others are said to be their antagonists.
Were the diaphragm to contract moderately the ribs would be but little drawn in, even if no muscles acted as antagonists. But, as a matter of fact, this domed muscle descends at the same time as the ribs ascend, because of the action of the muscles attached to them. The diaphragm being concave below toward the abdomen, the contents of this cavity fit closely to its under surface. There are found the liver, stomach, intestines, etc.—a part of great practical importance, as will be shown presently.
Naturally, in breathing, the organs of the abdomen, especially those above, are pressed down somewhat with the descent of the diaphragm in inspiration, and, in turn, push out the abdominal walls. If, however, the midriff contract so powerfully that the lower ribs are drawn inward, the abdominal walls follow them. Although the actual extent of the descent of the diaphragm is small in itself, since the total surface is large it effects a very considerable enlargement of the chest in the vertical diameter.