The relationship between the action of the diaphragm and the intercostal muscles, as well as the muscles of less importance, in making respiration possible by rhythmically enlarging and decreasing the volume of the thoracic cavity has been the subject of discussion by clinicians, surgeons and physiologists. Many valuable observations have been made which, at the present time, form a background for further observation and a fund of information from which to draw, as need arises, during the progress of research. In time, certain fixed opinions become accepted as facts, often because they are stated with emphasis, owing to the conviction of their authors. As truth is relative and corresponds with the best thought of a period, these opinions are true unless they can be proved incorrect, inaccurate or incomplete. For one to replace an old truth by a wider and more accurate conception of fact requires that the observer make