The changes of the respiratory pressure within the esophagus, as well as within the posterior mediastinum, differ greatly in their various levels. They are best in the section between the heart and the diaphragm; they are moderate from the upper aperture of the thorax to above the tracheal bifurcation, and they are. very much reduced in the region of the bifurcation and the heart. In the latter case the normal respiratory changes are reduced in their transmission to the mediastinum by the intervention of the inelastic tissues of the bifurcation and the heart. The difference in the changes of the respiratory pressure between the low^er and the upper part of the mediastinum is due to a difference in the respiratory changes of pressure between the lower and upper parts of the lungs. The changes in the respiratory pressure which begin in the lower part of the lungs lose some of their force on their way to the apices of. the lungs. In artificial respiration by intratracheal insufflation there is no difference in the respiratory pressures between the upper and the lower parts of the lungs.
This investigation supports the view, frequently entertained by clinicians, that the respiratory changes in the apices of the lungs are not as good as those of the rest of the lungs, and it disproves the claim of some physiologists that a decrease or increase of pressure at any part of the lungs must be equally distributed through all parts of the lungs, a claim based upon merely a priori physical considerations.