Judging from some recent publications on physical examination of the lungs, there seems to be a want of appreciation of the nicely balanced relation between the muscles of inspiration and bilateral extensibility of the lungs.
All the physical signs we seek in lung examinations are directed toward the study of three attributes; namely, volume, density and extensibility. Extensibility is the quality that is first modified in all diseases of the pleura, parenchyma and bronchial tree. Alterations of volume and density of the lungs come as later changes in disease. The student of pulmonary physical signs will find inspiratory extension of the thorax a very rich field for clinical study. For number and variety of signs and satisfaction in clinical observation, the interrelations between the actions of inspiratory muscles, the excursions of different parts of the pulmonary boundaries and the extensibility of the lung are more productive sources than any other
C. F. HOOVER. THE IMPORTANCE OF THE PATIENT'S POSITION IN STUDYING LUNG EXCURSION. JAMA. 1923;81(19):1602–1603. doi:10.1001/jama.1923.02650190032010