Nearly all modern authorities, including Peter, Gerhardt, and Paul Niemeyer, admit that fluids in the pleural sac assume more or less irregular curves, and not a hydrostatic, horizontal level. Whatever may be the nature and consistence of the effusion, fibro-serous, sero-purulent, or purulent, it does not behave in its distribution as if it were in an open vessel. But few writers, however, have troubled themselves to ascertain the causes of this apparently abnormal condition. They appear to have completely overlooked the facts that had been discovered in regard to the mechanics of the chest in connection with respiration and the circulation. Physiology had shown, especially by Marry's researches, the negative force of the lungs in aspirating the blood from the large venous trunks into the right side of the heart, and thus assisting the whole venous circulation. John Hutchinson16 drew attention to the antagonism existing between the expansion of the chest by muscular action and that of the lungs and the chest-walls. Hyde Salter17 showed that at the commencement of inspiration thoracic elasticity was favorable to inspiration, but as it advanced it became an expiratory force with lung-tension against further expansion. R. Douglass Powell18 drew further attention to these facts in connection with respiration and its modification by disease. Le Gros Clarke19 showed that atmospheric pressure over the abdomen kept the diaphragm in a condition of arched passive tension. He claimed that this negative force resisted the elasticity of the lung, and was the means of retaining the supplemental air in the lung and limiting the encroachment of abdominal organs.
16 Trans. Med. and Clin. Soc., 1846.
17 Lancet, Aug., 1865.
18 Trans. Clin. Soc., 1870.
19 Trans. Roy. Soc., 1872.
Douglass Powell in March, 1876,20 in an elaborate and very suggestive paper on "Some Effects of Lung Elasticity," gives the practical bearing of these physiological facts in clinical medicine, as indicating a better insight as to the true mechanism and relative value in diagnosis of some signs of chest diseases, especially as to the importance of thoracic resilience as a force in respiration.
20 Trans. Roy. Med. and Clin. Soc., vol. lix.
W. H. Stone early in 187721 reported his experiments on sheep as to the amount of negative pressure exerted by the lungs, and concluded that it was equal to four to five inches of water. He moreover showed that even when the effusion was considerable in the pleural cavity, the lung still had contractile force sufficient to support two inches of water, so that to evacuate the fluid it was necessary to use external suction sufficient to overcome this lung-traction. In December, 1877, G. M. Garland22 gave to the public the results of his observations and experiments in regard to the form of the curve of distribution assumed by the pleural fluid, and its causes. He demonstrated that "the lung, by virtue of the strength of its contractility, takes the effusion along with it in its retraction, and that thereby assumes a pneumono-dynamic instead of a hydrostatic level," and that the Ellis curve was the true line of the upper level of the fluid in free, uncomplicated pleuritic effusion. Thus the physical cause of this condition was the retractile force of the lung lifting up the fluid. This is aided by the elastic resistance of thoracic walls and the negative pressure exercised by the effused liquid. The normal line on right side of demarcation between lung and liver is the letter S curve drawn out, the summit being high and the anterior branch correspondingly depressed. The modifications of this normal line in pleuritic effusions represent the effect of the negative pressure of the fluid. The decline in the Ellis curve toward the sternum shows that the elastic energy of the anterior part of the lung is feeble compared with that in the axillary region. "The layer of fluid is of less thickness above than at the base of the lung against the diaphragm. The upper surface takes its shape from the lung, which lifts it up by its retractility, and the effusion by its weight exerts a negative pressure upon the lung. The mass of the fluid is held when in moderate quantity in the supplemental space between the lower border of the lung and the diaphragm" (Garland). The atmospheric pressure from the interior of the lungs and from the exterior of the chest-wall keeps the costal and parietal surfaces of pleura together. Skoda, Powell, Stone, Homolle, and Quincke have shown the retractile energy of the lung, but the credit of drawing especial public attention to it, and of afterward elucidating the subject in its practical application to the study of pleurisy and in putting the whole subject upon a scientific basis, is unquestionably due to G. M. Garland of Boston.
21 London Lancet.
22 Pneumono-Dynamics, Boston, 1878.