To the Editor.
—Snider1 has restated and convincingly supported the fact that tall, lean tuberculin reactors have a higher risk for subsequent active tuberculosis than more rotund individuals and that poor nutrition is not to blame.West2 has shown that alveoli in the upper zones of the upright lung are exposed to large, expanding stresses due to gravity, which may predispose to the formation of blebs at the superior margin of the lung. Particularly in lean, tall individuals, these blebs can form and rupture, causing spontaneous pneumothorax. Gravity also affects blood flow, which is diminished in the upper lung zones.3 Pulmonary artery pressure has to overcome the gravitational pressure of a column of blood extending to the apexes in the upright lung; the greater the lung height, the more difficult it would be to perfuse the upper lung zones. Low pulmonary blood flow causes the alveoli to