= Solution
\b[“Atelectasis pulmonum” means incomplete expansion or collapse of <lung> air spaces; in the newborn it classically describes portions that have not expanded adequately after birth.] Modern <atelectasis> also includes collapse of previously aerated <lung>. The older distinction between primary non-expansion and secondary collapse is useful when reading this paper.
The affected <lung> is airless, often dark and firm, with reduced volume, a depressed surface and little or no crepitation. Failure of inspiratory effort, obstruction of a <bronchus> by secretions, and external compression are different routes to this appearance. Modern physiology adds the role of <pulmonary surfactant> in opposing alveolar collapse; deficiency in premature infants belongs to <neonatal respiratory distress syndrome>, not an explanation the 1867 question itself supplies.<Atelectasis by Merck Manual Professional Edition>[Definition; factors maintaining alveolar expansion; Etiology]{ref}
The main historical confusion was with <pneumonia>. A pre-1867 review of <Charles West>'s work distinguishes primary non-expansion from secondary collapse, and contrasts both with pneumonic <hepatization>. A collapsed region loses air and volume; inflammatory <pulmonary consolidation> fills air spaces with <exudate>. Both can look solid.<Lectures on the Diseases of Infancy and Childhood (review, 1855)>[pp. 109–112, primary atelectasis, secondary collapse and confusion with lobular pneumonia]{ref}
The classical experiment introduces air through the <trachea> or an appropriate <bronchus> while the affected region is observed. Uncomplicated collapsed lobules can expand and regain buoyancy; inflation does not simply remove inflammatory <exudate>. The historical review describes this distinction.<Lectures on the Diseases of Infancy and Childhood (review, 1855)>[pp. 109–111, artificial inflation and buoyancy]{ref} The experiment's interpretation requires more than one observation: whether a region takes air, whether its volume changes, and whether its contents show <inflammation> are different measurements. Cut-surface inspection and <histology> test the last point directly. Floating or sinking alone cannot distinguish all mixtures of collapse, fluid and <pneumonia>, and cannot reliably establish whether an infant was born alive.
Other potential confusions include <pulmonary oedema>, <pulmonary haemorrhage>, and compressed <lung> beside a <pleural effusion> or <pneumothorax>. Fluid or <blood> in the air spaces differs from collapse alone, although the conditions can coexist; external compression can actually cause <atelectasis>. <Pulmonary hypoplasia> is different again: an underdeveloped organ is not merely an otherwise normally formed organ awaiting inflation.<Atelectasis in StatPearls>[Etiology; Differential Diagnosis]{ref}
\b[Reduced aeration is the observation; non-expansion, obstruction, compression and inflammatory consolidation are competing or coexisting explanations.] Gross inspection, controlled inflation and <histology> help discriminate them.
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