from The Century Dictionary.
- noun In ornithology, a large air-cell; an air-space, air-sac, or pneumatocyst.
Sorry, no etymologies found.
Fish are said to rise and sink in the water by altering the volume of an internal air-receptacle; and there may be many ways science, as yet, knows nothing of, by which we, who live at the bottom of an ocean of air, may do the same thing.
Now, in the vertebrated creature, the chest is merely the grand air-receptacle into which the blood is sent to be aërated; while in the insect, the chest contains but its own proportional share of the great air-system.