What is "lung sac"

Lung \Lung\ (l[u^]ng), n. [OE. lunge, AS. lunge, pl. lungen; akin to D. long, G. lunge, Icel. & Sw. lunga, Dan. lunge, all prob. from the root of E. light. [root]125. See Light not heavy.] (Anat.) An organ for a["e]rial respiration; -- commonly in the plural.

My lungs began to crow like chanticleer.
--Shak.

Note: In all air-breathing vertebrates the lungs are developed from the ventral wall of the esophagus as a pouch which divides into two sacs. In amphibians and many reptiles the lungs retain very nearly this primitive saclike character, but in the higher forms the connection with the esophagus becomes elongated into the windpipe and the inner walls of the sacs become more and more divided, until, in the mammals, the air spaces become minutely divided into tubes ending in small air cells, in the walls of which the blood circulates in a fine network of capillaries. In mammals the lungs are more or less divided into lobes, and each lung occupies a separate cavity in the thorax. See Respiration.

Lung fever (Med.), pneumoni

1. Lung flower (Bot.), a species of gentian ( Gentian Pneumonanthe).

   Lung lichen (Bot.), tree lungwort. See under Lungwort.

   Lung sac (Zo["o]l.), one of the breathing organs of spiders and snails.