| 释义 |
air sac
noun
1. : a cavity in the pollen grain of pines
2.
a. : one of the spaces in different parts of the bodies of birds that are air-filled and connected with the air passages of the lungs and usually with cavities in the bones and that serve to reduce the specific gravity of the body and assist respiration by the great increase of surface thus exposed to the air as well as by providing a space for the residual air so that the lungs may be completely filled with fresh air at each respiration
b. : paired bladderlike backward extensions of the branchial cavity of certain Indian fishes that are lined with vascular tissue and serve as accessory breathing organs
c. : the internal subumbrellar portion of the specialized inverted pneumatophore in siphonophores that contains air and keeps the colony afloat
3. : a thin-walled dilation of a trachea occurring in many insects
4. : a pulmonary alveolus |