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Key:
1. skull; 2. cervical vertebrae; 3. furcula; 4. coracoid; 5. vertebral ribs; 6. sternum and its keel; 7. patella; 8. tarsus; 9. digits; 10. tibia (tibiotarsus); 11. fibia (tibiotarsus); 12. femur; 13. ischium (innominate); 14. pubis(innominate); 15. ilium (innominate); 16. caudal vertebrae; 17. pygostyle; 18. synsacrum; 19. scapula; 20. dorsal vertebrae; 21. humerus; 22. ulna; 23. radius; 24. carpus (carpometacarpus); 25. metacarpus (carpometacarpus); 26. digits; 27. alula
The respiratory system of birds differs significantly from that found in mammals. Firstly they have rigid lungs which do not expand and contract during the breathing cycle. Instead an extensive system of air sacs(Fig. 15) distributed throughout their bodies act as the bellows drawing environmental air into the sacs, and expelling the spent air after it has passed through the lungs (Fig. 18). Birds also do not have diaphragms or pleural cavities.
Bird lungs are smaller than those in mammals of comparable size, but the air sacs account for 15% of the total body volume, compared to the 7% devoted to the alveoli which act as the bellows in mammals.
Inhalation and exhalation are brought about by alternately increasing and decreasing the volume of the entire thoraco-abdominal cavity (or coelom) using both their abdominal and costal muscles. During inhalation the muscles attached to the vertebral ribs (Fig. 17) contract angling them forwards and outwards. This pushes the sternal ribs, to which they are attached at almost right angles, downwards and forwards, taking the sternum (with its prominent keel) in the same direction (Fig. 17). This increases both the vertical and transverse diameters of thoracic portion of the trunk. The forward and downward movement of, particularly, the posterior end of the sternum pulls the abdominal wall downwards, increasing the volume of that region of the trunk as well. The increase in volume of the entire trunk cavity reduces the air pressure in all the thoraco-abdominal air sacs, causing them to fill with air as described below.
During exhalation the external oblique muscle which is attached to the sternum and vertebral ribs anteriorly, and to the pelvis (pubis and ilium in Fig. 17) posteriorly(forming part of the abdominal wall) reverses the inhalatory movement, while compressing the abdominal contents, thus increasing the pressure in all the air sacs. Air is therefore expelled from the respiratory system in the act of exhalation.
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