THE RESPIRATORY APPARATUS 199 



(Fig. 61, C). At this point the ciliated epithelial cells give way to 

 simple squamous epithelium. 



Thus we find three kinds of bronchi, or their ramifications, as 

 follows: the primary, the secondary, and the tertiary. 



The Air-sacs (Fig. 6 1 , A ) . The air-sacs are bladder-like structures 

 consisting of a delicate cellulo-serous membrane, an extension from 

 the bronchial tubes, in some places strengthened by an external 

 envelope of elastic fibrous tissue. Long thin blood-vessels are dis- 



6 



FIG. 61, A. Diagram of air-sacs and their location, i, The proximal end of the 

 humerus. 2, The proximal end of the right clavicle. 3, The cervical air cell. 

 4, The right coracoid bone. 5, The anterior thoracic air cell. 6, The right side 

 of the sternum. 7, The right side of the liver. 8, The peritoneum. 9, The 

 right abdominal air cell. 10, The coccyx, n, The proximal end of the right 

 femur. 12, The right supero-posterior air-sac. 13, The right infero-posterior 

 air-sac. 14, The right lung. 15, The axillary extension of the air-sac. 16, The 

 obturator foramen, 17. The pelvis. 



tributed in the substance of these walls. They are branches from 

 vessels of the general circulation and not extensions from those of 

 the lungs. No lymphatics have been found in the air-sacs. 



These sacs do not communicate with each other and normally 

 they are not fully inflated. In some locations they extend into the 

 bones and are in communication with the extensions of the bronchial 

 tubes. In fact, by some anatomists they have been called " bladder- 

 like, extra-pulmonary expansions of the bronchial tubes, free from 

 cartilage." The air-sacs make the bird's body lighter, thus making 



