134 



ANIMAL PHYSIOLOGY. 



while in birds there are no longer any dilated cavities, but 

 every air tube is surrounded with a system of air cells, com- 

 pletely separated by septa of connective tissue from those 

 which surround others. In the lungs of man and other 

 mammals, the air tubes go on dividing and subdividing till 

 they terminate in minute tubules, which open into irregular 

 passages, surrounded with air cells : and in the manner of 

 their development they exhibit both the modes of increase 

 in complexity observed in the zoological series; for they take 

 origin in the embryo as a pair of simple pouches coming off 

 from the throat, and these branch out into smaller pouches 

 budding in like manner till lobules are formed, which, 

 instead of branching outwards, have septa growing inwards 

 into their cavities. 



73. DUCK'S LUNG; section, magni- 

 fied twenty diameters. 



Pig. 72. FROG'S LUNG, 

 opened from behind, 

 enlarged. 



98. The main air tube, the trachea or windpipe, is about four 

 and a half inches long, and is surmounted by the larynx, the 

 part in which the glottis is placed, and which constitutes the 

 organ of voice. The constant patency of the trachea is 

 insured by a series of cartilaginous hoops, which in some 

 animals form complete rings, but in the human subject are 

 deficient behind. This tube divides below into a right and 

 left bronchus of similar structure; and each of these divides 



