LUNGS 349 



this animal, and so small the demand for oxygen by the 

 tissues, that a single bubble of air furnishes oxygen enough 

 for a considerable period of time. 



In the frog, which in its earlier stages breathes by gills, 

 we find a much more complex lung than this. Here, in- 

 closed in the thoracic part of the body cavity, are two sacs 

 connected at the throat by a common tube and lined with 

 blood vessels. These sacs have their inner surface raised 

 in many ridges that increase their internal sm-face without 

 increasing the size. As we go higher in the animal scale, 

 these ridges increase in depth, until finally, in mammals, 

 the interior of the lung sac is subdivided into thousands 

 of tiny air sacs connected by tubes to the common trachea. 

 The frog has no diaphragm, and in order to force air into 

 the lungs he first fills the mouth cavity through the nasal 

 passages, then, closing these passages and elevating the 

 throat, forces this mouthful of air down the tube into the 

 lungs. The air is expelled from these sacs by simply con- 

 tracting the body walls. In the frogs, too, we find the first 

 development of the trachea as a voice-producing organ. 



In the reptiles, instead of a ridged sac with a central 

 cavity, the ridges are developed so as to divide the sac into 

 pockets, and these pockets connect with the trachea by 

 tubes. In this case, the two branches of the trachea (the 

 bronchi) extend through the lung sac, and the branches 

 or bronchial tubes are given off to the pockets from the 

 bronchus as lateral branches. 



In birds we find the first development of true air sacs, 



* and a branching system of bronchial tubes similar to ours. 



The bird differs from man in having the lungs connected, 



by extensions, with air cavities in various parts of the body, 



and through these it is able to give the body a buoyancy 



