FOOD AND AIR TURNED INTO FLESH AND ENERGY 85 



the mouth, the valves in the nostrils close, and it is thus 

 pushed down into the lungs. The muscles in the walls 

 of the body now contract and squeeze upon the air in 

 the lungs, the nostril valves open, and the air is forced 

 out. This method is gradually improved upon in the 

 vertebrates until in the mammals we find a bony basket 

 of ribs and sternum, the thorax (fig. 49), containing the 

 lungs, with two sets of mus- 

 cles between the ribs, which 

 by their alternate contractions 

 and expansions first elevate 

 and extend the ribs, then 

 lower and draw them in, thus 

 enlarging and diminishing (- 

 the thoracic cavity. We find 

 further a muscular partition 

 in the thorax, the diaphrag-m, ^ ^ , , , 



^ ^ Fig. 49. — Tracheal tube, lungs, 

 separating it from the ab- heart, and diaphragm of mouse. 



dominal cavity. When the (F^om specimen.) 

 diaphragm, which is convex on the upper side, contracts 

 it lowers the floor of the thorax, thus enlarging the 

 thoracic cavity; the muscles in the walls of the abdomen 

 then contract and press upon the stomach, intestines, 

 and liver, pushing up the floor of the thorax and so 

 diminishing the thoracic cavity. Thus in two ways this 

 is enlarged, and in two ways diminished. As it enlarges, 

 the pressure of the outside air expands the elastic sacs of 

 the lungs; as it diminishes, the air is pressed out again. 

 Along with great increase of surface and great complexity 

 of mechanism for moving the air goes, as has been 

 pointed out, a perfecting of the circulatory apparatus for 

 bringing the blood to the respiratory surface, and a pro- 

 portionate complexity of the nervous system for producing 

 and regulating the movements necessary. It is to be 

 kept in mind, however, that the respiratory apparatus 



