ANIMAL PHYSIOLOGY 59 



upon in the vertebrates until in the mammals we 

 find a bony basket of ribs and sternum, the thorax (fig. 

 20), containing the lungs, with two sets of muscles 

 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 diaphragm, separating it from the abdominal 

 cavity. When the 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 wails 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 go, as has been pointed 

 out, a perfecting of the circulatory apparatus for bringing 

 the blood to the respiratory surface, and a proportionate 

 complexity of the nervous system for producing and regu- 

 lating the necessary movements. It is to be kept in mind, 

 however, that the respiratory apparatus only brings oxygen 

 to the respiratory surface, and before the real respiration 

 at the tissue-cell can take place the oxygen must be carried 

 by the blood to the cell. This process we shall later discuss 

 under the head of circulation. 



Now having seen how animals get the necessary oxygen 

 we may next inquire how they obtain and make use of 

 the equally necessary substances to be oxidized and to 

 build the body out of, that is, their food. 



How animals obtain and digest food. Amoeba eats 

 without a mouth. It extends any part of its soft body 

 over the little plant or animal it feeds upon. In many 



