502 APPLIED BIOLOGY 



opposed to the use of meats. On scientific grounds there is 

 no known objection to the proper use of meats, but simply 

 an objection to meats in excess of actual protein requirements 

 (i.e., above 60 to 100 grams a day). 



References : Those who are interested in questions of 

 diet should read the chapters on " Nutrition," and " Hygiene 

 of Feeding " in Hough and Sedgwick's " Human Mechan- 

 ism." Also obtain from the Department of Agriculture, 

 at Washington, the bulletins on the nutritive values of 

 foods. 



OXYGEN-SUPPLY 



426. Respiration. This has already been defined as 

 including the functions of obtaining oxygen and eliminating 

 carbon dioxide. In some lower animals (e.g., earthworm) 

 the skin is the respiratory organ ; in fishes and others there 

 are gills; amphibians breathe with both skin and lungs; 

 but in the vertebrates higher than the amphibians lungs 

 are the sole respiratory organs. In all these cases the mem- 

 branes which take up oxygen also give out or excrete carbon 

 dioxide. For greater convenience in study, we shall in this 

 lesson confine our attention to the supplying of oxygen to 

 cells in the human body; and deal with the excretion of 

 carbon dioxide in the next lesson. 



The respiratory organs consist of nasal passages, pharynx, 

 larynx, trachea (windpipe), bronchi (right and left branches 

 of the trachea), bronchial tubes (branches of bronchi), air- 

 chambers at ends of smallest bronchial tubes, diaphragm, 

 and wall of the thorax- or chest-cavity. 



426. Respiratory Passages. We commonly think of 

 the nose as an organ for the sense of smell ; but the fact is 

 that only a limited amount of epithelium (lining membrane) 

 in some of the upper nasal cavities has nerve-endings con- 

 nected with the olfactory part of the brain, and hence has 

 the power of perceiving odors. Most of the cavities in the 



