THE AIR 



i6i 



which also takes place in water. Protoplasm oxidation probably 



depends upon the action of special ferments, or enzyms (see 



section 105). (i) It involves (a) material that can act as fuel, 



and ib) oxygen. (2) It results in the formation (a) of carbon 



dioxid (since all the fuel 



contains carbon), (b) of 



water (oxid of hydrogen, 



since all the fuel contains 



hydrogen), and (c) of other 



oxids, depending upon the 



character of the fuel. 



The familiar fires give off 

 heat and light. Oxidation 

 in protoplasm may also re- 

 sult in other forms of energy. 

 Some of these are motion 

 (as in muscles), electricity, 

 and the peculiar processes 

 that are confined (so far as 

 we know) to nerve and brain 

 cells, such as thinking, wish- 

 ing, suffering, enjoying. 



135. Cell respiration. In 

 an engine the oxidation 

 takes place in the fire box 

 or the cylinder. In a living 

 plant or animal oxidation 

 takes place in every single 

 cell. In plants and animals 

 that consist of very many 

 cells the innermost cells are too far from the surface to get their 

 oxygen directly from the surrounding air or water in this manner. 

 In such cases the air either diffuses through special spaces (in 

 plants) or special tubes (in insects: see Fig. 7), or it travels 

 in a solution (blood) that reaches all parts of the body. In 

 every case, then, the protoplasm of the individual cell ( i ) gets 



Fig. 83. The human lungs 



The arrows show the course of air from the 

 outside, m, mouth; «, nostrils: p. pharynx: 

 /, laryrLx; t, trachea; b, bronchi. The right 

 lung is shown cut open; the bronchi branch 

 again and again, the last tubules ending in 

 delicate expansions, a, the air cells, or sacs; 

 epi, the epiglottis, which closes over the air 

 pipe when food passes from the pharynx to 

 the esophagus, e 



