EXTENT OF THE RESPIRATORY EXCHANGE OF GASES. 235 



the respiratory activity and the excretion of carbon dioxid diminish, 

 while the pulse remains nearly constant. It has been shown that when 

 there is a sudden change from cold to warm surroundings, the carbon- 

 dioxid output diminishes considerably; and, conversely, when the 

 change is from warm to cold, the excretion increases considerably. 



6. Muscular Exertion produces a considerable increase in oxygen- 

 consumption and carbon-dioxid elimination, which, for instance, may 

 be three times as great in walking as in a quiet, recumbent position. 

 Every kilogrammeter supplies 3^ milligrams of carbon dioxid; therefore, 

 each additional gram of carbon dioxid formed is the equivalent of 300 

 kilogrammeters. The establishment of a certain degree of tension in 

 the muscles requires more metabolic change than the maintenance of 

 this tension. 



The increase in the interchange of oxygen and carbon dioxid begins almost 

 immediately after the work commences. In a few minutes it attains a constant 

 height of at most from seven to nine times the amount during rest. After the 

 work is finished, the consumption of oxygen falls in from 3 to 15 minutes to 

 the rate during rest. The respiratory quotient remains essentially unchanged 

 during work. During light work there is relatively a little more oxygen consumed 

 than during heavy labor. The production of carbon dioxid is diminished with 

 practice, that is, with a more economically applied exertion of the muscles. 



The gaseous exchange is to a certain extent under the influence of the vagus 

 nerve, which in part inhibits and in part accelerates the heart's activity. Irrita- 

 tion of this nerve may produce a diminution in metabolism, characterized by a 

 more pronounced fall in the absorption of oxygen than in the excretion of carbon 

 dioxid; or it may call forth an increase in metabolism, distinguished by a greater 

 rise in the output of carbon dioxid than in the oxygen taken in. 



7. Ingestion of Food causes a not inconsiderable increase in the 

 carbon-dioxid excretion, which is in general governed by the quan- 

 tity of food. Hence, the increase is generally most pronounced (about 

 25 per cent.) from one-half to one hour after the chief meal (dinner). 

 The increase in the consumption of oxygen that follows the intro- 

 duction of food into the stomach depends in part upon the increased 

 muscular activity of the alimentary canal; nevertheless, the increased 

 exhalation of carbon dioxid cannot be attributed to this alone. It is also, 

 and to a greater extent, dependent on the heat-producing activity of the 

 digestive glands as in the case of the salivary glands. In addition, 

 some of the carbon dioxid is derived from oxidation, in the course of 

 urea-formation, of a part of the carbon contained in the proteids. 



The quality of the food also has some influence. According to 

 Magnus-Levy a proteid diet causes a much greater increase in the con- 

 sumption of oxygen (about from 70 to 90 per cent.) than does carbo- 

 hydrate food (which increases the consumption about 39 per cent.), or 

 a fat-diet (which causes an increase of only 15 per cent.), as experiments 

 on dogs show. 



A fasting adult weighing 50 kilos inspires in one hour eight liters of air for 

 each kilo; he consumes 0.45 gram of oxygen, and forms 0.5 gram of carbon dioxid. 

 The ingestion of food raises these figures to nine liters of air, 0.5 gram of oxygen 

 and 0.6 gram of carbon dioxid. The deposition of fat following a carbohydrate 

 diet, is attended with an increase in the amount of carbon dioxid given off. 

 results partly from combustion of the carbohydrates, and partly from their trans- 

 formation into fat, during which process carbon dioxid is separated, 

 tory quotient is also increased as a result of fat-formation following an abundant 

 carbohydrate diet; the quotient under such conditions may even rise above 1.2. 



The absorption of oxygen is uninfluenced by direct injection into the 



