THE GASEOUS EXCRETION. 383 



worn which permits a collection of the carbon dioxide and 

 vapor from the lungs, and of the total expired air. A deter- 

 mination of the oxygen and carbon dioxide is accurately made 

 and this furnishes all the data necessary for the calculation. 

 The nose is closed in this experiment; the mouthpiece is so 

 arranged that air may be drawn in without allowing the excre- 

 tory products to escape. 



DEDUCTIONS FROM RESPIRATION EXPERIMENTS. 



These are undertaken to answer a number of important 

 questions. The weight of carbon dioxide excreted may reach 

 fifteen hundred grams or more daily and it is interesting to 

 know under what circumstances it is increased and when 

 diminished. Very simple observations show that the body at 

 rest produces much less of the gas than does the body at work. 

 In the latter condition the destruction of food stuffs is called 

 for to liberate mechanical energy. This is practically possible 

 only through oxidation, and carbon dioxide is the first tangible 

 result of the oxidation. 



The question also comes up, what kind of organic matter 

 is most readily or most commonly oxidized when work is 

 done ? On this question much has been written and our views 

 have undergone various changes through the years. Liebig 

 considered the proteins as the foods which must be burned to 

 enable us to do mechanical work, but in a famous experiment 

 by Pick and Wislicenus, undertaken to throw some light on 

 this question, no great excess in the excretion of urea was 

 found in the work of ascending the Faulhorn, and the protein 

 oxidized was far too little to account for the work done. 

 Other investigators reached the same conclusion, but it has 

 been found that under certain conditions the proteins may be 

 consumed to do work. Ordinarily fats and carbohydrates are 

 used in preference and no large amount of protein is used if 

 the other substances are present in sufficient quantity. 



The question of what kind of foodstuff is oxidized through 

 periods of work and rest may be answered by experiment. As 



