INTERNAL RESPIRATION. 38? 



in a given time bears no constant ratio to the amount of 

 oxygen taken up by it simultaneously. This is certainly 

 true of muscle, for experiment shows that muscular work, 

 Awhile it continues, leads to an elimination of carbon dioxide 

 containing more oxygen than the total oxygen taken up 

 from the lungs in the same time. The balance is of course 

 made up in subsequent periods of rest, when more free 

 oxygen is taken up than is eliminated in combination 

 during the same time. Moreover, a frog's muscle excised 

 from the body and put in an atmosphere containing no 

 oxygen and made there to contract, will evolve with each 

 contraction considerable quantities of carbon dioxide 

 although from the conditions of the experiment it can 

 receive from outside no uncombined oxygen, and other 

 experiments show that it contains none. Hence the living 

 muscular fibre must contain a substance which is decom- 

 posed during activity and yields carbon dioxide as one pro- 

 duct of decomposition; and this quite independent of any 

 simultaneous direct oxidation. 



2. What is true of muscle is probably true of most of 

 the tissues. During rest they take up oxygen and fix it 

 in the form of complex compounds, bodies which, like gun- 

 powder, are readily decomposed into simpler, and in such 

 decompositions liberate energy which is used by the work- 

 ing tissue. One product of the decomposition is the 

 highly oxidized carbon dioxide, and this is eliminated; 

 other products are less oxidized, and p3ssibly are not elimi- 

 nated but built up again, with fresh oxygen taken from the 

 blood and fresh carbon from the food, into the decomposa- 

 ble substance. 



3. During the day a man gives off from his lungs more 

 <oxygen in carbon dioxide, than he takes up by the same 

 organs from the air. During the night the reverse is the 

 case. This, however, has nothing to do with the alternating 

 periods of light and darkness, as it has in the case of a 

 green plant, which in the light evolves more oxygen than 

 it consumes and in the dark the contrary. It, depends, 

 xather, on the fact that during the day more muscular effort 

 is exerted than at night, and the meals are then taken 



