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Muscle may accordingly be taken as a type of tissues in which 

 the oxygen pressure is so low that the blood-flow through them 

 cannot be much diminished without interference with their absorp- 

 tion of oxygen, while in tissues like the resting submaxillary gland 

 a considerable diminution of the blood-flow can occur without 

 diminution of the oxygen consumption of the tissue. Whether 

 this difference is merely a quantitative one, depending upon the 

 great oxygen requirement of active muscle, or a qualitative difference 

 connected with the peculiar relations of oxygen to the muscular 

 contraction, is unknown. These relations have so great an interest 

 in connection with the problem of intracellular oxidation, and have 

 been so much more minutely studied than the relations of oxygen 

 to the functional work of other tissues, that a few words on the 

 respiration of muscle may fitly be introduced here. The subject 

 must be returned to later on (Chapters XII. and XIII.). 



Fig. 123. Fatigue of a Pair of Sartorius Muscles (Fletcher). A, in an atmosphere of 

 oxygen; B, in an atmosphere of nitrogen. A is partially restored by a rest of 

 five minutes. 



Respiration of Muscle. It is a remarkable fact that an excised 

 frog's muscle is capable of going on yielding carbon dioxide for a 

 long time, in the entire absence of oxygen, in a chamber, for instance, 

 filled with nitrogen or other indifferent gas. Not only so, but it 

 can be made to contract many times in this oxygen-free atmosphere, 

 although it loses its power of contraction sooner than in oxygen, 

 and does not show the same capacity for recuperation during an 

 interval of rest. In mammals the muscles can also be made to 

 contract repeatedly when the dissociable oxygen has, as far as pos- 

 sible, been got rid of from the blood by asphyxiating the animal, and 

 to give off a correspondingly large quantity of carbon dioxide, 

 although they lose their contractibility much more rapidly than the 

 muscles of the frog. This has usually been interpreted as meaning 

 that the carbon dioxide does not arise, so to speak, on the spot, from 



