366 RESPIRATION 



in it or artificially added, there is no doubt that the cells of the body 

 are the busiest seats of oxidation. This is shown by the presence 

 of carbon dioxide in large amount in lymph and other liquids which 

 are, or have been, in intimate relation with tissue elements ; by its 

 presence, also in considerable amount, in the tissues themselves 

 in muscle, for instance ; by its continued and scarcely lessened pro- 

 duction not only in a frog whose blood has been replaced by physio- 

 logical salt solution, and which continues to live in an atmosphere 

 of pure oxygen, but in excised muscles ; and by the remarkable con- 

 nection between the amount of this production and the functional 

 state of those tissues. In insects the finest twigs of the tracheae, 

 through which oxygen passes to the tissues, actually end in the cells ; 

 and in luminous insects, like the glow-worm, it has been noticed 

 that the phosphorescence, which is certainly dependent on oxidation, 

 begins and is most brilliant in those parts of the cells of the light- 

 producing organ that surround the ends of the tracheae. Microscopic 

 evidence has been obtained that intracellular oxidation proceeds 

 most rapidly near surfaces like the nuclear and plasma membranes 

 e.g., in the indophenol (p. 272) and similar reactions the coloured 

 oxidation products are deposited chiefly in and around the nuclei of 

 such cells as liver and kidney cells and frog's red corpuscles (Lillie). 



The Passage of Oxygen from the Blood into the Tissues. A 

 fundamental fact of tissue respiration is that the amount of oxygen 

 taken up by the cells depends essentially upon their needs, and not 

 upon the amount of oxygen offered to them in the blood. In every 

 case studied, an increase of functional activity on the part of an 

 organ leads to an increased call for oxygen by that organ, and an 

 increased consumption of oxygen in it. The cells cannot be cajoled, 

 so to say, into consuming more oxygen merely by increasing the 

 available supply. Nor can they be prevented from absorbing and 

 consuming more of whatever supply is available when they are 

 caused to work harder. 



For skeletal muscle at rest, Barcroft gives 0-004 c.c. per gramme 

 per minute as the oxygen consumption; during maximal activity 

 twenty times as much (oo8 gramme) . In the heart of a small dog 

 through which blood was pumped by a larger dog the oxygen intake 

 when the heart was beating feebly was, on the average, about 

 o-oi c.c. per gramme of heart-muscle per minute. When the heart 

 was caused to beat very strongly under the influence of adrenalin, 

 the oxygen intake rose in one case to 0-08, and in two others to 0-04. 

 In the resting pancreas the oxygen intake has been found to be 0-03 

 to 0*05 c.c. per gramme per minute; in the active pancreas, o-i c.c. 

 The corresponding number for the submaxillary gland at rest is 

 0-03, and in activity 0-09; for the kidney, 0-03 at rest or during 

 scanty secretion, and 0-07 or even 0-09 during active secretion; for 

 the liver in fasting animals o-oi, in fed animals 0-035. 



