260 A MANUAL OF PHYSIOLOGY 



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 the 

 nucleus plays a predominant part in intracellular oxidation ; e.g., in 

 the indophenol (p. 265) and similar reactions the coloured oxida- 

 tion products are deposited chiefly in and around the nuclei of such 

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



The fact observed by Bohr that an increase in the carbon 

 dioxide tension of blood diminishes its combining power for 

 oxygen, and therefore favours the giving up of oxygen to the 

 lymph and tissues, may have an important influence on internal 

 respiration. The effect is much more marked where the oxygen- 

 tension is low than where it is high, so that in the lungs the taking 

 up of oxygen is scarcely interfered with even by a high carbon 

 dioxide tension. Lymph, bile, urine, and the serous fluids con- 

 tain very little oxygen, but so much carbon dioxide that the 

 pressure of that gas in all of them is greater than in arterial 

 blood, while in lymph alone (taken from the large thoracic duct) 

 has it been found less than that of venous blood. And it is 

 probable that lymph gathered nearer the primary seats of its 

 production (the spaces of areolar tissue) would show a higher pro- 

 portion of carbon dioxide. Strassburg found that with a pressure 

 of carbon dioxide in the arterial blood of 21 mm. of mercury, the 

 pressure in bile was 50 mm., in peritoneal fluid 58 mm., in urine 

 68 mm., in the surface of the empty intestine 58 mm. Saliva, 

 pancreatic juice, and milk, also contain much carbon dioxide, and 

 only a little, if any, oxygen. From muscle (to facilitate pumping, 

 the muscle is minced and warmed) no free oxygen at all can be 

 pumped out, but as much as 15 volumes per 100 of carbon dioxide, 

 some of which is free, that is, is given up to the vacuum alone, while 

 some of it is fixed, and only comes off after the addition of an acid. 



Muscle may be safely taken as a type of the other tissues in 

 regard to the problems of internal respiration. It is instructive, 

 therefore, to observe that the great scarcity of oxygen in the 

 parenchymatous liquids which bathe the tissues, here in the 

 tissues themselves deepens into actual famine. The inference 

 is plain. The active tissues are greedy of oxygen ; as soon as 

 it enters the muscle it is seized and ' fixed ' in some way or other. 

 The traces of oxygen in the lymph cannot therefore be journeying 

 away from the tissue elements ; they must have come from another 

 source, and this can only be the blood. Could we gather tissue 

 lymph for analysis directly from the thin sheets that lie between 

 the blood capillaries and the tissues, we might find more oxygen 

 present as well as more carbon dioxide. But if we did find more 



