12 METHODS OF INVESTIGATION 



In other instances the parasites are present in the 

 semi-fluid intestinal contents. The oxygen tension of this 

 habitat {cf. von Brand and Weise, 1932) can well be de- 

 termined with the van Slyke manometric apparatus {cf. 

 Peters and van Slyke, 1931). It is to be noted here, 

 however, that the enormous numbers of bacteria present 

 in intestinal material quite generally exhibit an intensive 

 oxygen consumption which invalidates the results gained 

 from any but the very fresh samples. 



The determination of the oxygen content of body fluids 

 like urine, bile, etc., or of tissues — all important parasitic 

 habitats — offers singular difficulties. One method that 

 was much used in former years and which dates back to 

 Ehrlich (1885), consisted in injecting dyes into the tis- 

 sues : the absence of molecular oxygen was then assumed 

 when the dyes became reduced. It is now generally 

 agreed that this procedure does not give reliable re- 

 sults since, as Krogh (1916) remarked, ''the stains em- 

 ployed may very well become reduced even if free mole- 

 cular oxygen is present." 



The more modern procedures are based on entirely 

 different principles. Their classification as given below 

 is taken from Campbell (1931), who has had wide ex- 

 perience with this problem. 



(a) Gas extraction method. The gases 

 dissolved in the secretions and excretions — they are in 

 equilibrium with the gases present in surrounding tis- 

 sues — are extracted by means of gas pumps and then 

 analyzed. Campbell regards the results obtained in this 

 way as doubtful because, on the one hand, oxygen may 

 diffuse out and disappear spontaneously from the fluids 

 during the manipulations and, on the other, small leaks 

 frequently occur in the pumps. This may be a serious 

 drawback since the absolute amounts of gases dissolved 

 in the fluids are very small. In the present writer's 

 view, however, these objections can hardly apply to an- 

 alyses performed with the van Slyke manometric appa- 



