78 RESPIRATORY EXCHANGE OF ANIMALS AND MAN 



maintained in the sample for over an hour it probably represents the 

 actual tension in the kidney at the time. 



The experiments of Verzar [1912] show conclusively that in the 

 salivary glands the oxygen tension is comparatively high, while it 

 appears to be low (less than 20 mm.) in the muscles. In the latter 

 organs the oxygen tension is clearly the limiting factor for the oxida- 

 tions, but it is obvious that a great deal of exceptionally difficult work 

 will have to be done before we can say anything quantitatively about 

 the relation between oxygen tension and rate of oxidations in the 

 warm-blooded organism. 



THE Toxic EFFECT OF VERY HIGH OXYGEN PRESSURES. 



At pressures above 1500 mm. (O 2 in the inspired air) oxygen has 

 a toxic effect upon warm-blooded animals. The temperature falls and 

 the respiratory exchange becomes diminished according to the experi- 

 ments of Paul Bert [1878]. The technique of these determinations 

 does not, however, inspire much confidence. The mechanism of the 

 poisonous action of oxygen is unknown. The symptoms bear some 

 resemblance to the symptoms of prussic acid poisoning and it has 

 been argued that oxygen under high pressure might inhibit oxidations. 

 Pfliiger [1873] points to the analogy with phosphorus, which does not 

 absorb oxygen from an atmosphere of the pure gas but burns readily 

 at lower partial pressures. 



THE EFFECTS OF VARIED OXYGEN TENSIONS ON COLD-BLOODED 



ANIMALS. 



Henze has made a number of experiments which show very clearly 

 the effects of varied oxygen pressures and the manner in which they 

 act. In experiments on eggs of sea urchins the same respiratory 

 exchange was observed when the oxygen quantity of the water varied 

 from about double the normal (35 per cent, tension) to about half the 

 normal amount, when the water was agitated. When the eggs were 

 lying quietly on the bottom of a bottle in a layer -J cm. high the re- 

 spiratory exchange was much diminished compared with experiments 

 in which the water was gently agitated (4*9 against 11*2), and when 

 comparative experiments with oxygen-rich and oxygen-impoverished 

 water were made without agitation the eggs in the oxygen-rich water 

 showed the larger oxygen absorption (37 against 2). In animals 

 with a good circulation and branchial respiration (e.g. the crustaceans 

 Cardnus mcenas and Scyllarus latus, the molluscs Aplysia limadna, 



