OXYGEN. 411 



of this gas. This oxygen must come from the circulation, and is undoubt- 

 edly to be regarded as oxygen that has not been consumed by the oxida- 

 tion processes taking place in the cells of the salivary glands. 



Paul Ehrlich 1 has proposed a very pretty method for following the 

 course of the oxidation processes taking place in the tissues. If a dye- 

 stuff which becomes decolorized on reduction and again resumes its color 

 on oxidation is injected into the veins of an animal, it is easy to recognize 

 the presence of oxidizable substances in the tissues. Methylene blue is 

 especially suited for such experiments. If this has been injected into the 

 veins, it will be found that a freshly-killed animal will be of normal color; 

 but after being exposed to the air for some time, the color of methylene 

 blue eventually appears; showing that the tissues have contained this 

 dyes tuff in a reduced form. 



The assumption that the consumption of oxygen must actually take 

 place in the tissues and cells has been based frequently upon numerous 

 observations concerning the oxygen supply of lower organisms. Thus 

 the observations of Kupffer 2 and of Max Schultze 3 regarding the direct 

 supply of oxygen to the cells of the body are a good example. The former 

 showed that insects which had no real vascular system conduct the atmos- 

 pheric oxygen directly to the tissues by means of an infinitely-fine tracheal 

 system. The finest little runners of the tracheae send out branches to the 

 individual cells, so that the latter by means of these tiny tubes take the 

 oxygen directly from the air. Again, Schultze showed that in the organs 

 of phosphorescence of Lampyris splendidula, the finest ends of the tracheae 

 lead to the individual cells, which cause the phosphorescence. 



Although these observations undoubtedly indicate the ability of highly 

 organized animals to take up and utilize directly the oxygen of the air, 

 yet they do not prove conclusively that also in the highest organized 

 animals such a direct introduction of the oxygen to the cells actually 

 takes place. In the ascending series of animal species, with the division 

 of labor and specialization of the separate cell groups becoming more and 

 more complicated, it would not seem impossible that perhaps one par- 

 ticular cell group may be limited to quite restricted functions; and that, 

 for example, the cells receive their energy, in the more highly developed 

 organisms, from certain cleavage processes, while the energy produced 

 by oxidation serves merely as the source of heat for the organism. We 

 have already seen 4 that intestinal parasites, and even frogs, can live for 



1 Med. Zentrb. 1885, 113. Das Sauerstoffbediirfnis des Organismus, Berlin, 1885. 

 Cf. C. A. Herter: Z. physiol. Chem. 42, 493 (1904), and Am. J. Physiol. 12, 128 (1904). 

 Herter and Richards: ibid. 12, 207 (1904). C. A. Herter: ibid. 12, 457 (1905). 



2 Beitrage zur Anatomie und Physiologic (1875). Cf. E. Pfliiger: Pfliiger's Arch. 

 10, 251 and 270 (1875). 



3 Arch, mikros. Anat. 1, 124; 5, 186. 



4 Lecture IV, p. 74. 



