METHODS OF INVESTIGATION 13 



ratus, provided that the samples introduced are abso- 

 lutely fresh. This method is so rapid that the spontan- 

 eous disappearances of oxygen ought to be negligible. 



(b) T o n o m e t r i c methods (c/. Krogh, 

 1908). The tonometric methods consist essentially in 1. 

 introducing a small gas bubble (air, nitrogen, etc.) into 

 the fluid that had been previously withdrawn from the 

 body without coming in contact with the air, 2. shaking 

 this bubble with the fluid to permit the oxygen dissolved 

 in the latter to mix with the introduced gas and come to 

 a pressure equilibrium with it, and 3. analyzing the mix- 

 ture. Campbell is of the opinion that the oxygen tensions 

 found in this way may be too low since a complete equili- 

 bration requires a fairly long time and consequently the 

 spontaneous disappearance of oxygen may vitiate the 

 results. 



(c) V e r z a r ' s method, (cf. Verzar, 1912), 

 This complicated method relies in principle on oxygen 

 determinations carried out on blood flowing to and from 

 an organ while the experimental animal breathes air con- 

 taining various percentages of oxygen. It has not come 

 into general use and, according to Campbell, it fails in 

 most cases to give reliable results. 



(d) Gas injection method. This is the 

 most widely used procedure and seems to give the most 

 dependable results (Campbell). The principle is to in- 

 ject gas (air, nitrogen or other gases) into the body cav- 

 ities, the urinary bladder or under the skin, to let it re- 

 main long enough (usually a few days) in the body to 

 allow for the establishment of an equilibrium of pressure 

 of the two gases, and then to withdraw the mixture and to 

 analyze it. The amounts of gas injected are usually 

 fairly large because much of it is resorbed during the 

 equilibration period. By resorting to micro analytical 

 procedures smaller volumes may be used. Thus, Meyer 

 (1935) injects only 50 cc. of nitrogen into the subcu- 

 taneous tissues in human beings. In invertebrates sim- 



