404 GASOMETRIC-POLAROGRAPHIC METHODS 



F, one of the reservoirs of the pressure-regulating system; G, crank-actuating 

 reservoirs of pressure-regulating system; //, optical box; /, control panel; J, 

 guide block for accurately placing frame bracket; K, stand for frame bracket 

 when not in use; L, brass tube projecting from underside of optical box; M, 

 black felt hood protecting ground-glass screen from stray light; N, dial operat- 

 ing divided mirror; 0, pulley over which passes cord for rotating frame; P, 

 revolving frame for brass cases; Q, ratchet arm for rotating frame; R, cord 

 attached to latter; S, support for frame bracket during experiment. 



The constants of the respiration chambers are determined as for 

 "Warburg vessels, mercury being used to measure the total volumes 

 of the chambers. 



C. POLAROGRAPHIC 



Polarographic methods have been employed for the determina- 

 tion of certain elements in small amounts of tissue, e.g., the pro- 

 cedures of Carruthers for sodium (1943a), magnesium (1943b), 

 and copper (1945), which were used for studies of carcinogenic 

 changes in mouse epidermis. These procedures require several 

 hundred milligrams of tissue for analysis and hence are suited to 

 histochemical work only when a relatively large quantity of histo- 

 logically well-defined material is available. Because of the limited 

 application these methods will not be given here. 



On the other hand, polarography has been applied more readily to 

 respiration studies on the histochemical level. The mercury from the 

 dropping electrode may affect biological systems, but this possible 

 difficulty has been eliminated by Laitinen and Kolthoff (1941a,b), 

 who developed a method for the estimation of oxygen in solution 

 using a platinum wire electrode in place of the mercury type. Con- 

 tact between mercury and the biological material is also avoided in 

 the double vessel of Selzer and Baumberger (1942). The determina- 

 tion of the oxygen content of body fluids by means of the polaro- 

 graph was described by Beecher et al. (1942), but the method 

 of Davies and Brink (1942) would appear "to offer the best oppor- 

 tunity for the application of the polarographic technique to 

 respirometry on a scale suitable for histochemical work. The ap- 

 paratus of the latter investigators will be discussed in detail. 



Microelectrode Measurement of Local 

 Oxygen Tension in Tissue 



Davies and Brink (1942) described two types of stationary 

 platinum microelectrodes by means of which local oxygen tensions in 



