QUARTZ ROD TECHNIQUE 



210 



RADIOACTIVE ISOTOPES 



pathologic processes (a) as they de- 

 velop, (b) as they proceed at sublethal 

 degrees of intensity, and (c) as they 

 accumulate toward lethal combinations 

 of factors, but are still reversible, that 

 is while the animal's life can still be 

 saved, and (d) as they accumulate into 

 non-reversible stages. Further, the 

 method permits study of the results of 

 experimental therapeutics on visible 

 pathologic processes. For demonstra- 

 tions and elaboration of this theme see 

 Knisely, M. H., Stratman-Thomas, 

 W. K., Eliot, T. S., and Bloch, E. H., 

 1945, cited above. 



It may seem to some that the above 

 discussion is too critical or unjustly 

 critical of the histological sectioning 

 techniques, or that the author is trying 

 to belittle their use. This I do not be- 

 lieve to be so. The best histologists 

 have always studied sections not for the 

 structure of the dead sections them- 

 selves, but rather to determine as closely 

 as possible the structure and functions 

 the tissues had had when last alive . Pre- 

 cision and accuracy in developing con- 

 cepts from the evidences gathered by a 

 technique can never be greater than the 

 user's understanding of the inherent 

 limitations of that technique. The ac- 

 curacy of a technique cannot be deter- 

 mined simply by repeating its steps an 

 infinite number of times; its limitations 

 and degrees of accuracy must also be 

 cross-checked by other and, if possible, 

 quite different techniques. Each useful 

 technique delineates one or more aspects 

 of the original tissue more accurately 

 than do other techniques. Obviously 

 the most accurate and comprehensive 

 concepts of micro-anatomy, microscopic 

 physiology and microscopic pathologic 

 physiology can be developed only by 

 synthesis; by putting together in the 

 mind of the student the most accurate 

 of the available individual aspects. For 

 this purpose each technique has special 

 values of its own; for this purpose not 

 enough different techniques are yet 

 available. 



Quinoline Dyes. Only pinacyanol is of ap- 

 parent value to histologists. 



Quinone-Imine Dyes. Possess 2 chromo- 

 phores : indamin-N= and quinoid ben- 

 zene ring. They are divisible into 

 Azins, Indamins, Indophenols, Ox- 

 azins, Thiazins. 



Quinone Oximes, see Nitroso Dyes. 



Rabbit Ears, see Sandison's Technique for 



inserting transparent chambers in. 

 Rabies, see Negri Bodies. 

 Rabl's Fluid is sat. aq. mercuric chloride, 



1 part; sat. aq. picric acid, 1 part; aq. 



dest., 2 parts. 

 Radiation. Methods and results of radia- 



tion of normal tissues reviewed (Warren, 

 S. and Dunlap, C. E., Arch. Path., 

 1942, 34, 562-608 and earlier papers). 

 Radioactive Isotopes as tracer substances 

 (from Dr. W. L. Simpson of The Bar- 

 nard Free Skin and Cancer Hospital). 



In the 20 years that have elapsed since 

 Hevesy first used a radioactive isotope 

 of lead to trace the lead metabolism of 

 plants, advances in nuclear physics have 

 made available to biologists materials 

 that appear to open up new approaches 

 to a variety of problems limited only by 

 the ingenuity of the investigator and 

 the availability of the tracer substances 

 he desires. Discovery of the phe- 

 nomenon of artificial radioactivity in 

 1934 by I. Curie and F. Joliot and the 

 development of the cyclotron by E. O. 

 Lawrence and his associates at the Uni- 

 versity of California are acknowledged 

 generally to be the chief factors that 

 have produced these important ad- 

 vances. 



The assumption is made that an iso- 

 tope is accepted by tissues without 

 discrimination, and that its distribu- 

 tion, metabolism, and elimination will 

 be the same as that of the non-radioac- 

 tive form of the element. This appears 

 valid except perhaps for the lightest 

 elements in which relatively great dif- 

 ferences of atomic weight exist between 

 the radioactive and the stable isotopes. 

 Although radiations from large (thera- 

 peutic) doses of some isotopes do exert 

 profound effects on tissues, the concen- 

 tration of those employed as tracer sub- 

 stances is usually so low (often less than 

 one part to several billion of the stable 

 isotope) that no tissue changes can rea- 

 sonably be attributed to the radiation 

 accompanying their decay. 



Less than 5% of the cyclotron pro- 

 duced radioactive isotopes have been 

 employed in biological studies. Among 

 the limitations to their use are the fol- 

 lowing : (1 ) They are sometimes difficult 

 to obtain. Isotopes that decay rapidly 

 are available only to experimenters 

 near the cyclotron. (2) The rate of 

 decay of unstable isotopes must be slow 

 enough to permit the measurement of 

 their radiation at the end of an experi- 

 ment. While larger quantities can be 

 employed to offset rapid decay, a limit 

 is soon reached beyond which further 

 increases in concentration is either not 

 possible because of difficulties in pre- 

 paring them or is not desirable because 

 of the effects produced by radiation of 

 tissues. The length of experiment 

 should not be longer than 5 or 6 times the 

 half life of the element used. (3) The 

 form in which the radioelement is de- 

 sired places a limit on some investiga- 

 tions. Since they are usually pre- 



