EXPERIMENTAL PSYCHOLOGY 



insight of the microscopist alone. And even for him 

 the task of investigation was far from facile, for the 

 central nervous tissues are the most delicate and fragile, 

 and on many accounts the most difficult of manipula- 

 tion of any in the body. 



Special methods, therefore, were needed for this es- 

 say, and brain histology has progressed by fitful im- 

 pulses, each forward jet marking the introduction of 

 some ingenious improvement of mechanical technique, 

 which placed a new weapon in the hands of the inves- 

 tigators. 



The very beginning was made in 1824 by Rolando, 

 who first thought of cutting chemically hardened pieces 

 of brain tissues into thin sections for microscopical ex- 

 amination the basal structure upon which almost all 

 the later advances have been conducted. Miiller pres- 

 ently discovered that bichromate of potassium in solu- 

 tion makes the best of fluids for the preliminary preser- 

 vation and hardening of the tissues. Stilling, in 1842, 

 perfected the method by introducing the custom of 

 cutting a series of consecutive sections of the same 

 tissue, in order to trace nerve tracts and establish 

 spacial relations. Then from time to time mechanical 

 ingenuity added fresh details of improvement. It was 

 found that pieces of hardened tissue of extreme deli- 

 cacy can be made better subject to manipulation by 

 being impregnated with collodion or celloidine and 

 embedded in paraffine. Latterly it has become usual 

 to cut sections also from fresh tissues, unchanged by 

 chemicals, by freezing them suddenly with vaporized 

 ether or, better, carbonic acid. By these methods, 

 and with the aid of perfected microtomes, the worker 



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