PROGRESS IN EXPERIMENTAL PSYCHOLOGY 



in order to trace nerve tracts and establish spacial rela- 

 tions. Then from time to time mechanical ingenuity 

 added fresh details of improvement. It was found that 

 pieces of hardened tissue of extreme delicacy can be 

 made better subject to manipulation by being impreg- 

 nated with collodion or celloidine, and embedded in par- 

 affine. 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 of recent periods avails himself 

 of sections of brain tissues of a tenuousness which the 

 early investigators could not approach. 



But more important even than the cutting of thin sec- 

 tions is the process of making the different parts of the 

 section visible, one tissue differentiated from another. 

 The thin section, as the early workers examined it, was 

 practically colorless, and even the crudest details of its 

 structure were made out with extreme difficulty. Remak 

 did, indeed, manage to discover that the brain tissue is 

 cellular, as early as 1833, and Ehrenberg in the same 

 year saw that it is also fibrillar, but beyond this no great 

 advance was made until 1858, when a sudden impulse 

 was received from a new process introduced by Gerlach. 

 The process itself was most simple, consisting essentially 

 of nothing more than the treatment of a microscopical 

 section with a solution of carmine. But the result was 

 wonderful, for when such a section was placed under 

 the lens, it no longer appeared homogeneous. Sprinkled 

 through its substance were seen irregular bodies that had 

 taken on a beautiful color, while the matrix in which they 

 were embedded remained unstained. In a word, the cen- 

 tral nerve cell had sprung suddenly into clear view. 



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