294 THE BODY AT WORK 



large cells are picked out, in another small ones ; in a third 

 no nerve-cells are blackened, but connective tissue is brought 

 into view. When the block of tissue soaked with bichromate 

 of potassium is immersed in a solution of nitrate of silver, the 

 chromate escapes from it into the surrounding liquor much 

 more quickly than the nitrate gets in ; and when at last the 

 nitrate of silver enters, it finds that some of the cells have 

 fixed the chromate in their substance. This retained chromate 

 combines with silver. The product is rapidly reduced to a 

 black subchromate. No explanation of the fixing of the 

 chromate by individual cells has yet been offered. It is a 

 remarkable fact that another process which similarly makes 

 choice amongst the elements has since been introduced, giving 

 even more Valuable results. Pieces of fresh tissue are placed 

 in a very dilute solution of methylene blue. When staining is 

 satisfactory, nerve-cells alone take up the dye. The selection 

 of individual nerve-cells is not carried so far as it is by the 

 chrome-silver method, but it is exhibited to a certain extent. 

 It is probable that nerve-cells live (in a physiological sense) 

 longer than other tissue elements. Methylene blue contains 

 some easily removable oxygen of which the oxygen-starved 

 nerve-cells take advantage. The reduced methylene blue 

 remains in their substance, so that when the preparation is 

 reoxidized by exposure to air the pattern of the nerve-cells 

 is rendered conspicuous. When a few cells are selected, it is, 

 presumably, because they were the only ones alive at the time 

 when the dye entered the tissue. Preparations made from 

 the wall of the alimentary canal seem to justify this simple 

 explanation. They show patches in which muscle-fibres are 

 stained, patches in which there is no staining, and intermediate 

 zones in which nerve-cells are coloured and muscle-fibres are 

 not. But the hypothesis is inadequate to meet all cases. 

 When first employed, the blue was injected into the animal 

 in successive doses until it killed it. The staining was believed 

 to occur intra vitam. Subsequently it was found that its 

 application to fresh tissue, or, for certain results, to tissue 

 which has been kept for some hours, is equally effective. 

 ^ Without an understanding of the nature of the two new pro- 

 cesses, and of the character of the results which they yield, it 

 would bo impossible for the reader to realize the extraordinary 



