FRESH TISSUES. 4OI 



point beneath the right edge of the cover enables one to have perfect con- 

 trol of it while it is being lowered. The contact between the cover and the 

 mounting medium (salt solution in this case) spreads gradually from left 

 to right as the cover is lowered, expelling the air as it advances. If bubbles 

 are caught in the medium, the cover may be alternately raised and lowered 

 a little until they escape, but once the cover is flat upon the specimen it 

 should not be lifted. 



Connective tissue, medullated nerves, fat, desquamated epithelial 

 cells and blood should be examined in the fresh state by every student as 

 showing certain features better than the preserved specimens. Chorionic 

 villi may be identified in this way, and the cells in urine are studied un- 

 stained. A drop of acetic acid (from i to 5 per cent.) placed upon connective 

 tissue causes the white fiber to swell and disintegrate, but renders the elas- 

 tic tissue and the nuclei more distinct. A few drops of stain may be placed 

 upon the tissue for some minutes and then washed off in order to bring 

 out the nuclei. Methylene blue (i per cent, aqueous solution) and methyl 

 green (i per cent, solution in 20 per cent, alcohc 1 ) or the haematoxyline 

 solutions may be used for this purpose. If sections are overstained a more 

 dilute solution or shorter application is indicated, but if the section is pale, 

 prolonged staining or stronger solutions are required. Thus the time 

 limits given with the various dyes are only approximate a^ the response of 

 different tissues is not uniform, and different samples of given solution 

 vary in their staining capacity. 



ISOLATION. 



Some tissues cannot properly be separated into their elements in the 

 fresh condition but may be shaken or teased apart after preliminary 

 treatment. Epithelial cells become separable after remaining from 5 to 

 24 hours in 33 per cent, alcohol (40 cc. of 95 per cent, alcohol and 60 cc. 

 of water). The pieces of epithelium used should be small (5-10 mm. 

 square). The same treatment prolonged for one or two weeks is employed 

 in isolating the nerve cells of the spinal cord. Muscle cells may be pulled 

 apart after remaining some hours in a fresh 35 per cent, solution of potas- 

 sium hydrate. The muscle fibers should be examined in a few drops of the 

 same solution, since they disintegrate if it is diluted. They may however be 

 transferred to solutions of potassium acetate which neutralizes the potash 

 and prevents further maceration. The elements of nails may be scraped 

 off from fragments boiled in a test tube containing a concentrated solution 

 of potassium hydrate. Immersion in cold concentrated sulphuric acid is 

 recommended for the same purpose. 



Another solvent for the intercellular substances of muscle is a satu- 

 26 



