AND HOW TO USE IT. 231 



brings out the medullary sheath of nerves. Specimens must 

 remain in it for from a quarter of an hour to several hours. In 

 hardening tissues, always insert in your note-book the name of 

 animal, tissue, date of injecting, of changing or substituting fluids, 

 and any facts of special interest ; or, if the student prefers, or will 

 give a little extra time, I should recommend him to copy his des- 

 cription also on to the label of the bottle in which the tissue is 

 put, but not to rely on the label only, as it may become unfastened 

 or get defaced. The following is a rough copy of the way in 

 which I insert notes in my little book, and if I afterwards feel 

 inclined I can add to them, and so make the directions, etc., quite 

 complete : — 



Osmic Acid, (Os 0^). 



Of great service for embryonic tissues, nerve-fibres, retina, 

 connective tissue, corpuscles, epithelium, epidermis, testis, etc. 



It quickly colours fat-cells and medullary nerve-matter. 



It more sloivly colours ganglion cells, axis cylinders, muscles, 

 and cells rich in protoplasm, and the internal ear. 



It most sloivly colours connective tissue. 



Very small pieces may be immersed in a from i per cent, to 2 

 per cent, solution for from a quarter of an hour to twenty-four 

 hours, and these solutions also harden, but i — 10 per cent, solu- 

 tion which does not harden may also be used. After hardening, 

 first wash in distilled water, then place in rectified spirit. Secondly, 

 sections are made, and mounted in a saturated solution of acetate 

 of potash or in Farrant's fluid. The former is the best, for glyce- 

 rine renders them too transparent. 



N.B. — Avoid Osmic Exhalations. 



The last line stands out distinct and clear, so that no mistake 

 ca7i be made. The vapour is dangerously irritating to the conjunc- 

 tiva and nostrils, and is also excessively poisonous; great care 

 must therefore be taken. 



Picric Acid.— A cold, saturated aqueous solution ; keep more 

 crystals in the bottle to ensure saturation. Small pieces of the 

 material to be hardened are put in a large quantity of the fluid, 

 and ought only to be left in it for a day or two, or they will be 

 rendered too brittle. Its stain is of a bright yellow colour, which 



