242 THE ESSENTIALS OF HISTOLOGY 



Tissue or Organ Hardening Fluid 



Spinal cord .... Bichromate of ammonia. 



Spleen ..... Bichromate of potash. 



Stomach ..... Distend with chromic acid or with 



alcohol. 



Suprarenal capsule . . . Alcohol. 



Tendon and ligament . . Alcohol. 



Testis Alcohol. 



Thymus gland .... Alcohol. 



Thyroid gland .... Alcohol. 



Tongue Bichromate of potash. 



Tonsils ..... Alcohol. 



Trachea ..... Chromic acid. 



Ureter Chromic acid. 



Uterus ..... Chromic acid. 



Tissues to be hardened in alcohol may either be placed at once in strong 

 spirit (90 per cent, alcohol), or the hardening may be effected gradually, the 

 tissue being placed first in weak spirit (50 per cent.) for twenty-four hoiirs, 

 then in somewhat stronger, and finally in strong spirit or absolute alcohol. 

 They are ready for cutting after having been twenty-four hours in strong 

 spirit. 



For tissues that are to be hardened in \ per cent, chromic acid, an immer- 

 sion of from 7 to 14 days is generally necessary ; they may then be washed 

 with water, and placed in alcohol for preservation and to complete the pro- 

 cess of hardening. 



Organs placed in bichromate of potash or Miiller's fluid are ready for 

 sections in a fortnight or three weeks ; they may, however, be left for a much 

 longer time in those fluids without deterioration. With picric acid the 

 hardening process is generally complete in two or three days ; the organs 

 should then be washed for some hours under a tap and transferred to spirit. 



The hardening of the brain and spinal cord in bichromate of ammonia 

 takes three or four weeks. These organs should not be left too long in the 

 solution, since they are apt to become brittle, but sections should be prepared 

 from them as soon as ready. 



In no case should the pieces of tissue to be hardened be too thick for the 

 fluid readily to penetrate to every part. 



Embedding of Hardened Tissues, and Preparation of Sections. Sections are 

 most advantageously made with some form of microtome. It is generally 

 needful to support the hardened tissue whilst it is being cut, and with this 

 object it is embedded in some fatty or other substance which is applied to it 

 in the fluid condition and becomes solid on standing. The embedding sub- 

 stance can either simply enclose the tissue, or the tissue may be soaked in 

 it : the latter method is the one most commonly employed. 



The embedding substance chiefly used is paraffin. 



Embedding in paraffin. Before being soaked in melted paraffin, the 

 piece of tissue is stained with borax-carmine or haematoxylin, dehydrated by 

 absolute alcohol, and is then soaked in turpentine. From turpentine it is 

 transferred to melted paraffin, which should not be too hot, and soaked in this 

 for an hour or more, according to thickness. It is then placed in any desired 

 position in a paper tray or on the microtome and surrounded by melted 



