INFILTRATION AND IMBEDDING. 2"J 



mixture, in which they float for a short time. They are then washed in 

 running water for from twelve to twenty-four hours, and transferred to 

 gradually concentrated alcohols. Crystals of sublimate which may be 

 present are removed with iodized alcohol. Zenker's fluid penetrates 

 easily, and fixes nuclear and protoplasmic structures equally well without 

 decreasing the staining qualities of the elements. 



Formalin (Formol). Of recent years formalin, which is a 

 40% solution of the gas formaldehyd in water, has been much used as a 

 fixing fluid. It is best employed in the form of a solution made by add- 

 ing 10 parts of formalin to 90 parts of water or normal saline solution. 

 Small pieces of tissue remain in this solution for from twelve to twenty- 

 four hours, larger pieces or organs a number of days or weeks, and are 

 then transferred to 90^ alcohol. 



Potassium Bichromate and Formalin. 



Potassium bichromate, 2% to 3^ aqueous 



solution 90 parts. 



Formalin 10 " 



Tissues remain in this fluid from several days to several weeks, de- 

 pending on their size. Wash thoroughly in water and dehydrate in 

 alcohol. Useful for fixation of central nervous system. 



We have attempted to give only the fixing and hardening fluids com- 

 monly employed for general purposes. There are numerous other fluids 

 used for special purposes ; these will be noticed under the headings of the 

 corresponding tissues and organs. 



INFILTRATION AND IMBEDDING. 



Few tissues have a consistency, even after fixation, which enables 

 them to be cut into sections thin enough to be studied under high magni- 

 fication, without being especially prepared for this purpose. To admit 

 of sectioning, it is generally necessary to imbed them in media which 

 offer no resistance to the knife, while giving them firmness, and do not 

 obscure the structure of the sections when cut, or which may be removed 

 from the sections by methods which are not harmful to them. The media 

 used for imbedding may be classed under two heads : ( i ) Such as are 

 fluid when warm, and may in this state be caused to penetrate the tissue, 

 and are solid when cold ; (2) such as are fluid when in solution, and in 

 this state will penetrate tissues, but which become solid on the evaporation 

 of the solvent. The best example of the former class of substances is 

 paraffin ; and of the latter, celloidin (collodion or photoxylin). 



i. PARAFFIN IMBEDDING. 



In describing the method of paraffin infiltration and imbedding it 

 is assumed that the tissues have been previously fixed and hardened and are 

 in alcohol ready for further manipulation. From the hardened tissues 

 small flat pieces are cut with a sharp knife or razor. If possible, they 

 should be square, rectangular, or triangular in shape, their surfaces not 

 exceeding y^ square inch, and their thickness from ^ to ^ of an inch. 

 Pieces of larger size may be imbedded, if desired, provided the requisite 

 care be exercised. The pieces selected are placed in absolute alcohol, in 

 which they remain until thoroughly dehydrated. From the latter they 



