72 A RESUME OF THE USES OF FORMALIN. 



2. Water .. ... ... 30 c.c. 



Alcohol (ninety-five per cent.) ... 15 „ 



Formalin ... ... ... 5 „ 



Hydric acetate ... ... ... i „ 



Probably the most successful use of formalin in histological 

 technique is its substitution for osmic acid in the osmium-dichro- 

 mate fluid used in Golgi's silver method for the central nervous 

 system. This substitution was probably first made by Dr. O. S. 

 Strong, though it has been recommended by Lachi and others. 

 Strong employs the following mixture : 



Potassium dichromate (3 "5 to five 



per cent, solution) ... ... 100 vols. 



Formahn ... ... ... 2-5 to 5 vols. 



After the specimen has been in the solution for several days it is 

 transferred to a one per cent, silver nitrate solution ; or, at the 

 end of two days, it is transferred from the formaUn dichromate 

 mixture to the following : 



Potassium dichromate (five per cent, sol.) 2 vols. 



Formahn ... ... ... i » 



After remaining in this fluid for from twelve to twenty-four hours, 

 it is placed in the silver solution. The advantages of this method 

 are, that the stage of hardening is prolonged, the stage favourable 

 to impregnation lasts longer, and the results are more certain. 

 For embryonic tissue he does not consider it as good as the osmic 

 dichromate mixture. Fish has used the above-described method, 

 but thinks he has obtained better results with the following : 



Miiller's fluid ... ... ... 100 c.c. 



Formalin (ten per cent.) ... ... 2 „ 



Osmic acid (one per cent.) ... ... 2 „ 



Strong has also used formalin as an injection medium for 

 hardening brains in situ. He uses formalin diluted with an equal 

 volume of water. This he injects into the cerebral vessels until 

 it runs out of the cut jugulars. After a few minutes he makes a 

 second injection, then a third, and even a fourth, at intervals of 

 fifteen minutes. The brain is then removed from the cavity of the 

 skull. The swelling which usually occurs when formalin is used 

 does not take place. Sections from brains hardened in this man 



