Histology and Embryology. By C. S. Minot. 
107 
centrated solution of sulphate of protoxide of iron in distilled water, 
and pour it slowly into a concentrated solution of yellow prussiate 
of potassium; a precipitate of insoluble Prussian blue is formed. 
There should he a small excess of prussiate at the end of the opera- 
tion, to test which take out a drop and add to it a little of the 
sulphate. If there is any free prussiate still present, a blue pre- 
cipitate is thrown down. Filter through a felt strainer, below 
which a funnel with a paper filter has been placed. Pour water on 
to the strainer in small quantities at a time, and continue filtering ; 
this operation must be kept up for several days, until the liquid 
below the second filter appears distinctly blue. The matter on the 
felt strainer is then removed and dissolved in distilled water. This 
solution is admirable for cold injections or for filling the lymph- 
vessels, as will be described presently. There should always remain 
an excess of blue in the vessel in order to he sure that the solution 
is saturated ; as the solution is removed it may be replaced by dis- 
tilled water, as long as there is any blue left. To make the 
“ warm ” injection mass, take twenty-five parts of the Prussian-blue 
solution and one part gelatine. The latter must be of the finest 
quality, as otherwise it produces a granular precipitate which 
renders it useless for histological purposes. Put the gelatine to 
soak for half an hour in distilled water, then remove and wash it ; 
place it in a glass vessel and warm it in a water-bath, when it will 
melt in the water it has absorbed. The Prussian blue is put in 
another vessel in the same water-bath, so that the two liquids are 
at the same temperature. Pour the gelatine, little by little, into 
the blue, stirring constantly with a glass rod. Keep on warming 
and stirring until the granular precipitate formed at first dis- 
appears. Upon being filtered through a piece of clean flannel, the 
mass is ready for use. 
It requires only to be slightly warmed to become liquid, and 
the organ to be injected does not need to be heated to so high a 
temperature as is necessary in using many other injection masses ; 
there is therefore no danger of injuring the tissues by subjecting 
them to too high a temperature. The injection should be con- 
tinued until a little while after the mass begins to come out 
through the veins, in order to allow all the capillaries time to fill 
themselves. When the injection is finished, the organ may be 
placed to advantage for twenty- four hours in a 2 to 1000 solu- 
tion of bichromate of potassium in distilled water, and then be 
removed to alcohol ; or it may he put at once in alcohol, and, when 
hardened, sections made of it. The sections should be pretty thick, 
and may or may not be stained as is desired. If too thin, they do * 
not show the connections of the vessels. 
As an example of the clearness with which the blood-vessels 
may be traced in a successful preparation, a section through an 
vol. xvur. x 
