13. _Yellow injection._--To prepare this, take--
Acetate (sugar) of lead 380 grains. Bichromate of potash 152 ” Size 8 ounces.
Dissolve the lead salt in the warm size, then add the bichromate of potash finely powdered.
Some of the chromic acid remains free, and is wasted in this solution, so the following is given:--
Acetate of lead 190 grains. Chromate of potash (neutral) 100 ” Size 4 ounces.
The first of these has the deepest colour, and is the most generally used.
14. _White injection._--This is a carbonate of lead:--
Acetate of lead 190 grains. Carbonate of potash 83 ” Size 4 ounces.
Dissolve the acetate of lead in the warm size, and filter through flannel; dissolve the carbonate of potash in the smallest quantity of water, and add to the size: 143 grains of carbonate of soda may be substituted for the carbonate of potash.
15. For blue injection, which is not, however, much used with reflected light, as before stated, take--
Prussian blue 73 grains. Oxalic acid 73 ” Size 4 ounces.
The oxalic acid is first finely powdered in a mortar, the Prussian blue and a little water added, and the whole then thoroughly mixed with the size.
16. It may here be repeated, that it is only when the capillaries are to be filled that there is any need to be at the trouble to prepare the colours by this double decomposition; and, indeed, colours ground so finely may be procured that the above instructions would have been omitted, had it not been supposed that some students might find a double pleasure in performing as much of the work as possible by their own unaided labours.
17. The process of injection may now be considered; but it is impossible for written instructions to supply the place of experience. I will do my best, however, to set the novice at least in the right way. There are two kinds of injection--one where the object and colours are opaque, and consequently fit for examination by _reflected_ light only; the other, where the vessels are filled with transparent colours, and must be viewed by _transmitted_ light. The first of these is most frequently employed, so we will begin with it. In the object which is to be injected a vessel of the kind which we wish to be filled must be found; an opening must then be made in it to allow one of the small pipes before mentioned to be thrust some distance within it. When this is accomplished, thread the curved needle with a piece of silk thread, or very fine string, which some operators rub well with beeswax. This thread must not be too thin, else there is danger of cutting the vessel. The cord is then carried under the inserted pipe, and the vessel bound tightly upon it, the ends being brought up round the transverse arms, and there tied; so that all danger of accidentally withdrawing the pipe is obviated. Care must now be used in closing all the vessels which communicate with that where the pipe is placed lest the injecting fluid escape; and this must be done by tieing them with silk. Should, however, any of these be left open by accident, the bullnose forceps must be made use of, as before recommended.