21. Such is the method recommended by the “Micrographic Dictionary,” and perhaps it is as good as any mechanical plan could be; but where the operator is willing to undergo the labour of performing all this with the hand, he has a much better chance of succeeding, because the pressure can be regulated so accurately, and changed so quickly when requisite, that no mere machine can compete with it, however well contrived.
22. When the beginner attempts to inject a subject, one of his difficulties is finding the vessel from which to commence. Another consists in distinguishing the arteries from the veins; but this is partly removed by making a longitudinal incision in the vessel, and with a blunt thick needle probing a little distance into the tube. The artery will be found thicker in the coating than the vein, and the difference is easily perceived by this mode of testing: the vein is also of a bluer colour than the artery. I say above, a “longitudinal incision” must be made: the reason for this is, the artery when cut across contracts considerably, and is lost in the adjoining substance; but where the opening is made longitudinally all danger of this contraction is obviated.
23. The different systems of vessels are often injected with various colours, so that their relative positions, &c., may be shown most clearly. In some specimens, the veins are injected with white, and the arteries with red; in the kidney, the urinary tubes are often filled with white, and the arteries with red. Then, again, the liver affords tubes for three or four colours. But no written instructions on this point can benefit the young student, and he must be content for a while to employ himself with single colours until he has gained the mechanical skill and the primary knowledge which are necessary before he can make any advance.
24. We will now consider the best methods of mounting injected objects. They must always be well washed in water after they have been kept in any preservative liquid, using a camel-hair pencil to clean the surface if necessary. Many parts when injected are in masses, such as the lungs, liver, &c., of animals, and consequently sections of these must be cut. For this purpose Valentine’s knife is very convenient, as the thickness can be regulated so easily; but where the injections are opaque, there is no need to have the sections very thin. Some few of this kind undergo comparatively little change in drying, so that the section may be well washed and floated upon the glass slide in the place desired, where it will dry perfectly and adhere to it. It must be then moistened with turpentine and mounted in Canada balsam like other objects. No great heat should be used with these preparations, as it is very liable to injure them; and some of the colours seem to suffer a slight contraction when any great degree of warmth is applied. There are many objects, however, which must be seen in the mass to be understood, and, indeed, lose all their form and beauty in drying, such as certain parts of the intestines, &c. These must be mounted in fluid, with the precautions noticed at length in [Chapter IV.], and for this purpose either Goadby’s fluid, the chloride of zinc solution, or spirit diluted with ten parts of distilled water, may be employed. It is a good thing, when practicable, to mount similar objects on two separate slides, using different preservative liquids, and taking the precaution of marking each with the kind of liquid employed. This not only serves as a guide to what is best for certain subjects, but if one is injured, there will probably be a good specimen in the other.
25. It may be here mentioned that many are now mounting sections of injected substances with the balsam and chloroform before mentioned, instead of using balsam alone, and consider that the labour is much lessened thereby.
26. A description of that mode of injection which is most generally employed has now been given, but this is not the only method of effecting our object. A most ingenious process was invented by M. Doyers, requiring no artificial warmth, by which many beautiful objects have been prepared. Make a solution of bichromate of potash, 524 grains to a pint of water, and throw this into the vessels to be injected; then take 1,000 grains of acetate of lead dissolved in half a pint of water, and force this into the same vessels. A decomposition now takes place in the vessels, and the yellow chromate of lead is formed. In this decomposition, however, the acetate of potash also is formed and as this salt has an injurious action upon the cells, Dr. Goadby recommends nitrate of lead to be used, which preserves rather than destroys them. He also advises the addition of two ounces of gelatine dissolved in eight ounces of water, to eight ounces of the saturated solution of each salt; but with this addition the hot-water bath would be required to keep the injecting fluid liquid.
27. Many of these are best mounted in balsam, in the same manner as those made in the ordinary way; whilst others are best shown when preserved in liquids, for which purpose Goadby’s fluid may be employed.
28. This mode of making injections with chromate of lead is deemed by many the best, especially where one colour only is employed. But it must be allowed that there is a little more danger of failure where two separate fluids are used for the same vessels.
29. We will now consider the best manner of making transparent injections, which, for many purposes, possess an undoubted advantage over the opaque ones. But it must be remembered that there are certain subjects to which no transparent injection could be applied, as they are too thick when in their natural state, and cutting would destroy all that beauty which is shown by the different parts in their relative adaptation. For those objects, however, which must be cut into sections to display their wonders, or are naturally thin—such as some of the finer tissues, livers, kidneys, &c.—transparency is a great acquisition, and enables us to understand the arrangement of the vessels more perfectly. Again, another advantage is the simplicity of the process; no hot water is needed with some preparations, either for the subject or the injecting fluid, which runs into the minute vessels thoroughly and easily, whilst the cost is small.
30. For this kind of injection no colour is so commonly made use of as Prussian blue. It is not a good one, as was before stated, for any opaque object, as the light reflected from it appears almost black; yet by transmitted light no colour is more useful, because its distinctness is equally great by artificial light and ordinary daylight. The method of preparing this, as given by Dr. Beale, is as follows:—