34. A few subjects may be noticed which are very beautiful when injected, and amongst these are the eyes of many animals. They must be injected by the artery in the back part, and when the blue transparent liquid is employed, nothing can exceed the delicate beauty which some of the membrane bears. It must, however, be dissected with care, but well repays us for the trouble. Water-newts and frogs are not difficult subjects, and in their skin and other parts are many interesting objects. Amongst the commoner animals—rats, rabbits, cats, &c. &c.—almost endless employment may be found, making use either of portions or the whole animal at once. The intestines of many of these are very beautiful. We must divide them with a pair of scissors along the tube, and cleanse them from all the matter; the coating may then be laid upon a slide, and any remaining impurity removed with a camel-hair pencil and water. When dried it may be mounted in balsam, and having been injected with the transparent blue, its minute beauty is shown most perfectly. In injecting a sheep’s foot, which is a good object, the liquid should be forced into it until a slight paring of the hoof shows the colour in the fine channels there.
35. When the lungs of small animals are injected, the finest fluid must be used, as some of the capillaries are so small that it is not an easy matter to fill them properly. And before entering upon these subjects, a certain proficiency in the mode of using the syringe, &c., should be obtained by practising upon simpler parts.
36. No subjects are more difficult to inject than fish, owing to the extreme softness of their tissues. Dr. Hogg recommends the tail of the fish to be cut off, and the pipe to be put into the divided vessel which lies just beneath the spinal column; by which method beautiful injections may be made. The gills, however, are the most interesting part as microscopic objects.
37. These instructions may seem very imperfect to those who have had much experience in this branch; but they will remember that their own knowledge was not gained from any written descriptions, but was forced upon them by frequent failures, some of which probably were very disheartening. As I before stated, it is very difficult (if not impossible) to accomplish much without some knowledge of anatomy.
38. I may here mention that the transparent injections sent over from the Continent are beautifully executed by Hyrtl of Vienna (who states that the injected fluid is composed of gelatine and carmine), Dr. Oschatz of Berlin, the Microscopic Institute of Wabern, Schaffer and Co. of Magdeburg, and others. Some of these will bear examining with a high power. A friend informs me that he measured a vessel in a rat’s tongue by Hyrtl, which was 1-7200th of an inch in diameter, and had a clear outline with quarter-inch objective. He has also made many experiments with the same materials, but has as yet failed in producing perfectly distinct outlines, there being a tendency of the colouring matter (magenta, carmine. &c.) to diffuse itself through the coats of the vessels into the surrounding tissues, although he has varied the pressure from one half a pound to sixty pounds. He believes the vessels are first washed out (injected with warm water and pressure applied), then some fluid introduced which renders the arteries impervious to the coloured fluid afterwards injected.
39. He finds that after washing out the vessels as above, the injecting fluid is much more easily introduced. He has used a strong solution of gallic acid previously to injecting with the colouring matter (in one experiment only), and the result was satisfactory. He puts the query,—Might not carbolic acid have a similar effect? He has often used it with injections to preserve the specimens, but not in sufficient quantity to act in the way indicated above.
Since writing the above, Mr. J. G. Dale, F.C.S., and I have made numerous experiments with carmine injection, and have at length been favoured with what we deem success. Some of the vessels in a kitten lately injected do not exceed 1-2000th of an inch in diameter, and present a clear outline with one-fifth objective. There is no extra-vasation, neither does the colouring matter show any grain except when a very high power is employed. The following is our process:—
| Take | 180 grains best carmine. |
| ½ fluid ounce of ammonia, commercial strength, viz., 0·92, or 15° ammonia meter. | |
| 3 or 4 ounces distilled water. |
Put these into a small flask, and allow them to digest without heat from twenty-four to thirty-six hours, or until the carmine is dissolved. Then take a Winchester quart bottle, and with a diamond mark the spot to which sixteen ounces of water extend. The coloured solution must be filtered into the bottle, and to this pure water should be added until the whole is equal to sixteen ounces.
Dissolve 600 grains potash alum in ten fluid ounces of water, and add to this, under constant boiling, a solution of carbonate of soda until a slight permanent precipitate is produced. Filter and add water up to sixteen ounces. Boil and add the solution to the cold ammoniacal solution of carmine in the Winchester quart, and shake vigorously for a few minutes. A drop of this placed upon white filtering-paper should show no coloured ring. If much colour is in solution the whole must be rejected, because, although it is possible to precipitate all the colouring matter by the addition of ammonia or alum, it is not well to do so, as the physical condition of the precipitate is thereby altered.