Alcohol mixes both with water and oil, and consequently has been employed to fill the capillary vessels, but it coagulates the animal fluids it meets, and often blocks up the canal. It will not suspend durably coloured powders, and, at last, evaporates, leaving little more than the colours of those to which it had been united. Melted tallow, with a little mixture of oil, is often useful; but it sometimes stops too soon, where it meets with animal fluids, and becomes, by time, very brittle. Oil of turpentine, recommended by Dr. Monro, is generally employed to fill the finer vessels. It suspends the colouring matter; and, when the more volatile parts are evaporated, enough of the grosser particles remain, to retain the powder, and keep the vessels sufficiently full. After this is injected, it is confined by filling the larger vessels with a coarser injection, with which it unites very accurately.

Anatomists have preferred for the colour of their injections such pigments as most nearly imitate the natural contents of the vessels; the red for the arteries, and the blue for the veins. The vegetable colours are apt to concrete, and are destroyed by insects. The mineral are therefore preferred. The red is generally vermilion, a substance which in a small proportion gives a very considerable body of colour; and the green consists of distilled verdigris, which is brighter than the common sort, and dissolves in oil; the blue of verditer or smalt; the yellow of king's yellow; the black of lamp black or burnt ivory are used.

The properties required in the injecting matter are fluidity; and they must likewise grow stiff, but tough and flexible when cold; for were they too hard the smaller vessels would be frequently broken. The following possess these properties:

Fine injection. By Dr. Nicho/ls. - Take hard white Spanish varnish, and hard brown Spanish varnish, of each equal parts; turpentine varnish and vermilion, of each a sufficient quantity. Mix them.

Coarse Injection. By Dr. Nicholls. - Take of yellow resin two pounds; of yellow wax one pound; of turpentine varnish a sufficient quantity.

These injections may be coloured with vermilion or with verdigris. Whatever colouring matter is used, it must be ground extremely fine.

Dr. Monro recommends for the fine injection a pound of oil of turpentine, gradually poured on the colouring matter finely powdered. To procure -the vermilion or verdigris very fine, it may be agitated with the oil, and, after standing at rest a little time, poured off; the coarser parts will by that means be separated, as they will have subsided.

Dr. Monro's coarser injection consists of tallow one pound, white wax five ounces, common oil three ounces, melted over a lamp, adding Venice turpentine two ounces. When this is dissolved, the whole must be strained through a warm linen cloth; and, if designed to run far, some oil of turpentine must be added when it is used. The fine injections, it is said, should be thrown in as warm as the finger can well bear; the coarser nearly at the boiling point. In general, however, these directions are erroneous; for, by such heats, the colour will be changed, and the coats of the vessels injured. It will be safer to give them only so great a degree of heat as is sufficient to render them perfectly fluid.

Quicksilver is frequently vised for injections, and it is excellently adapted for this purpose, from its admitting of the minutest division. Were it possible to render it solid, and to impart to it any given colour, its advantages would be very considerable. May it not be possible to oxidate it within the vessels ? Its great fluidity is, however, inconvenient, as the slightest puncture empties all the vessels filled with it; and its weight renders the preparation so heavy, that it is liable to strike against the glass, and to rupture the distended vessels. In injecting with quicksilver no impulse of a piston is necessary, for its own weight is sufficient; but the operator must recollect, that the momentum is in proportion to the perpendicular height of the column, not its diameter. Quicksilver is chiefly used in injecting the lacteals and lymphatics, the vessels of the parotid glands, of the testis, and of the mammae, sometimes the arteries and veins of the hand.

In general, the younger the animal is the injection will go farther, and the same will happen when the fluids have been exhausted by disease. In the first case, the small vessels are larger; in the second, they are more empty. The less solid the part is, more vessels will be filled; and the more membranous, the brighter and more beautiful the preparation will appear. The great object in injections, therefore, is, \o empty the vessels, to relax the solids, and prevent the too rapid coagulation of the injected fluids. Water is, therefore, first injected, till it returns colourless by the veins; the water is propelled by injecting air, and the air is afterwards squeezed out. But the water cannot be wholly separated, and the particles of this fluid interposed between those of the injection occasion its breaking. It is, therefore, more common to trust to maceration for some time in the water, and squeezing the vessels, so as to evacuate the fluids by the divided end.

It is not easy to detail with advantage, in this place, the minuter regulations of this operation. It must be learnt from the works of practical anatomists, and from experience. The arterial system, after death, is usually empty; and the injection runs freely through it. To inject the veins from the trunks the valves must be forced, which is difficult, and generally impossible; for the coats will yield rather than the valves, so that one of the smallest branches which will admit the pipe must be opened. It must be recollected, however, that the veins of the abdominal viscera have no valves, so that they may be injected in any direction.

The subjects to be injected, after having their vessels cleared of their contents, should be warmed in water.

A foetus may be injected by the umbilicus; a child by the aorta ascendens from the left ventricle; an adult in the same manner as a child. Injection by the aorta fills only the arteries; but by the umbilicus of a foetus both arteries and veins are injected. When the arteries in the cornea are filled, the injection should not be pushed farther. When finished, the subject should be cooled suddenly in cold water.

If the body is macerated a day or two in cold, before it is put into warm, water, the blood will be more effectually dissolved, and the vessels more effectually emptied than by any other method. When put into warm water it may continue thirty-six or forty-eight hours, the water being kept as hot as one can bear a hand in it.

A preparation is best dried by a current of free air, avoiding dust; when dry, it must be varnished. The shining varnish may be laid on it with a brush. While drying, if animalcules appear, the part may be wetted with a solution of hydrargyrus muriatus dissolved in rectified spirit of wine.

Muriatic or nitrous acid diluted is proper for destroying the soft parts of injected preparations.

The rectified spirit of malt is the best for preserving these or any other anatomical preparations.