The most common, the most solid, and the most convenient injections, are made of fatty and resinous matter. Volatile oils, balsams, resins dissolved in alcohol, fats, wax, and the most ordinary fixed oils, are principally used. These different substances are combined, and the compositions of them are varied according to the nature of the injections, which it is desirable to prepare, and above all, according to the manner which it is proposed to preserve them. The nature and the preparation of the colouring matters, ought also to vary according to the kind of fatty medium which is used. The volatile oils being nearly equally penetrating, turpentine is generally chosen, because it is cheaper. Nevertheless, for small objects, the citron, or that of a species of lavender (aspie of the shops) is preferred, on account of their odour, which are besides not very expensive. When it is intended to inject only with one of these oils, which makes a liquid matter extremely penetrating, after having dissolved a colouring matter previously ground in a fixed oil, the mixture is slightly heated. This liquor is generally employed to render perceptible the small vessels of membranes, which are not to be dissected, but well preserved in their integrity. If it is intended to inject the large trunk which supplies these membranes towards the end of the operation, it is necessary to inject a little essence of varnish, charged with much resin, and before drying the piece, let it soak a day or two in an aqueous solution of the deuto-chloride of mercury, according to the process of Chaussier. The matters with which the volatile oils are to be coloured, should be previously ground with the greatest care. It is easy to procure those which are prepared with nut oil, and which are sold in little bladders to be employed upon palettes. Colours, thus prepared and intimately amalgamated with fixed oils, remain much better suspended; the heaviest oxides, even those of lead and mercury, are not then subject to become deposited. Resins, dissolved in spirits of wine, are also sold by the pint, all prepared, under the name of varnish, and in general are not costly. Those which the anatomist can turn from the ordinary arts to his own profits, are employed principally in pieces which it is intended to preserve dry. Perfect success attends the varnish, named in the shops _fat_, _wood-red_, _à la copale_, and with some others which remain a long time flexible. These fluids are difficult to colour; it is necessary, for the first, to grind the colouring matter with essence, and for the others with alcohol; and to incorporate them afterwards with varnish after having slightly heated them. The carmined lakes, thus suspended in fat varnish, produce absolutely the effect of arterial blood; this colour preserves very well, and with such like injections it is unnecessary to colour the surface of the arteries.