_a._ _Dissection._--It consists in stripping the part which it is intended to preserve, of the tissues and organs which are foreign to it: if the object be the preparation of muscles, for example, these organs are left alone with their insertions in the bones, or rather, the vessels, previously injected, preserve their relations with the muscles and the bones. Nevertheless, in the dissection of the hard parts, whether it is proposed to follow the branches of the vessels and nerves which penetrate, or are distributed in their substance, or whether it is desired to develop and render their organization more apparent; it is less convenient to have recourse to instruments than to chemical re-agents, which bring into view the parts which it is desirable to study. When the object is the preparation of a bone only, the operation consists of two parts, _excarnation_, and _etiolation_, the details of which will be presented in the article upon bony tissues. _b._ _Macerations and corrosions._--These operations are frequently brought into use by the naturalist: water, acids, alkalies, volatile oils, &c., serve to produce varied effects in the preparation of the different tissues. The maceration of different portions of the skeleton is produced by water. The employment of other liquids has for object, in attacking several parts which they dissolve, to expose others which it is desirable should be left bare. Thus, in order to absorb the grease which exudes from the skeletons of certain fish, or of bones, the maceration of which has not been perfected, it is useful to steep the piece in a marly alluminous paste, which must be alternatively put to dry and soften in the sun, in order that the clay may absorb the fetid oils with which the bones are impregnated. In order to dissolve the grease with which certain parts are covered some time after their preparation, as happens to some natural skeletons, it is often necessary to steep the piece in an alkaline liquor, or rather, to allow it to macerate for some weeks in a very penetrating volatile oil. It is only by the aid of such processes that we are able to follow encephalic nerves in many of the cetacea, although these parts present in these animals extremely singular dispositions. It is with the same view that should be macerated either in water elevated to a certain degree of temperature, or in acid liquors, the hard tissues, in the interior of which it is proposed to denude certain parts. Thus the nerves and vessels of the roots of the nails, the horns, the skin, cannot be well exposed but by this process. The canals, which traverse certain bones, cannot, as we have already shown, be easily followed, unless the piece has remained for a longer or shorter time in an acid liquor. Macerations in alkaline and etherial liquors are still of great assistance, as the researches so happily conceived and executed by Bichat have proved.