The means of preservation may be arranged under two principal heads, as we have said, according as the anatomist intends to expose his preparations to the open air, or to preserve them from insects and render them more transparent by the aid of certain liquors, in which it is intended to keep them continually.
Preservation by desiccation.—When applied to soft parts is only applicable to anatomy, properly so called, and to natural history; it cannot be employed for specimens of pathological anatomy.
Desiccation is preceded by a more or less prolonged immersion, according to the thickness of the organs in acid or saline solutions, &c.; that which presents the greatest advantage for the nerves, according to Dumèril, is diluted nitric acid. The salts commonly employed present some inconveniences. Corrosive sublimate hardens too much, and causes the parts to contract on each other; the triple sulphate of alumine, (alum,) often chrystalizes in drying, which produces in the interior of the piece, which ought to be pellucid, saline vegetations, which not only elevate the organic lamina, often rendering the surface tuberculous, but further deprive the part of the transparency necessary to see the texture of it; the muriate of soda, (white kitchen salt,) attracts the humidity of the air, and causes the varnish to scale off, which can have no hold upon the preparation. Diluted nitric acid, with which the parts are washed, does not expose them to these inconveniences: the preparation preserves, it is true, a certain degree of suppleness; it tarnishes a little, but is never humid.
The numerous means used for disposing the preparations to desiccation, may be reported under four series:
Alcohol, where expense is no object, is preferable to all the others; its affinity for water gives it the property of absorbing humidity from anatomical pieces.
The deuto-chloride of mercury, the proto-nitrate of the same base, the solutions of acetate of lead, and of the proto-nitrate, merit the preference among metallic substances.
Marine salt and alum are nearly those alone among the earthy salts which have been employed for this object. M. Breschet advises that, according to the method followed by the tanners, the piece be permitted to remain for several days in powdered sea salt, and to immerge it afterwards in a strong solution of alum for fifteen days, when it is to be withdrawn and dried.
In fine, tanning is still a preparatory method for desiccation.
Desiccation.—Anatomical pieces may, says M. Doct. Patissier, be dried in the open air, in a stove, in a vacuum, and by employing substances very avaricious of water, and in a bath of sand, or of absorbing powders; but desiccation by means of an oven is the best process: the heat of the oven must be neither too weak nor too strong; the most convenient temperature is that of 45° to 55° of centigrade.
When the parts have been dried by one of the processes just mentioned, if they be abandoned to themselves, they would become injured in a little time by humidity and insects. There remains, then, some care to be taken before depositing them in a cabinet; they should be washed in a liquid containing a preparation of arsenic, or of sublimate, or rather by applying to them a varnish containing one or both of these substances. We shall not reconsider here the compositions of varnishes, having already given several formulæ for them when speaking of Swan’s method, and we shall have occasion to refer to them again when passing in review the different methods of preparing objects of natural history.