The stage should either be made of glass, or provided with a glass dish for dissecting under water, or preservative fluid. A pair of aplanatic lenses, mounted on a focussing bar as shown in Fig. 230, will be found the most convenient to work with.
Investigations of this nature should be always carried out in the manner described, but preparations of the kind cannot be preserved any length of time, unless properly hardened in spirit or Formalin solution. The method of teasing out under the light of a condensing lens is shown in Fig. 231.
[Illustration: Fig. 231.--Method of teasing out Muscular Fibre, &c., in a fluid medium under Condensed Light.]
It may be as well to state at the outset that physiological and pathological tissues can be hardened by immersion in methylated spirit alone, or a saturated solution of picric acid in methylated spirit in about a week, and it is said to yield satisfactory results, even some of the tissues being ready in twenty-four hours. The only drawback is that sections thus quickly hardened must be stained with picro-carmine. But, whatever method of hardening adopted, the tissue should be washed by means of a stream of water for half an hour, to remove all traces of the hardening agent, and on its removal pressed between folds of cotton cloth or fine Swedish filtering paper.
The principal hardening re-agents usually kept in bulk ready for use are the following:--
_Absolute Alcohol._--This is suitable for the internal organs of animals, glands, &c. These organs must be perfectly fresh, and should be cut into small pieces, so that the alcohol may penetrate them as quickly as possible. The hardening is usually complete in a short time.[45]
_Chromic Acid and Spirit._--Chromic acid one-sixth per cent., water solution two parts, and methylated spirit one part. This reagent hardens in about ten days. Then transfer to methylated spirit, which should be changed every day until all colour is discharged from the tissue. This is a suitable reagent for the preparation of cartilage, nerve trunks, heart, lips, blood vessels, trachea, lungs, tongue, intestines, and gullet.
_Potassium Bichromate._---Make a two per cent. water solution of this salt. This will harden specimens in about three weeks. Then transfer the preparation to methylated spirit, and change it every day until all colour is discharged. This is suitable for spinal cord, medulla, cerebellum, and cerebrum.