Formal.—An aqueous solution containing about 35 per cent. of formaldehyde. It is a rapid hardening agent, causes very little shrinkage of the tissues, and does not discharge the colour of the specimens as much as alcohol. For hardening formal should be used as a two to five per cent. solution in distilled water. It may also be used as a ten per cent. solution for mounting museum preparations, but there is some tendency for a cloudy deposit to form on the glass after a time. It is the most suitable hardening agent at our disposal for eyes. It rapidly fixes the tissue elements, but does not cause much contraction. It may also be used for hardening the brain and spinal cord. Large quantities of fluid must be used for the latter purpose and it must be frequently changed. As soon as they are sufficiently hardened they should be transferred to methylated spirit.
Marchi’s fluid.—This consists of:—
| Müller’s fluid | 2 | parts. |
| Osmic acid solution (one per cent.) | 1 | part. |
It is used for hardening specimens as a preliminary to Golgi’s method for staining nerve cells (p. [97]), and also to complete the hardening of sections of spinal cord, &c., before employing Schäfer’s modification of the Weigert Pal hæmatoxyline method (p. [91]).
It is also used as a stain for recently degenerated nerve tracts and fibres, especially after experimental lesions.
The fluid has little penetrating power, and therefore tissues must be cut into small pieces, about 3/8 inch cube. It is not necessary to place them in this fluid at once on removal from the body, but the preliminary hardening must be in Müller’s fluid and not in alcohol, &c.
Special Hardening Reagents for Rapid Fixation in Order to Study Cell Structure.
1. Alcohol.
2. Flemming’s solution (modified by Friedmann):—
| Osmic acid (one per cent.) | 3 | c.c. | (♏xxx.). |
| Glacial acetic acid | 2 | c.c. | (♏xx.). |
| Chromic acid (one per cent.) | 42 | c.c. | (℥j.) |