hydrated sections of formaldehyde-fixed tissue are usually pre-treated with bouin solution to obtain correct results in the trichrome stains for contrasting colours in collagenous and cytoplasmic (muscle) fibres. the trichrome methods were devised for tissues fixed in acidic mixtures containing mercuric chloride, which are now used only on a small scale.) only the picric acid component of bouin's solution is needed to bring about this correction of trichrome staining.