42. _Skin._—To prepare skin for section a piece is to be selected which, after having been boiled for a few seconds in vinegar, must be stretched out on a bit of flat wood, and being maintained in position by pins be allowed to remain until thoroughly dry. Then imbed in paraffine, and cut _exceedingly_ thin transverse sections. These may be stained in carmine, but more beautiful results are obtained if picro-carmine be employed. Sections of skin, when stained by this agent are much increased both in beauty and instructiveness; for the several constituents of the tissue becoming tinged with different colours are readily distinguishable from each other, whilst the contrast of colouring forms a pleasing picture to the eye. The method of preparing picro-carmine is very simple, though it sometimes yields a solution not altogether satisfactory. The best formula with which we are acquainted is that given by Rutherford,[16] and if due care be taken in following it out failure will generally be avoided. “Take 100 c.c. of a saturated solution of picric acid. Prepare an ammoniacal solution of carmine, by dissolving 1 gramme in a few c.c. water, with the aid of excess of ammonia and heat. Boil the picric acid solution on a sand bath, and when boiling add the carmine solution. Evaporate the mixture to dryness. Dissolve the residue in 100 c.c. water, and filter. A clear solution ought to be obtained; if not, add some more ammonia, evaporate, and dissolve as before.” Sections may be exposed to the action of this fluid for a period varying from fifteen to thirty minutes, then rapidly washed in water, and mounted in glycerine. They may also be mounted in balsam, care being taken in that case to shorten as much as possible the period of their immersion in alcohol, so that no risk may be run of the picric acid stain being dissolved out. Footnote 16: “Practical Histology,” 2d edit. p. 173. If it is intended to study the structure of the skin with anything like thoroughness, portions must of course be examined from different localities, in order that its several varieties and peculiarities may be observed. Thus the _sudoriforous_, or sweat glands, may be found in the sole of the foot, whilst the _sebaceous_ glands are to be sought in the skin of the nose. The _papillæ_ are well represented at the tips of the fingers,[17] whilst the structure of the shaft of the _hair_, together with that of the follicle within which its root is enclosed, as also the muscles by which it is moved, are to be studied in sections of skin from the scalp or other suitable locality. Footnote 17: