46. Tongue.—Harden in chromic acid, imbed and cut transverse sections in paraffine. As, however, the paraffine is apt to get entangled amongst the papillæ, whence it is afterwards with difficulty dislodged, it will be as well before imbedding to soak the tongue in strong gum for a few minutes, and afterwards immerse in alcohol till the gum becomes hardened, so that the delicate papillæ may thus be protected from the paraffine by a surface-coating of gum. The best staining agent is picro-carmine (§ 42). Sections of cat’s tongue near the root, when thus stained, furnish splendid objects. Sections should also be made of the taste-bulbs, found on the tongues of rabbits. These are small oval prominences, situated one on each side of the upper surface of the tongue near its root. They should be snipped off with scissors, and vertical sections made in the direction of their long axis. Stain with carmine or picro-carmine, and mount in glycerine or balsam.
47. Vegetable Ivory.—After prolonged soaking in cold water may readily be cut in the microtome. The sections should be mounted in balsam, and though not usually regarded as polariscopic objects, nevertheless, when examined with the selenite, yield very good colours.
48. Wood.—Shavings of extreme thinness may be cut from large pieces or blocks of timber, by means of a very sharp plane. In this way very good sections may be procured of most of the common woods, as oak, mahogany, “glandular wood” of pine, etc. Where however, the material to be operated upon takes the form of stems, roots, etc., of no great thickness, they should, after having been reduced to a suitable consistence (§ 4), be imbedded in paraffine, and cut in the microtome. Before imbedding it must not be forgotten to immerse the wood to be cut in weak gum-water (§ 11), this precaution being of great importance, especially in the case of stems, etc., the bark of which is at all rough and sinuous. If the sections are to be mounted unstained, they are usually put up in weak spirit (§ 26). A very general method also of dealing with this class of objects is to mount them dry (§ 26). This plan, however, cannot be recommended, for however thin the sections may be, the outlines, when this process is adopted, always present a disagreeable black or blurred appearance. To avoid this we may have recourse to Canada balsam, but the ordinary method of employing it must be slightly modified, a drop of chloroform being substituted for the clove oil (§ 23), otherwise this latter agent will cause the section to become so transparent as to render minute details of structure difficult to recognize. A better plan, perhaps, is to stain the section with carmine or logwood, and mount in balsam by the ordinary process. The best course to follow, however, especially in the case of transverse sections, is the double staining method.[[18]] For this purpose the sections in the first place must be subjected to the action of a solution of chloride of lime (1/4 oz. to a pint of water) until they become thoroughly bleached. They must then be soaked in a solution of hyposulphite of soda (one drachm to four ounces of water) for an hour, and after being washed for some hours, in several changes of water, are to be transferred for a short time to methylated spirit. Some red staining fluid is now to be prepared by dissolving half a grain of Magenta crystals in one ounce of methylated spirit. A little of this solution being poured into a small vessel of white porcelain (§ 14), the sections are to be immersed in the dye for about thirty minutes. They are now to be removed, and after rapid rinsing in methylated spirit to remove all superfluous colour, they must be placed in a blue staining fluid made by dissolving half a grain of aniline blue in one drachm of distilled water, adding ten minims of dilute nitric acid and afterwards sufficient methylated spirit to make two fluid ounces. The sections must be permitted to remain in this solution for a very short time only, one to three minutes being generally sufficient, for as the action of the dye is very energetic, it will, if too long exposure be allowed, completely obliterate the previous coloration by the magenta. After being again rapidly rinsed in methylated spirit, as much of this as possible must be drained off, and the sections put into oil of cajeput, whence, in an hour, they may be transferred to spirits of turpentine, and after a short soaking, mounted in balsam.
[18]. See a paper by Mr. Styles in the “Pharmaceutical Journal,” also “Monthly Microscopical Journal” for August, 1875. [For a very exhaustive paper on this subject by the late Dr. Beatty, of Baltimore, Md., see “American Journal of Microscopy” for June, 1876.
If the student will carefully carry out the above process, his trouble will be amply repaid by the beautiful results obtained, for by its means he may, with ease, prepare for himself a series of slides of such value as to constitute a worthy addition to his cabinet.
The preceding list by no means represents all the objects, sections of which will be found interesting to the microscopic student. Such was not its purpose—had it been so, the enumeration might have been prolonged almost indefinitely. The end in view was to bring under the notice of the reader only those substances the cutting of which is accompanied by difficulty; and even of this class the space at our disposal has been so limited that we have been unwillingly compelled to pass over many, and dwell only on such as possess a typical character.