39. Porcupine Quill.—Soften in hot water, cut in paraffine, and mount in balsam. Much (in our opinion too much) lauded as an object for the polariscope.
40. Potato.—From the large amount of water which it contains thin sections cannot be cut from the potato in its natural state. It must, therefore, be partially desiccated, either by immersion in methylated spirit for a few days or by exposure to the air. Sections may then readily be obtained by imbedding and cutting in paraffine. Such sections mounted in balsam are very beautiful, the starch being seen in sitú, whilst if polarized light be employed each granule gives out its characteristic black cross.
41. Rush is to be prepared and cut as orange-peel (§ 37). Transverse sections of this “weed” furnish slides of the most exquisite beauty.
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.
[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.
[17]. It is well, in connection with these papillæ, to bear in mind a fact pointed out by Frey, namely, that the tips of the fingers frequently become, post-mortem, the seat of extensive natural injections; hence, in sections from this region, we frequently obtain good views of distended capillaries without having been at the trouble of previously injecting them.—Frey, “Microscopical Technology.”
43. Spinal Cord.—The spinal cord, say of a cat or a dog (or if procurable, of man), after being cut into pieces about half an inch in length, may be hardened in the usual chromic acid fluid (§ 5). As it is peculiarly liable to overharden and become uselessly brittle, the process must be carefully watched. Its further treatment is the same as that of brain. These sections may be stained very satisfactorily by the ink process, for communicating details of which we are indebted to the kindness of Dr. Paul, of Liverpool. The agent usually employed is Stephenson’s blue-black ink, which, for this purpose, must be quite fresh. As in the case of carmine, two methods of staining may be adopted—either rapid, by using concentrated solutions, or more prolonged, according to degree of dilution. For the reasons previously given (§ 14), slow methods of staining are always to be preferred, as yielding the most beautiful results, yet, for the purposes of preliminary investigation, it is often convenient to have recourse to the quick process. To carry out the latter plan, an ink solution of the strength 1 in 5—10 parts of water is to be freshly prepared, and the sections exposed to its action for a few minutes. For gradual staining the dilution must be carried to 1 in 30—50, and the time of immersion prolonged to several hours, the sections being occasionally examined during the staining, so that they may be removed just as they have acquired the desired tint. When a satisfactory coloration has been obtained, the preparations should be mounted in dammar or balsam (§ 23). One advantage of this method of staining is, that definition is almost as good by artificial light as by day.
44. Sponge may readily be cut after being tightly compressed between two bits of cork; or its interstices may be filled up by immersion either in melted paraffine (§ 11) or in strong gum (§ 18), and then cut as usual.
45. Stomach requires no special method of hardening (chromic acid). Sections should always, when practicable, be cut in the freezing microtome. In default of this, proceed in the manner as directed for lung (§ 35). Both vertical and horizontal sections will, of course, be required. If the preparation has been injected, the latter are particularly beautiful. Stain with carmine or aniline blue (§ 27), and mount—if for very close study, in glycerine—if injected and for a “show” slide, use balsam.