Frazer has introduced a “student’s sliding microtome” on the same principle as the Schanze which costs about £3.
The Cambridge Rocking microtome.—This instrument, as made by the Cambridge Scientific Instrument Company, or the slightly modified form made by Messrs. Swift (fig. [8]), is the best instrument for cutting sections of small objects embedded in paraffin. Ribbons of serial sections can be obtained from it with greater ease and certainty than with other microtomes. This microtome differs from those which have been previously described in that the knife is fixed, while the object is moveable. The microtome consists of an oblong heavy metal stand. A long bar is arranged so that it rides in see-saw fashion on two strong vertical pillars arising from the frame. One end of this bar is hollow, and receives the piece of wood carrying the tissue embedded in paraffin, which is firmly clamped in position. This end is depressed by means of a strong spiral spring. In order to raise it there is an arrangement by which the other end of the bar is depressed by a cord which revolves round a pulley. When the handle is turned, the tissue is raised, and when the cord is relaxed, the spring pulls the tissue firmly and steadily down. The razor, which must have a straight edge, is fixed firmly by screws, with its edge upwards at the end of the microtome. The object is then adjusted so that in its descent a thin slice is taken off by the razor. There is an ingenious arrangement by which the depression of the bar to raise the section pushes it a little further in the direction of the razor. The distance can be graduated from 1/500 to 1/3000 inch. The actual working of the machine is therefore very simple. The position of the block containing the tissue to be cut having been adjusted so that the razor just cuts it, the free end is depressed by means of the pulley. This also pushes the section a little beyond the razor. The strong spring then draws the tissue steadily past the edge of the razor, and a thin section is left on the blade. This may be at once transferred to a slide, or if the paraffin be of the proper consistence, another cut may be made, when the two sections should adhere by their edges, and so by repeating the movement a continuous ribbon may be obtained. If there is difficulty in obtaining a good ribbon, it will usually be got over by taking a little soft paraffin and attaching it by means of a hot needle to the lower end of the paraffin block. The cost of the instrument is about £5.
Fig. 8.—Swift’s Modification of the Cambridge Rocking Microtome.
Fresh sections.—Although these are not so satisfactory as hardened specimens for accurate histological work, it is often very useful to make them both in the post-mortem room where an immediate opinion of the nature of the tumour or diseased organ is desired, and also in the operating theatre. With a little practice sections may be cut, stained, and mounted, within ten minutes of the removal of the specimen from the body. In this way important information may be afforded to the operating surgeon, and in not a few cases it has caused the proposed treatment to be entirely altered. Thus, in one case, a supposed chronic periostitis was shown to be a sarcoma, and the limb was amputated. In another, a supposed sarcoma of the thigh was found to be a gumma, when a portion was removed and microscopically examined.
A portion of the specimen should be placed without any preparation on the zinc plate of the freezing microtome, and some gum painted round it. It is then frozen. The serum in the tissues is not in sufficient mass to injure the knife when it is frozen. The knife should be wetted with, and sections transferred to, either pericardial serum, or 3/4 per cent. solution (70 grains to the pint), of common salt, neither of which causes the cells to swell up as plain water does. They should be carefully floated out on a glass slide, an operation which requires much more patience than in the case of hardened sections, as fresh sections are less coherent and also more sticky, so that the edges tend to curl up on the knife, &c. They should then be examined, one unstained, simply mounted in salt solution; another stained with picrocarmine and examined in the saline solution; and a third stained in picrocarmine, mounted in Farrant’s solution, and preserved. The last usually gives the best results, the picrocarmine staining becoming quite brilliant after a week. The glycerine, however, is apt to make the sections shrink a good deal, and the weight of the cover-glass tends to break up the unhardened section.