If the specimen is very delicate, and likely to be spoiled by being curled up on the knife, the latter should be kept cold by frequently dipping it in a vessel containing lumps of ice in water. The gum will then remain frozen after cutting, and support the tissue better. Each section should be at once transferred to a glass slide from the knife, washing it off with a stream of ice water from a pipette.
The knife that is used may be an ordinary razor, with the edge ground straight. It requires to be held steadily with both hands. As this is rather inconvenient, Dr. Sheridan Délépine suggested the employment of an ordinary plane iron such as is used in a carpenter’s plane. This only requires one hand, and the other can be kept on the head of the screw beneath to raise the plate at once after each stroke of the knife. Its disadvantages are that it is rather heavy for prolonged working, and that it is less easy to “set” than a razor.
A. Frazer has recently introduced a valuable improvement in the Cathcart microtome (fig. [5]).
In this the brass frame carrying the zinc plate and ether spray tubes is surrounded by a brass cylinder, in which it fits accurately, and is pushed up as desired by turning the screw beneath the instrument. This brass frame and with it the zinc plate, &c., can be easily drawn altogether out of the outer tube, and replaced by a second brass well, which exactly fits its place and can be raised by the screw as desired. In this is a small toothed clamp which can be screwed up so as to hold a piece of wood carrying a piece of tissue embedded in paraffin. Sections can also be cut in celloidin with this instrument, but as oblique strokes with the knife cannot be made, it is impossible to get very thin sections. The combined microtome can be obtained for a guinea from Frazer, 22 Teviot Row, Edinburgh.
Fig. 5.—Frazer’s Modification of Cathcart’s Microtome.
A. Microtome arranged for ether spray. B. Cylinder with clamp for holding object embedded in celloidin, &c. to replace ether spray apparatus.
There is another modification which is more generally useful, and at the same time more expensive than the original model. In this, instead of glass runners to support the knife, there is a flat glass plate about eight inches square sufficiently large to allow of “Swift’s plough” (fig. [6]) being used for the purpose of cutting sections. This instrument consists of a triangular brass frame, supported on three legs, each of which is a screw, tipped with ivory. There is one screw in front and two behind. Beneath the plate, and held in position by the posterior screws in front, and a little clamp behind, is a razor with the edge directed forwards. The edge can be raised or depressed by turning the anterior screw, on which the frame is supported. Before sections are cut the edge of the razor should be brought down to the level of the tissue, taking care that all the legs are equal in length. The plough should then be firmly grasped with both hands, (the index finger of one hand being left free to turn the anterior screw) and pushed rather obliquely through the tissue. The edge of the razor is then slightly lowered by turning the screw through a very small angle, and another section made, and so on. With a little practice very thin uniform sections may be made with great rapidity.
Another useful ether spray microtome is that made by Jung of Heidelburg. The knife swings round a pivot, and there is an ingenious ratchet arrangement which works synchronously with each swing of the knife, to raise the tissue automatically the requisite distance for the next section to be made. The exact thickness of the sections can be graduated with great nicety by a simple contrivance. The instrument can be obtained in this country for about £2. It works satisfactorily, but, with practice, the student will get equally good results with the cheaper “Cathcart.”