Before mounting, the paraffin must be removed from the sections. This is easily done on the slide in the case of single sections and of ribbons. If the sections are curled, a little warmth will make them unbend and lie flat. The slide is warmed over a spirit lamp until the paraffin just melts. The sections will keep their places owing to the celloidin beneath. Xylol is then allowed to flow over the slide from a pipette, until the paraffin has been completely dissolved, which can be ascertained by glancing at the sections under the low power of the microscope. The slide is placed in an almost vertical position to let the xylol drain off, excess is wiped off from the edge of the slide with blotting paper, a drop of Canada balsam solution (p. [61]) is run on the slide, and a cover-glass of suitable size is applied.
Microtomes.—After a large amount of practice, persons with a fair amount of manual dexterity may acquire sufficient skill to be able to cut very satisfactory sections of specimens embedded in paraffin, &c., by hand. In the Pathological Laboratory of a large German University, until quite recently the use of a microtome was prohibited by the Professor, who is himself a most distinguished histologist. The amount of time expended before one acquires the necessary skill, and the cheapness and great convenience of the modern microtome have combined to throw hand cutting into the background, and some form of microtome is now almost universally adopted.
Fig. 4.—Cathcart’s Ether Spray Microtome.
A, B. Wooden frame and supports. C. Glass runners. G. Screw for raising the zinc plate H. J. Ether bottle. L. Tube from air bellows.
Of these there are a very large number in the market, each having special advantages, and often special drawbacks. A few of the more generally useful only will be described. We have microtomes for cutting in gum frozen by ether spray or ice, and those intended for cutting in paraffin or celloidin.
Cathcart’s ether spray microtome (fig. 4).—This, or its more recent modifications (see later), is perhaps the most useful and economical microtome for the purposes of the student. Its prime cost is low, it is small and portable as well as being clean and inexpensive to work with.
It consists of an oak frame which can be firmly clamped on to a table. On this frame are two narrow parallel supports about two inches high, which are covered by strips of plate glass, and serve as smooth rests along which the razor may glide in making sections. Between them is a brass well and in this a zinc plate firmly fixed in the horizontal position, which is almost at the level of the glass runners. It is capable of being raised or lowered through about 3/8 inch by means of a screw with a very fine and accurate thread. This screw is turned by a large milled wheel beneath the microtome. Just beneath the zinc plate are two small tubes, one connected with an india-rubber bellows, the other with a bottle at the side which contains ether. As the air issues from the first tube, it passes over the open end of the second, and thus draws the ether out and makes it play on the zinc plate, and at the same time causes it rapidly to evaporate, and so reduces the temperature of the zinc plate.
In cutting sections with this microtome the tissue is taken out of the gum and placed on the zinc plate. The bellows are then worked until the gum on the zinc plate is completely frozen. The plate should be lowered by means of the screw until the surface of the piece of tissue is on a level with the glass runners. These and the razor should then be wetted with water. The razor being held firmly in the hand is pushed along the glass runners in a rather oblique direction. The plate should then be raised by turning the screw below through a very small arc, another section taken off and so on. Sections are carefully removed from the razor to a vessel of water by means of a soft wet camel’s hair brush. The needle should never be used for this purpose.