Fig. 227.—Section Scissors and Forceps.
The use of the razor for cutting sections has not been wholly abandoned, the method of using which is as follows:—Take the tissue between the thumb and finger of the left hand, hold the finger horizontally, so that its upper surface may form a rest for the razor to glide upon, take the razor firmly, and keep the handle in a line with the blade, then draw it through the tissue from heel to point and towards yourself. While cutting keep the razor well wetted with diluted methylated spirit.
Fig. 228.—Dissecting Knives.
Some preparation is required for cutting sections with the single microtome. The substance to be cut must be embedded in some other material, as carrot, turnip, potato, alder pith, paraffin, or thick gum, with either of which the cylinder or well of the microtome must be so nearly filled as to leave only an excavation in the centre for the specimen to be operated upon to occupy. The various forms of microtomes in use, and the selection of the most suitable, is therefore a matter of some difficulty. I must content myself by particularising two or three typical forms in general use. As all the substances intended for cutting require preparation, it will be first necessary to attend to the following directions given by one experienced in section cutting, Mr. M. J. Cole[44]:—(1) Always use fresh tissues. (2) Cut the organs into small pieces with a sharp knife. (3) Never wash a specimen in water; when it is necessary to remove any matter, allow some weak salt solution to flow over the surface of the tissue, or wash it in some hardening re-agent. (4) All specimens should be hardened in a large quantity of the re-agent; too many pieces should not be put into the same bottle, and keep them in a cool place. (5) In all cases the hardening process must be completed in spirits. (6) Label the bottles, stating the contents, the hardening fluid used, and when changed. Attention to details is necessary, as if hardening is neglected, good sections cannot be made.
Embedding in Paraffin Wax or Lard.—Melt together, by the aid of gentle heat, four parts of solid paraffin and one part of lard. A quantity of this may be made and kept ready for use. Melt the paraffin mass over a water bath, take the specimen, and dry it between the folds of a cloth to remove the spirit, so that the paraffin may adhere to its surface, place it in a small chip-box, in the desired position, and pour in enough melted paraffin to cover it, then set aside to solidify; when quite cold break away the box, and cut sections from the embedded mass with a sharp razor.
To infiltrate a tissue with paraffin, place the specimen in absolute alcohol or chloroform for an hour or two, then transfer to a bath of melted paraffin, at its melting point (about 110° F.), and keep it at this temperature for several hours, so that the paraffin may penetrate to the middle of the tissue. Then remove the specimen from the paraffin and put it into a small chip-box, pour in enough paraffin to cover it, and set aside to cool. When quite cold, make sections as before, with a razor, or fix it into a microtome, with a little melted paraffin. The sections when cut must be placed in turpentine to remove the paraffin, and then into absolute alcohol to remove the turpentine, and finally in distilled water to remove the alcohol, when they may be forthwith stained. It is often found better to stain the tissue in bulk before embedding. In this case the sections will only require the turpentine to dissolve away the paraffin, and may then be mounted in Canada balsam.
Hardening and Preparing Animal Tissues for section cutting and microscopical examination.—Fresh tissues are not well suited for microscopical examination, but it is sometimes advisable to observe the appearances of a fresh specimen, especially if it is suspected to contain amaloid bodies or parasites. It will then be necessary to tease out a small portion of the tissue immersed in a weak solution of salt and water by the aid of a pair of fine needles ([Fig. 229]) and the dissecting microscope ([Fig. 230]).
Fig. 229.—Needles for teasing out Sections.