The Preparation & Mounting of Microscopic Objects

by microscopist Thomas Davies

Dissection.—As I stated at the commencement of this chapter, no written instructions can enable any student to become an adept in this branch without much experience and no little study. I will, however, describe the necessary apparatus, and afterwards mention the mode of treatment which certain objects require.

A different microscope is manufactured for the purpose of dissection, most first-rate makers having their own model. The object-glasses of many of these are simple, and consequently not expensive; but one of the great requisites is a stage large enough to hold the trough, in which the operation is often performed. Where this is the case it would scarcely be worth the expense of getting a dissecting microscope if the student were pursuing no particular study, but merely made use of the instrument when an object to be operated upon turned up accidentally. The ordinary form is much improved for this purpose, by having two wooden rests placed at the sides of the microscope, upon which the hands may be supported when working upon the stage. These should be weighty enough to be free from danger of moving. These supports will also be found to be a remedy against much of the weariness which inevitably arises from having to sustain the hands as well as work with them. The erector, as I before observed, is necessary to a young student; but with a little practice he may work very well without it.

We will now notice some of the instruments which are most useful in dissection. Two or three different sizes of ordinary scissors should be possessed, but the shapes must be modified in others for many purposes, as those used by surgeons; a pair with the cutting parts bent in a horizontal direction, and another pair slightly curved in a perpendicular; so that parts of the substance operated upon may be reached, which it would be impossible to touch with straight scissors. One point of these is sometimes blunt, and the other acute, being thus made very useful in opening tubular formations. Another form of these is made, where the blades of the scissors are kept open by a spring, the handles being pressed together by the fingers. Where it is desirable, one or both of these handles may be lengthened to any degree by the addition of small pieces of wood.

The Knives which are most useful are those of the smallest kind which surgeons employ in very delicate operations. These are made about the length of an ordinary pen-knife, and are fixed in rather long flattish handles; some are curved inwards, like the blade of a scythe, others backwards; some taper to a point, whilst others again are broad and very much rounded. Complete boxes are now fitted up by the cutlers, of excellent quality and surprisingly cheap.

Needles.—These are very useful and should be firmly fixed in handles as recommended in [Chapter I.] It is convenient to have them of various lengths and thicknesses. If curved by heating and bending to any required shape they may be re-hardened by putting them whilst hot into cold water. Dr. Carpenter also makes edged instruments by rubbing down needles upon a hone. They are more pleasant to work with when short, as the spring they have whilst long robs them of much of their firmness.

A glass syringe is also useful in many operations, serving not only to cleanse the objects but to add or withdraw liquids from the dissecting-trough. This trough will now be described, as many substances are so changed by becoming dry that it is impossible to dissect them unless they are immersed in water during the operation. If the object is opaque and must be worked by reflected light, a small square trough may be made to the required size of gutta-percha, which substance will not injure the edge of the knives, &c.; but where transparency is necessary, a piece of thin plate-glass must be taken, and by the aid of marine glue (as explained in [Chapter IV.]) the sides affixed of the required depth. As pins, &c., cannot be used with the glass troughs and the substance must be kept extended, a thin sheet of cork loaded with lead in order to keep it under water may be used; but this, of course, renders the bottom opaque. When working with many thin substances, a plate of glass three or four inches long and two wide will serve every purpose, and be more pleasant to use than the trough. A drop or two of water will be as much liquid as is needed, and this will lie very well upon the flat surface. As these are the principal apparatus and arrangements which are requisite in dissection, the method of proceeding in a few cases may now be noticed.

Vegetables.—The dissection of vegetable matter is much less complicated than that of animal; maceration in water being a great assistant, and in many cases removing all necessity for the use of the knife. This maceration may be assisted by needles, and portions of the matter which are not required may be removed by them. When, for instance, the spiral vessels which are found in rhubarb are wanted, some parts containing these are chosen and left in a small quantity of water until the mass becomes soft, and this is more quickly effected when the water is not changed. The mass must be then placed upon a glass plate when practicable, or in the trough when large, and with the aid of two needles the matter may be removed from the spiral vessels, which are plainly seen with a comparatively low power; and by conveying these to a clean slip of glass, repeating the process, and at last washing well, good specimens may be procured. Most of these should be mounted in some of the preservative liquids in the manner described in [Chapter IV.] Many, however, may be dried on the slide, immersed in turpentine, and then mounted in balsam; but liquid is preferable, as it best preserves the natural appearance. Certain kinds of vegetables require a different treatment to separate these spiral vessels. Asparagus is composed of very hard vegetable matter, and some have recommended the stems to be first boiled, which will soften them to such a degree that they may easily be separated. Dilute acids are also occasionally used to effect this; and in some instances to obtain the raphides caustic potash may be employed; but after any of these agents have been made use of, the objects must be thoroughly cleansed with water, else the dissecting instruments (and perhaps the cell) will be injured by the action of the remaining portion of the softening agent.

For the dissection of animal tissues it is necessary that the instruments be in the best order as to sharpness, &c.; and as the rules to be observed must necessarily be somewhat alike in many instances, the treatment required by some of the objects most frequently mounted will now be described. We may here remark that cartilage can be best examined by taking sections, which will show the arrangement of the cells very perfectly. This, however, is plainly seen in the mouse’s ear without any section being necessary. Glycerine, the preservative liquids before mentioned, and Canada balsam are all used to mount it; but perhaps the first named may be preferred in many cases.

Muscle.—This is what is commonly called the “flesh” of animals. If a piece be laid upon the slide under the microscope, bundles of “fibres” will be perceived, which with needles and a little patience may be separated into portions, some of these being “striated,” or marked with alternate spaces of dark and light. Some of the non-striated or smooth class of muscle, such as is found in intestines, may be prepared for the microscope by immersing for a day or two in nitric acid diluted with three or four parts of water, and then separating with needles and mounting as soon as possible. Sometimes boiling is resorted to to facilitate the separation, and occasions little or no alteration in the material. Specimens are often taken from the frog and the pig, as being amongst the best, Goadby’s solution being generally used in mounting them. The muscle of insects also shows the striæ very perfectly.

Nerve-tissue.—This is seldom mounted; as Dr. Carpenter observes, “no method of preserving the nerve-tissue has been devised which makes it worth while to mount preparations for the sake of displaying its minute characters,” but we will mention a few particulars to be observed in its treatment. The nerve should be taken from the animal as soon as possible after death, and laid upon a glass slide, with a drop or two of serum if possible. The needles may be used to clean it, but extreme delicacy is necessary. It will be found that the nerve is tubular and filled with a substance which is readily ejected by very slight pressure. When the nerve is submitted to the action of acetic acid, the outer covering, which is very thin, is considerably contracted, whilst the inner tube is left projecting; and thus is most distinctly shown the nature of the arrangement. Mr. Lockhart Clarke, who has made great researches into the structure of the spinal cord, gives a part of his experience as follows:—He takes a perfectly fresh spinal cord and submits it to the action of strong spirits of wine. This gives the substance such a degree of hardness that thin sections may be readily cut from it, which should be placed upon a glass in a liquid consisting of three parts of spirit and one of acetic acid, which renders them very distinct. To mount these sections, they must now be steeped in pure spirit for two hours and afterwards in oil of turpentine, and lastly must be mounted in Canada balsam.