СНАРТER III.

ON CUTTING SECTIONS.

SECTIONS may be cut either by hand with a razor, or with the microtome.

1. In cutting sections by hand it is necessary to imbed the tissue in some material which will cut easily, and at the same time hold it firmly.

The best substance for this purpose is a mixture of wax and olive oil.

Take equal parts of white wax and olive oil by weight and melt them together, pour into a shallow vessel, and when cold cut into small blocks.

Small tin boxes with a removable bottom are required to hold the mixture while the tissue is being imbedded, and the best size is two inches long, by three-quarters of an inch wide, and three-quarters of an inch deep.

It will be necessary also to have a small porcelain ladle and a stand to raise it above a spirit lamp or gas jet. Melt some of the wax mass in the ladle, and be careful not to make it too hot.

Prepare the tissue so that the face which is to be cut can be easily recognized, stick a needle into it away from the part which is to be cut, drain off most of the spirit by laying it on filter-paper, and then immerse in the melted wax mass, so that it is perfectly covered; take it out and let it cool. Take a small piece of filterpaper and place it over the removable bottom of the tin box, and then fix it in its place; the filter-paper will prevent the wax from running out if the bottom fits loosely. It is also useful to leave a little of the paper projecting on which to write the name of the material imbedded. Then half fill the box with melted wax-mass and hold the material in it, keeping it quite steady until the wax hardens; then by gently screwing the needle round it can easily be removed, and the box filled up with wax-mass. It will be found a saving of time to imbed a portion of material at each end of the box. When the wax-mass has become thoroughly hard, which will take some time, especially in warm weather, pull off the bottom and push the

wax-mass, with the filter-paper adhering, out of the box. It can then be laid by until wanted, the name of the material imbedded being written on the paper.

For cutting sections thus imbedded, a hollow-ground razor is necessary; the razor must be very sharp. A small glass capsule about three-quarters of an inch deep filled with spirit is also required to put the sections into when cut, and to moisten the razor in.

Take the wax-mass and with a scalpel carefully remove small slices from one end, until the imbedded mass can just be seen, then take the razor and dip it into the capsule, taking up a little spirit ; let this run along the edge so as to thoroughly moisten it, and commence cutting as thin sections as possible, by drawing the razor diagonally across the mass with a steady sweep; this must not be done too quickly, and the amount of pressure to be put on the razor will depend on the tissue imbedded and can only be learnt by practice. As each section is cut dip the razor into the capsule of spirit and wash it off. Wipe the razor occasionally and remove adhering portions of wax-mass, and always keep the edge wetted with spirit. When a sufficient number of sections have been cut, the thinnest should be selected and removed to a watch-glass containing clean spirit.

Great care is required in cutting sections by hand, to hold the razor firmly yet lightly, so as to cut them thin and at the same time even, and this cannot be done without a great deal of practice.

For larger sections the boxes must be proportionately increased in size, and it will be found convenient, when the wax-mass is as wide or wider than the razor, to cut off slices from each side, so as to reduce the surface to be cut as much as possible without interfering with the stability of the imbedded material.

A small flat spear-headed needle will be found useful for taking up very small sections.

THE FREEZING MICROTOME.

A much easier method of cutting sections is by using a microtome. Of these there are a number made, in some of which the material is imbedded in wax-mass, or a mixture of paraffin and lard, and raised gradually by a screw, while a razor is worked on a flat plate shaving off sections; these are also made with a chamber to contain a freezing mixture so that the material can be imbedded and frozen.

The best microtome for pathological use is the Williams.

It consists of a tub to contain the freezing mixture, with a brass standard into which is to be screwed the brass circular plate on which the material is frozen. A top with a glass surface fits on to this, having a hole through which the circular plate projects. The knife is fixed into a triangular frame having screws at each angle by which it is raised and lowered.

To prepare the microtome for use:

1. Have the knife as sharp as possible.
2. Pound some ice finely in a cloth.

3. Scrape some salt into a fine powder.

With a spoon put a layer of ice into the tub and then some salt on it, mix with the spoon, and so on, until the tub is about half full, then ram it hard with a stick and fill again; put in the salt and ice in about equal proportions, leave room for the top, wipe off the salt and ice from the edge, put on the top and fix it with the screw for that purpose. Screw the circular plate into its place, and the microtome is ready for use.

An India-rubber tube must be fixed to carry away the drainings. as the ice melts.

Mr. Groves, of King's College, London, has adapted an ether spray to this machine, by which it can be used at any moment without the trouble of procuring ice. This instrument will be found a great convenience to those who only do occasional work. The ether fumes are carried away by a waste tube, which can be led outside a window so that no unpleasant smell is developed in the

room.

Dr. Roy has invented a microtome on a different principle, but which is used with the ether spray. It is made by the Cambridge Scientific Instrument Company. It is invaluable when fresh material has to be examined, as a small portion is taken, laid on the brass plate, covered with mucilage, and frozen at once. This can be done more quickly than with either of the other instruments.

SHARPENING THE RASOR.

To cut even sections, the knife must be perfectly sharp and free from any notches, as a notch so minute as to escape the eye will leave a ridge across the section, looking like a thick band under the microscope.

The first thing to be done is to learn to sharpen the razor oneself;

even the best surgical instrument makers will return a razor with notches in it, like a saw; they cannot be seen, but on drawing the razor carefully over the edge of the thumb-nail, or a piece of cardboard, they can easily be felt. To sharpen razors a German hone is necessary; this can be procured from Eberbach & Son, Main Street, Ann Arbor. In choosing a hone, pick out the softest, one that can easily be marked by the finger-nail, one also that is free from dark streaks.

The hone must be moistened with water; if oil is used the mucilage in which the material is frozen will not adhere to the edge of the razor, and the sections will curl up. Having moistened the surface of the hone with water, lay the razor flat on it, and draw it across diagonally from heel to point, with the edge forward, first one way then the other.

When there are no notches in the edge, half a dozen times each way will be enough, but when there are notches the process must be continued until the edge is found to be perfectly smooth when drawn across the edge of the thumb-nail or a piece of cardboard. Then wipe the razor quite dry, and strop it in the usual manner. The best strop is one made with a screw handle, so that it can be tightened as required, but it should be of plain leather with no oil or composition on it.

TO PREPARE THE MATERIAL.

Any tissue which has been preserved in spirit must be soaked in water for about six hours to remove the spirit, and then placed in mucilage for about six hours.

It will be found a great saving of time, when a number of specimens are to be cut at one freezing, not to have the material too thick, as a piece a quarter of an inch thick will give an enormous number of thin sections, and take only a short time to freeze.

TO MAKE MUCILAGE.

Pour warm water ou picked gum acacia and make a solution rather thinner than the mucilage sold in the shops.

CUTTING THE SECTIONS.

The specimens having been soaked in the solution of gum acacia are now ready to be cut into sections. Take up one with a pair of

forceps, and lay it on the circular plate of the microtome, drop some gum solution on it with a small brush, and see that it runs down on to the plate all around the specimen so as to fix it firmly. When it is thoroughly frozen adjust the razor so that it will just pass over without touching. Then lower a little more and try each end of the razor against the frozen material to see if it is quite level. If one end is too low, raise it until they both make the same cut. Raise the frame again until it just clears the material, and then lower the screw at the apex each time, by giving it a slight turn to the left, so that the knife takes off a thin section of the frozen material. The knife must be pushed across in a diagonal direction. Very slight pressure must be made from above; the pressure necessary to carry the knife through the material is made by the two thumbs, applied to the base of the brass frame. The student should accustom himself to hold the frame with the fingers of the left hand in such a manner that he has a firm grasp of it, and can take off the sections with a brush, and not drop the razor and frame. Have a small vessel ready, containing warm distilled water. It is necessary to use distilled water, as in ordinary water the lime in solution is precipitated by boiling and the specimen will be covered by fine particles of carbonate of lime and utterly ruined. Moisten the upper surface of the razor with gum solution, and the sections as they are cut will slip up on it without curling; carefully remove them with a camel's-hair brush and place them in the warm distilled water and let them remain for ten minutes or longer, until the gum is dissolved out; this will take longer with some material, such as lung, than others.

With these microtomes the most beautiful sections can be cut, perfectly even throughout, surpassing anything that can be done by hand. With care very large sections may be cut quite as readily as smaller ones, but the razor must be very sharp, and the material not too hard; those hardened in chromic acid mixture seem to do best. Very little force is required in pushing the knife through the material, and if it is sharp a very slight turn of the screw each time will enable one to cut a section; which ought to be so thin as to be almost invisible, as the gum melts on the razor.

In cutting some material, such as retina, it is advisable to stain it en masse before freezing, otherwise the sections cannot be seen when placed in water.

In cutting, the razor must never be stopped in the middle of a section, as it will always produce a ridge where the edge of the razor stops; this shows as a dark line across the stained section.