lar cement between the cells of serous membranes and other parts. For this purpose it is very useful for demonstrating the existence of squamous epithelium in such situations as the surface of the mesentery, or the lining of blood vessels. A per cent. solution is used, made by dissolving I gramme of the salt in 200 c.c. of distilled water.

In the case of the mesentery, it is merely necessary to immerse it for two or three minutes in the solution, and then expose it to the light in distilled water for two or three days. It is necessary to change the water several times.

When the lining membrane of a blood vessel is to be demonstrated, or in any similar case where it would damage the part to remove it at once, the silver solution may be poured on it in situ, and after being allowed to remain for two or three minutes, washed off with distilled water, the tissue being afterwards dissected out.

In making specimens of mesentery it is necessary to be careful that they should be spread out without being stretched, which will destroy the silver lines.

The whole mesentery may be removed with the intestine and stained in silver, and then hardened in spirit. Some portions of mesentery will then be found stretched out in a natural manner, and these can be cut out and mounted in Canada balsam solution.

They may with advantage be stained in a 5 per cent. watery solution of eosin or a strong solution of soluble aniline blue, in this way the structure will be brought out.

9. Picric Acid. A saturated solution of picric acid

will decalcify small bones. It is also used in some cases as a hardening agent by adding part of water to 2 parts of a saturated solution; but it does not give such good results as the chromic acid mixture.

10. Osmic Acid. This can be procured as a 1 per cent. solution in water, and it is then diluted to various strengths as required. It blackens fat and the medullary sheath of nerves.

A piece of mesentery placed in a weak solution for half an hour will show the fat cells lying along the course of the blood vessels, as round black bodies.

It is also used for hardening the internal ear.

CHAPTER 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 of an inch wide, and 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 waxy 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 filtering 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 filter paper 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 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; a very good one for this purpose can be procured from Baker, High Holborn; the razor must be very sharp. A small glass capsule about of an inch deep filled with methylated spirit is also required to put the sections into when cut, and to moisten the razor in.

Take the wax mass and with a scapel 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 down 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