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cumstances is a piece of platinum wire mounted in an ordinary screw needle-holder. It is pliable and can be bent to any shape, will not break like a glass needle when dropped, and is not affected by acids. but serviceable substitute. desired shape by heating. many other instruments required in microscopical work do not need any special mention.

Ladies' hat-pins form a cheap They are readily bent to any Forceps, scissors, scalpels, and

Bottles. For cover-slip work and for staining on the slide dropping-bottles will be found extremely convenient. The patent T. K. pattern of 50 c.c. capacity is probably the best form and size.

EXAMINATION OF FRESH MATERIAL.

Fresh tissues may be examined either in teased prepara

tions or in sections.

Teased preparations are made by cutting out a very small bit of the tissue in question and dividing it as finely as possible, by means of two sharp, clean needles, on a slide in a drop or two of some indifferent fluid, such as the normal salt solution. Teased preparations are often made, for instance, of the heart-muscle when fatty degeneration is suspected. If the tissue is soft, the cells are easily obtained by simply scraping the cut surface with the edge of the knife.

Sections of fresh tissues can be made with a razor or with a double knife, but much the better way, at least for general diagnostic purposes, is to use frozen sections, which can be very quickly and perfectly made with the freezing microtome. The fresh sections are put into salt solution in a glass dish large enough to permit of a slide being dipped into it, so that a section can be floated and spread out evenly on its surface. The slide is then carefully raised, the excess of fluid wiped off, and a cover-slip put on.

If it is desired to stain the section, a few drops of Löffler's methylene-blue solution are poured over it after it is spread evenly on the slide. In a few seconds the coloring fluid is

thoroughly washed off with salt solution, a cover-slip put on, and the section examined in the salt solution. If sections of fresh tissues are put directly into a staining fluid in the ordinary manner, they pucker up and do not stain evenly.

Fresh preparations are often treated with chemicals for various purposes. Of these chemicals, acetic acid is the most generally useful in pathological work. It shrinks the nuclei and renders their outlines more distinct. It swells connective tissue, making it more transparent, so that the elastic fibers which are unaffected stand out distinctly. It precipitates mucin and dissolves or renders invisible the albuminous granules so abundantly present in the protoplasm in the cloudy swelling of various organs in disease. Its main use as a reagent for fresh tissues is to demonstrate fat and to differentiate that substance from albuminous granules.

Acetic acid is ordinarily used in a 1 to 2 per cent. aqueous solution, a few drops of which are placed at one edge of the cover-slip, and then drawn beneath it by placing a piece of filter-paper on the opposite side. If in a hurry, however, stronger solutions, or even glacial acetic acid, may be used. Other reagents are of less importance, but are occasionally used.

Osmic acid is sometimes employed in a 1 per cent. aqueous solution to demonstrate fat, which it stains brown to black.

Hydrochloric acid in a 3 to 5 per cent. solution is used to demonstrate calcification. Phosphate of lime is simply dissolved, while from carbonate of lime bubbles of carbondioxid (CO2) are set free.

Indifferent Fluids.-Fresh tissues are usually examined in normal salt solution, a per cent. solution of common salt in water. It has the advantage over water that tissues do not swell up so much in it, blood-globules are unaffected, and the finer structures are better preserved. A very few drops of Lugol's solution added to the stock-bottle of salt solution will be found useful in preventing the growth of mould.

Serous fluids, such as hydrocele fluid, are occasionally used. Artificial serum is made by adding 1 part of eggalbumin to 9 parts of normal salt solution.

Macerating fluids are little used in pathology. Occasionally, however, when tissues are tough, so that they cannot be readily teased apart, they are macerated in certain fluids. which dissolve the substances that hold the different elements together. The reagents most commonly used are the following:

1. Ranvier's one-third alcohol is made by taking I part of 96 per cent. alcohol and 2 parts of water; twenty-four hours are usually enough.

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2. Very dilute solutions of chromic acid are recommended to go of I per cent.

3. 33 per cent. Caustic Potash.-Tissues are macerated in a few minutes to one hour: they must be examined in the same fluid, because the cells are destroyed if the solution is weakened.

Examination of Fluids.-Small fragments of tissue should be picked out with forceps. If much blood is adherent, wash the tissue well in salt solution. When the cellular elements are few in number they are obtained with a pipette, just as in urine-work, after allowing them to settle at the bottom of the glass. A centrifugal machine will be found of great service when the sediment is slight.

INJECTIONS.

INJECTIONS are not much used in pathology. The process is an art that requires much patience and considerable experience. The purpose of an injection is to render vessels and vessel-walls more visible than under ordinary circumstances. Transparent, deeply-colored fluid mixtures are used which will become hard in the vessels. Some injection-masses are employed cold, others warm. The warm injection-masses contain gelatin, and are much more troublesome to use, but give much the more perfect results. For

coloring the mass carmine is the best material, because it is a permanent color.

The instruments required are cannulas of various sizes and a syringe, or, better still, a constant-pressure apparatus. When a warm injection-mass is used the bottle containing the mass must be placed in a water-bath and kept at a temperature of about 45° C. The organ or animal to be injected must likewise be placed in a water-bath of the same temperature.

It is very important that in connecting the end of the tube carrying the injection-mass with the cannula inserted in the vessel no air-bubbles shall enter. When blood-vessels are to be injected it is advisable to wash them out first with normal salt solution.

Cold Injection-masses.—1. Blue Coloring Mass.—

Soluble Berlin blue,
Distilled water,

I;

20.

2. Carmine Injection-mass (Kollmann).-Dissolve I gram of carmine in I c.c. of strong ammonia plus a little water; dilute with 20 c.c. of glycerin. To this solution add 1 gram of common salt (NaCl) dissolved in 30 c.c. of glycerin. To the whole solution add an equal quantity of water.

Warm Injection-masses.-1. Berlin Blue.-Warm the solution of Berlin blue given above, and add it, with continual stirring, to an equal quantity of a warm, concentrated solution of gelatin prepared as follows: Allow clean sheets. of the best French gelatin to swell up for one to two hours at room-temperature in double the quantity of water. Then dissolve them by warming gently over a water-bath. Filter the combined solution through flannel.

2. Carmine-gelatin Mass.-This is by all means the best injection-mass to use, because it is permanent, but it is very difficult to prepare.

Dissolve 2 to 2.5 grams of best carmine in about 15 c.c. of water, to which just enough ammonia is added, drop by drop, to effect the solution. Filter the fluid obtained, and

add it, with continual stirring, to a filtered warm, concentrated solution of gelatin (prepared as above) over the waterbath. Then add acetic acid slowly until the color changes to a bright-red shade. The exact amount desired is when the solution loses its ammoniacal odor and has a peculiar sweetish aroma free from acid. Examined under the microscope, no granular precipitate of carmine should appear. If too much acetic acid has been added, so that the carmine is precipitated, the mass must be thrown away and a new lot prepared.

Organs which have been injected with a cold mass are. placed directly in 80 per cent. alcohol. After a few hours they are to be cut up into pieces that are not too small. After a warm injection-mass the organ or animal is placed first in cold water to hasten the solidification of the gelatin, and then transferred to 80 per cent, alcohol. Masses already prepared for injecting cold or warm can be obtained from Gruebler.

FIXING REAGENTS.

THE various reagents used for the preservation of fresh tissues possess the properties of penetrating, killing, fixing, hardening, and preserving in different degrees. Of these properties "fixing" is the most important, and to a certain extent implies or includes the others. The term "fixative" has been used more particularly, perhaps, for reagents which preserve faithfully the various changes of the nucleus in karyomitosis. In a broader sense, however, it refers to the faithful preservation of any tissue-element or pathological product, and of the chemical properties peculiar to that element or product. A good fixative is a reagent that penetrates and kills tissues quickly, preserves the tissue-elements, and particularly the nuclei, faithfully in the condition in which they are at the moment when the reagent acts on them, and hardens or so affects them that they will not be altered by the various after-steps of staining, clearing, and

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