almost colorless, and smaller than the opaque grayish-white colonies of the bacillus coli communis.

Of course suspected colonies must be shown to be composed of bacilli which conform to the various tests described elsewhere before they can be identified with any degree of certainty as those of the typhoid bacillus.

The Blood-serum Reaction in Typhoid Fever.-A few drops of the blood of a suspected case of typhoid fever are collected in a small test-tube, either from the finger or the ear. After clotting has taken place, transfer a drop of the serum by means of a medicine-dropper to 0.5 or 1.0 c.c. of a twenty-four-hour bouillon culture of the typhoid bacillus. After mixing, place a drop of the mixture on a slide, cover it with a cover-glass, and examine it with either an oil-immersion lens or a high-power dry lens. If desired, the mixture may also be examined as a hanging-drop preparation.

If the patient has typhoid fever, the majority of the bacilli on the preparation will be seen to lose their motility and to agglomerate" into clumps within a very few minutes. This constitutes the serum-reaction. It is only demonstrable, as a rule, after the first week of the disease.

The reaction may also be obtained from the dried blood. A few drops of the blood may be collected on a glass slide or a piece of paper and allowed to dry. It may then be brought to the laboratory, where the dried blood is extracted with 15 or 20 drops of water, and the test made with a drop. of the solution thus obtained, by mixing it with a drop of an eighteen to twenty-four-hour bouillon culture of the typhoid bacillus, and observing the mixture with the highpower dry lens after covering with a cover-glass.

Rabies (Hydrophobia).—The diagnosis of this disease from a pathological standpoint is usually made by the production of experimental rabies in a rabbit by intradural inoculation with material from the nervous system of the animal suspected to have died of it. The poison of the disease is found in the brain, spinal cord, salivary glands, and pancreas. For purposes of inoculation a piece (1 or 2 c.c.) of the medulla or brain, preferably the former, is rubbed up

in a sterilized mortar with about 10 c.c. of sterilized distilled water. The resulting fluid is filtered through absorbent cotton, and then through filter-paper, to remove tissueshreds. Of the clear fluid thus obtained 4 or 5 drops are injected beneath the dura of a rabbit by means of a hypodermic syringe, the skull being trephined with a small trephine about 4 mm. in diameter. The most favorable place for opening the skull is at a point in the median line just posterior to a line drawn through the middle of each eye.

The symptoms of experimental rabies in the rabbit first.

FIG. 76.-Bacillus of leprosy: section through a subcutaneous node, showing the bacilli in tissue-cells; X 500 (Fränkel and Pfeiffer).

manifest themselves after two weeks, never earlier, but they may not appear until later, not even until two months have passed. The first symptom is a weakness of the hind legs, followed by paralysis. The paretic condition soon extends to the fore legs, dyspnea appears, and death usually occurs in about three days after the onset of the symptoms. Paralytic symptoms developing before two weeks are not due to infection with rabies, but to some other cause; for instance, in

fection with the pneumococcus or other bacteria which may be present in the material inoculated.

During the course of the disease the animal never appears stupid, with dull eyes, as in other infections, but remains "conscious," so to speak, until the last.

Leprosy. The bacillus of this disease shows essentially the same staining reactions as the bacillus tuberculosis. In sections of the lesions the bacilli are found in large numbers, mostly inside the tissue-cells (see Fig. 76).

Actinomycosis.-The microscopical diagnosis has been discussed in the section on Bacteriological Diagnosis.

PART III.

HISTOLOGICAL METHODS.

Introduction.-The ideal function of the technique of pathological histology is so to fix tissues for microscopic examination that every tissue-element or pathological product is perfectly preserved with all its morphological and chemical properties intact, and so to stain tissues that every tissue-element or pathological product can be readily differentiated from any other tissue-element or pathological product that resembles it. In certain respects only has this ideal yet been reached, but the number of differential stains is increasing yearly.

In the following pages the various steps in the preparation and staining of tissues have been arranged, so far as possible, in logical sequence.

LABORATORY OUTFIT.

Microscopes.-The most important laboratory instrument is the microscope. It should be, so far as means will permit, the best that skill can produce. The two foreign makes of microscopes most in favor in this country are the Zeiss and the Leitz. Excellent microscopes are also made by Reichart, Hartnack, and Seibert. American microscopes have greatly improved during the past few years, but do not yet reach the standard set by the best foreign makes. Undoubtedly the best microscopes in every particular and the most expensive are those manufactured by Zeiss.

It is important for a beginner in microscopy, before buying a microscope of any make, to have it carefully examined and its lenses tested at a pathological or other laboratory by

some one skilled in its use. The continental form of stand of medium size is to be preferred to all others. The large stand is undesirable, because it is too heavy and too high for comfortable use. It should be furnished with rack and pinion, and with micrometer screw for coarse and fine adjustment, with a triple nose-piece, and with an Abbe condenser and iris diaphragm. The necessary objectives are a low and a high dry, and a oil-immersion. Two eye-pieces, a low and a high, will be found sufficient for all ordinary purposes. The stands, oculars, and objectives generally used are the following:

Even if all these different parts cannot be purchased at the same time, it is important to buy a stand to which they afterward may be added, for the list includes only what every medical practitioner should have at his service for the proper examination of urine, sputum, blood, etc.

The apochromatic lenses and compensation oculars are too expensive to come into general use. Fortunately, they are more important for photomicrography than for general microscopic work.

The oil-immersion lens should always be cleaned after using by wiping off the oil with an old linen or silk handkerchief or with the fine tissue-paper now manufactured for that purpose. If the lens is sticky, moisten the cloth with. benzol or xylol; Leitz recommends alcohol. The same process can be used if necessary for the dry lenses, but it must be done quickly, so as not to soften the balsam in which the lenses are imbedded. Ordinarily a dry cloth is sufficient.

In using the Abbe illuminating apparatus it is important. to bear in mind that the best results are obtained, according to Zeiss, by employing the plain mirror, for the condenser is