a methodical and systematic procedure is always requisite. So many unknown objects, so many strange and unusual forms, so many structural peculiarities are revealed to the eyes of the tyro in microscopy, that he is at once plunged into profound confusion, from which he can extricate himself only by adopting the most laborious and rigid system of observation. He should examine with the utmost care the physical appearance and character of the ultimate structures; he should note the exact shape of the object, whether round or oval, globular or flat, &c.; the peculiarities of its edge or border, whether fine and brilliantly illuminated, or dark and abrupt, whether smooth or rough, regular or irregular, serrated or beaded, &c. Peculiarities of color produced by strong and faint, and by reflected and transmitted light, should next claim his attention. The size of the object should, in all cases, be obtained by actual measurement, and all variations in diameter noted. The transparency must also be observed,-whether the body be opaque or diaphanous. If opaque, the degree of opacity must be stated, its causes, and the effects upon the transmission of the luminous rays. The superficial and deep-seated layers, and in the case of cellular and tubular bodies, the contents should also be investigated; and lastly, the effects of various reagents upon these physical properties must be ascertained with the same care and patience.

It will thus be seen that the successful application of the microscope to the diagnosis of disease, requires a very considerable acquaintance with the healthy appearance and structure of both the animal and vegetable tissues. Armed with this preliminary knowledge, the student will be surprised at the facility with which he will be enabled to distinguish from each other the various animal solids and fluids, different morbid products, the matters constituting food, &c., whether these be unchanged, or in a state of disintegration from the processes of mastication, digestion, ulceration, &c.

In view, however, of the great difficulty experienced in demonstrating accurately the histological character of the healthy tissues, and the still greater difficulty of making out the characteristics of morbid growths, the student should exercise great caution and deliberation in pronouncing an opinion upon the nature of any morbid tissues examined by him.

The student should early acquire the habit of recording all his observations in a note-book kept expressly for the purpose. He should exercise himself also in making drawings, as exact as possible, of all the objects he examines. Such a practice, though laborious, will leave upon his mind a more vivid and lasting impression of the various objects of his research, and gradually render him a very close and reliable observer.

EXAMINATION OF THE NERVOUS SYSTEM.

FIG. 346.

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B

Brain.-As the nerve-fibres rapidly undergo change, the brain should be examined very soon after death, by depositing minute portions upon a perfectly clean slip of glass, and moistening them with serum or a weak saccharine solution. If it is desired to examine the distribution and arrangement of nerve-fibres, the brain should be placed in a solution of chromic acid; by the hardening thus produced it can be easily cut into thin slices by means of a Valentin's knife. The addition of water to a portion of white cerebral matter changes the natural appearance of these fibres, by separating the oily and albuminous contents of C the tubular sheath. The oily matter collects into globules, giving a beaded appearance to the fibres. (Fig. 346, g, g'.) The nerve-tubes may be rendered very distinct by the addition of a dilute solution of caustic soda.

Small portions of the meninges of the brain may be examined in the same manner.

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A

To examine the cerebral vessels, a thin section must be well washed, and subjected to gentle pressure. The vessels are thus deprived of their investing neurine, and may now be rendered more distinct by the dilute caustic soda.

The corpora amylacea, or gritty particles found in the pineal gland and other parts of the brain (Fig. 347), must be separated from the nervous tissue for examination by repeated washing in water.

Spinal Cord.-To examine the spinal cord with advantage, it should first be hardened in a solution of chromic acid, or in spirits of wine. The structure of thin sections is thus rendered quite conspicuous. The following method was employed by Mr. J. S. Clarke.1

"A perfectly fresh cord was hardened in spirits of wine, so that extremely thin sections, in various directions, could be made by means of a very sharp knife. A section so made was placed on a glass slide, and treated with a mixture composed of one part of acetic acid and three of spirits of wine, which not only makes the nerves and fibrous portion more distinct and conspicuous,

1 Philosophical Transactions, 1851, Part ii.

but renders also the gray substance much more transparent. The section was then covered with thin glass, and viewed first

The lower figure represents a choroid plexus with several small tumors at * supposed at first to have been tubercular; they proved to consist of aggregations of concentric corpuscles, cholesterine, and pure oil, united by areolar tissue; the concentric corpuscles which are shown above the plexus are magnified 100 diameters.

by reflected light with low magnifying powers, and then by transmitted light with higher ones.

"According to the second method, the section is first macerated for an hour or two in the mixture of acetic acid and spirit. It is then removed into pure spirit, and allowed to remain there for about the same space of time. From the spirit it is transferred to oil of turpentine, which expels the spirit in the form of opaque globules, and shortly (sometimes immediately) renders the section perfectly transparent. The preparation is then put

FIG. 348.

up in Canada balsam, and covered with thin glass. By this means the nervefibrils and vessels become so beautifully distinct, that they may be clearly seen with the highest powers of the microscope. If the section be removed from the turpentine when it is only semi-transparent, we sometimes obtain a good view of the arrangement of the blood vessels. This mode of preparation succeeds best in cold weather; for in summer, the cord, fresh when immersed in the spirit, remains more or less spongy, instead of becoming firm and dense in the course of five or six days. The spirit should be diluted with an equal quantity of water during the first day, after which it should be used pure. Certain modifications of this mode of preparation

may be sometimes employed with advantage by a practised hand."

Nerves. The structure and arrangement of nerve-fibres are best studied in the mesentery of small animals, as the newt; though with a little care in manipulation they can be very well displayed in any part of the nervous system. Their ultimate distribution, however, presents greater difficulties. Phosphoric acid, and solutions of caustic soda, and iodine of different strengths, are of great use in rendering these fibres more distinct. According to Dr. Waller, the tongue of the frog is best adapted for examining the arrangement of nerve-fibres in papillæ. When the nerve-fibres are not quite fresh, or have been soaked in water, and where they have been stretched or subjected to pressure for some time, their structure will be found to have undergone certain peculiar changes, as complete conversion into fibrous tissue, fatty degeneration, &c. (Fig. 348.)

EXAMINATION OF THE MUSCULAR SYSTEM.

Muscular Fibre.-Sarcolemma.-Muscular fibre is of two kinds, -the striated, voluntary, or muscular fibre of animal life; and the unstriated, involuntary, or muscular fibre of organic life.

The voluntary muscles of man and the lower animals furnish specimens of the striated fibre. They may be prepared for examination by cutting out a small slice from a muscle, separating the fibres with fine needles, and placing them upon a glass slide, and adding a drop or two of water. Muscles which have been boiled or hardened in chromic acid, corrosive sublimate, or spirits of wine, yield excellent sections for examination. The general anatomy of voluntary muscular fibre is well displayed. in the thin slips of muscle lying just beneath the skin of small animals, as the frog; while the general arrangement and form of the fibres is well shown, according to Beale, in a transverse section of the pectoral muscle of a teal (Querquedula crecca), which has been put upon the stretch, and allowed to become perfectly dry.

The ultimate fibrille may be studied with advantage upon the muscular tissue of the eel and pig. The fibrillæ are separated and rendered distinct by maceration in chromic acid. From the back of the throat, after a meal of meat, in the discharges of cholera patients and in vomited matters, admirable specimens, showing the transverse striæ, may often be obtained. In examining the arrangement of the nuclei, solutions of caustic soda and acetic acid will be found very useful.

In the tongue of the frog, as shown by Kölliker, and in the upper lip of the rat, according to Huxley, peculiar fibres, known as branched muscular fibres, may be found. To obtain specimens of these fibres, the tongue of a frog is boiled for a short time in water. A piece of the mucous membrane is then dissected away,

and a very small portion of the submucous tissue cut from the edge of the tongue with a pair of sharp scissors. This is torn into fragments with very fine needles, and then placed in the field of a quarter-inch object-glass. If the tongue is boiled very long, the fibres become too brittle for separation and examination.

The crustacea, mollusca, and insecta present peculiarities in the structure of their voluntary muscular fibre which separate them in a marked manner from the higher divisions of the vertebrata.

The involuntary, smooth, or non-striated muscular fibres, though appearing like flattened bands (Fig. 350), in reality, according to Kölliker, consist of elongated cells. They are found in various situations, as in the alimentary canal, the large and

FIG. 350.

small arteries, veins and lymphatics, the trabecular tissue of the spleen, the uterus, bladder, and urethra, &c.

"The contractile fibre-cells have been arranged in three classes :

1. Short rounded or flattened cells, somewhat resembling epithelium.

2. Flattened bands, with fringed edges. 3. Long rounded or fusiform fibres, slightly wavy, and terminating at each end in a point.

"The first two varieties are obtained from the blood vessels. The last form is met with in the intestinal canal, uterus, &c. These cells may be readily isolated by macerating small pieces of the muscular coat of the alimentary canal, &c., in dilute nitric acid, containing about 20 per cent. of strong acid. By a little