The Brain and Spinal Cord.—In order to examine the structure of these parts, it is necessary to harden portions, by immersion in chromic acid or alcohol, until they are sufficiently firm to be cut with a razor. The sections are afterwards to be rendered transparent by being covered with glycerine.

Mr. Lockhart Clarke's method of making preparations of nerve structure has been found so effective, that the process is given in detail:

Portions of brain, medulla, or spinal cord, are to be cut into pieces, as small as convenient for making the necessary sections subsequently, and steeped in a solution of one part of crystallised chromic acid to 200 parts of water for two or three weeks, and then kept in a solution of about 1 part of bichromate of potash in 100 to 200 parts of water. If for rodents, birds, reptiles, and fishes, 1 part to 600 parts of water, gradually increasing the strength at the end of a week.

Sections may then be made by means of a knife or razor dipped in spirit. The slices are to be first placed in spirit for a few minutes, and then floated on to the surface of some turpentine, where they must be allowed to remain until nearly or quite transparent, when they are to be removed to glass slides on which a little Canada balsam has been previously dropped. Examined under the microscope, the preparations will probably present, at this time, but little trace of structure; but if set aside for some time and occasionally treated with a little turpentine and Canada balsam, the cells and fibres reappear, presenting a beautiful appearance. Before the preparations are finally covered they should be examined by the microscope at intervals.

The principle of this method is this: to replace the spirit by the turpentine, and this by Canada balsam, without drying the sections.

Another plan consists in steeping 'the sections of the hardened structure in the carmine solution for twenty-four hours, then drying them slowly, and afterwards washing them in turpentine, and mounting in Canada balsam rendered thin by the addition of turpentine or benzole.

The illustration represents a section of the spinal cord prepared in the manner directed, by treatment with chromic acid, and subsequently by turpentine and balsam.

Transverse section of human spinal cord, close to the third and fourth
cervical nerves; magnified ten diameters (from Stilling). f. Posterior
column. i, i. Gelatinous substance of the posterior horn. k. Posterior
root. 1. Supposed anterior root. a. Anterior fissure. c. Posterior
fissure. b. Grey commissure, in which a canal is contained, which, ac-
cording to this writer, extends through the length of the cord. g. An-
terior horn of grey matter containing caudate vesicles. e. Antero-lateral
column (from k to a).

The distribution of the bloodvessels can only be studied in transparent injections, of which very beautiful examples may be obtained of the different dealers in microscopic specimens.

OF THE EYE.

As the various appendages to the visual organ-the muscles, glands, and tarsal cartilages-differ in no respect from similar tissues in other parts of the body as to the method of examination, it will not be necessary to repeat directions that have already been given. One appendage, however, being peculiar to the eyes of animals, requires especial notice. In the horse, ox, sheep, and many others, there exists at the inner canthus a semilunar expansion of cartilage, connected by means of a long stem with the fatty tissue at the bottom of the orbit, and so arranged as to be protruded over the front of the eye when that organ is drawn inwards by the action of the retractor muscle. nous structure is termed the cartilago nictitans. It may easily be

This cartilagi

examination.

Under the microscope the ordinary groups of cartilage cells will be observed imbedded in a granular matrix.

Proceeding to the study of the eyeball and its contents, the cornea and sclerotic ought first to be examined in sections of dried preparations.

For this purpose take an eye of an ox or sheep, dissect off all the surrounding tissues, and cut away the posterior third of the globe to allow the contents to escape. The shell is then to be slowly dried until sufficiently firm to bear the pressure of the razor. Sections are to be made by first dividing the preparation across the centre of the cornea, and then cutting thin slices from the edge so as to include portions of the sclerotic and cornea.

Steeping these sections in carmine solution for a few hours not only adds to their beauty, but renders the several parts much more distinct.

The specimens may afterwards be dried and mounted in Canada balsam, or examined in a moist state on the slide in the usual way.

The drawing represents a section of the sclerotic and cornea prepared according to the method described, and shows the fibrous structure of these parts with the conjunctiva and the membrane of Demours.

Section of cornea and sclerotic. a. Conjunctival epithelium. b. Fibrous structure of sclerotic and cornea. c. Membrane of Demours.-Magnified 200 diameters.

The conjunctiva covering the cornea is composed of layers of elongated and roundish cells.

The membrane of Demours, or membrana humoris aquei, is attached to the inner surface of the cornea and lines the anterior chamber, in which the aqueous humour is contained.

These parts are seen very distinctly in sections that have been steeped in carmine solution, and are readily distinguished from the fibrous structure of the cornea.

The epithelium on the surface of the cornea can be readily examined by scraping the surface of a perfectly fresh eye, and placing the collected matter in a drop of water on the slide, covering it in the usual way, and examining the object with a high power.

Portions of the sclerotic may be teazed out in a drop of water on the slide, and examined with a high power. The student will recognise the characteristic fibres of connective tissue. A drop of acetic acid placed at the edge of the covering glass will render apparent the few fine elastic fibres which are intermixed with the white.

A small portion of the cornea similarly treated will prove to be also fibrous in structure; the fibres, however, are extremely minute and are arranged in parallel bundles.

The vessels and nerves of the cornea are best studied in perfect specimens in the following manner. Select the eye of a young white rabbit or lamb, and cut off the cornea by a circular section with scissors close to the ciliary ring, place it on the glass slide with a little of the aqueous humour, and cut several slits round the edges of the ciliary ring, in order that the cornea may lie flat. If necessary it must be divided into three or four portions. When the structure is found to lie perfectly flat apply the covering glass, and proceed to examine first with the low power in order to discover the nerves and vessels, which may afterwards be traced towards the centre of the cornea by means of a higher objective. If the epithelium is at all cloudy a drop of caustic soda should be added to clear it. The nervous trunks are mostly dark in colour at the border of the cornea, but they become much less distinct towards the centre, and hence are difficult to trace.

The illustration will convey an idea of the appearance the observer should perceive if his specimen has been well prepared.

Nerves of the cornea of the rabbit in their coarser ramifications. The dis-
tance to which the dark-bordered tubes extend in the trunks is here ex-
pressed by the darker portions of the lines.

The blood vessels of the cornea will be found only at the border, and in the adult subject they are very sparingly distributed.

The choroid coat is a vascular tunic extending from the entrance of the optic nerve to the iris, with which it is continuous.

Besides the blood vessels, which are exceedingly numerous, the choroid contains numerous pigment cells of peculiar form, which may be examined by cutting off a small portion of the coat, teazing it out in a little water on the slide, and applying the covering glass in the usual way.

The specimen will present under a high power a number of the cells represented in the next illustration, besides some epithelial cells filled with pigment as well as fibrous tissue and portions of vessels.

The cells existing in the substance of the choroid are distinguished by long caudate processes; they are filled with pigment granules, and possess a nucleus which is usually light, in contrast with the dark material surrounding it.