with a drop of glycerin or acetate of potash, and the cover-slip applied. These methods are employed in mounting sections colored with a stain that would be injured by contact with alcohol, and where clearing is not especially necessary.

98. Farrant's Gum Glycerin.

In place of pure glycerin the following mixture may be used:

Glycerin

Water

Gum-arabic (powder)

Arsenious acid

Dissolve the arsenious acid in water.

Place the gum-arabic in a glass mortar and mix it with the water; then add the glycerin. Filter through a wet filter-paper or through fine muslin.

99. To preserve such preparations for any length of time the coverglasses must be so fixed as to shut off the glycerin or acetate of potash from the air. For this purpose cements or varnishes are employed which are painted over the edges of the cover-slip. These masses adhere to the glass, harden, and fasten the cover-slip firmly to the slide, hermetically sealing the object. The best of these is probably Krönig's varnish, prepared as follows: 2 parts of wax are melted and 7 to 9 parts of colophonium stirred in, and the mass filtered hot. Before employing an oil-immersion lens it is advisable to paint the edge with an alcoholic solution of shellac.

F. INTRODUCTION TO METHODS OF INJECTION.

100. A few remarks on the injection of the vascular system will not here be amiss, as it is only by this method that the relations of the bloodvessels to the neighboring tissue elements can be clearly brought out. The process consists in filling the vessels with a mass that can be injected in a fluid state but hardens readily, and is at the same time suitable for microscopic purposes and for sectioning methods. Of such substances there are a large number, and the technic of injection has been developed to such a degree that it has become a very important part of anatomic technic in general. Gelatin masses of different composition have come into general use for injecting the vascular system; of these, we shall here mention a red and a blue mass.

101. Gelatin-carmin.-The first is a gelatin-carmin mass, and is prepared as follows: (1) 4 gm. of carmin are stirred into 8 c.c. of water and thoroughly ground. Into this a sufficient quantity of ammonia is poured to produce a dark cherry color and render the whole transparent. (2) 50 gm. of finest quality gelatin is placed in distilled water for twelve hours until well soaked. It is then pressed out by hand and melted at a temperature of 70° C. in a porcelain evaporating dish. The two solutions are now slowly mixed, the whole being constantly stirred until a complete and homogeneous mixture is obtained. To this mass is added, drop by drop, a 25% acetic acid solution until the color begins to change to a brick red and the mass becomes slightly opaque. should be very carefully done, as a single drop too much may spoil the whole. During this procedure the substance should be kept at 70° C. and constantly stirred. The change in color indicates that the reaction of the mass has become neutral or even slightly acid (an ammoniac solution should not be used, since the stain diffuses through the wall of the

This

vessel and colors the surrounding tissues); the whole is filtered through flannel while still warm.

102. The blue mass is prepared from an aqueous solution of Berlin blue. A saturated solution is made and poured (as above) into a solution of gelatin warmed to 70° C. until the desired intensity of color is obtained.

103. Injection masses already prepared are to be had in commerce. Besides those already mentioned, still others colored with China ink, etc., are in general use.

104. Small animals are injected as a whole by passing the cannula of a syringe into the left ventricle or aorta. In the case of large animals, or where very delicate injections are to be made, the cannula is inserted into one of the vessels of the respective organs. The proper ligation of the remaining vessels should not be omitted.

105. Before injecting, the animals or organs are kept warm in water heated to about 38° C. in order to prevent the injection mass from hardening before passing into the smaller vessels.

106. Before injecting, it is always desirable to thoroughly bleed the animal, or press out as carefully as possible all the blood contained in the organ.

107. Organs injected with carmin are fixed in alcohol and should not be brought in contact with acids or alkalies. Such parts as are injected with Berlin blue are less sensitive in their after-treatment. Pieces or sections that have become pale regain their blue color in oil of cloves.

108. If objects or sections injected with Berlin blue be treated with a solution of palladium chlorid, the bluish color changes to a dark brown which afterward remains unchanged (Kupffer).

109. In thin membranes and sections the vessel-walls can be rendered distinct by silver-impregnation, which brings out the outlines of their endothelial cells. This may be done either by injecting the vessel with a 1% solution of silver nitrate, or, according to the process of Chrzonszczewsky, with a 0.25% solution of silver nitrate in gelatin. This method is of advantage, since, after hardening, the capillaries of the injected tissue appear slightly distended. Organs thus treated can be sectioned, but the endothelial mosaic of the vessels does not appear definitely until the sections have been exposed to sunlight.

110. By means of the above injection methods other lumina can be filled, as, for instance, those of the glands. As a rule, these are only partially filled, since they end blindly, and their walls are less resistant and may be damaged by the pressure produced by the injection.

III. The injecting of lymph-channels, lymph-vessels, and lymphspaces is usually done by puncture. A pointed cannula is thrust into the tissue and the syringe emptied by a slight but constant pressure. The injected fluid spreads by means of the channels offering the least resistance. For this purpose it is best to employ aqueous solutions of Berlin blue or silver nitrate, as the thicker gelatin solutions cause tearing of the tissues.

112. To bring out the blood capillaries and the lymphatic channels, Altman's process (79), in which the vessels are injected with olive oil, is useful. The objects are then treated with osmic acid, sectioned by means of a freezing microtome, and finally treated with eau de Javelle

For

(a concentrated solution of hypochlorite of potassium). By this process all the tissues are eaten away, the casts of the blood-vessels remaining as a dark framework (corrosion). The manipulation of these preparations is extremely difficult on account of the brittleness of the oil casts. lymph-channels Altman (ibid.) used the so-called oil-impregnation. Fresh pieces of tissues, thin lamellæ of organs, cornea, etc., are placed for five to eight days in a mixture containing olive oil 1 part, absolute alcohol 1⁄2 part, sulphuric ether 1⁄2 part (or castor oil 2, absolute alcohol I, etc.). The pieces are then laid for several hours in water, where the externally adherent globules of oil are mechanically removed and those in the lymph-canalicular system are precipitated. The objects are

now treated with osmic acid, cut by means of a freezing microtome, and corroded. In this case, the corrosive fluid (eau de Javelle) should be diluted two or three times.

GENERAL HISTOLOGY.

I. THE CELL.

DURING the latter part of the seventeenth century, Hooke, Malpighi, and Grew, making observations with the simple and imperfect microscopes of their day, saw in plants small compartment-like spaces, surrounded by a distinct wall and filled with air or a liquid; to these the name cell was applied. These earlier observations were extended in various directions during the latter part of the seventeenth and the eighteenth century. Little advance was made, however, until Robert Brown (1831) directed attention to a small body found in the cell, previously mentioned by Fontana, and known as the nucleus. In the nucleus Valentin observed (1836) a small body known as the nucleolus. In 1838 Schleiden brought forward proof to show that plants were made up wholly of cells, and especially emphasized the importance of the nuclei of cells. In 1839 Schwann originated the theory that the animal body was built up of cells resembling those described for plants. Both Schleiden and Schwann defined a cell as a small vesicle, surrounded by a firm membrane inclosing a fluid in which floats a nucleus. This conception of the structure of the cell was destined, however, to undergo important modification. In 1846 v. Mohl recognized in the cell a semifluid, granular substance which he named protoplasm. Other investigators (Kölliker and Bischoff) observed animal cells devoid of a distinct cell membrane. Max Schultze (1861) attacked vigorously the older conception of the structure of cells, proclaiming the identity of the protoplasm in all forms of life, both plant and animal, and the cell was defined as a nucleated mass of protoplasm endowed with the attributes of life. In this sense the term cell is now used.

The simplest forms of animal life are organisms consisting of only one cell (protozoa). Even in the development of the higher animals, the first stage of development, the fertilized egg, is a single cell. This by repeated division gives rise to a mass of similar cells, which, owing to their likeness in shape and structure, are said to be undifferentiated. As development proceeds, the cells of this mass arrange themselves into three layers, the germ layers, the outer one of which is the ectoderm, the middle one the mesoderm, and the inner one the entoderm. In the further development, the cells of the germ layers change their form, assume new qualities, adapting

themselves to perform certain definite functions; a division of labor ensues, the cells become differentiated. Cells having similar shape and similar function are grouped to form tissues, and tissues are grouped to form organs.

We shall now consider the structure of the cell. Every cell consists of a cell-body and a nucleus.

A. THE CELL-BODY.

The body of the cell consists of a substance known as protoplasm or cytoplasm. This is not a substance having uniform

physical and chemical qualities, but a mixture of various organic compounds concerning which knowledge is not as yet conclusive, but which in general are proteid bodies or albumins in the widest

sense.

In spite of the manifold differences in its composition, protoplasm exhibits certain general fundamental properties which are always present wherever it is found. Ordinarily, protoplasm exhibits certain structural characteristics. In it are observed two constituents, threads or plates, which are straight or winding, which branch, anastomose, or interlace, and which are generally arranged in a regular framework, network, or reticulum. These threads probably consist of small particles arranged in rows, called cell-microsomes (vid. van Beneden, 83; M. Heidenhain, 94; and others). This sub