The art of filling the vessels of a tissue with some coloured fluid to represent the blood is at once a very important and difficult one. A natural injection of the capillaries may sometimes be obtained by ligatures; the plan, however, is only applicable to membranous structures, such as intestines or stomach. Artificial injection consists in forcing a fluid into the vessels of a part or the whole of the circulatory system. In the old method the injected medium was wax, fat, or gelatine, rendered fluid by heat and coloured with vermilion, chromate of lead, Prussian blue, or white carbonate of lead. This plan, distinguished by the term opaque injection, is now almost entirely abandoned, having been superseded by the more recent method of transparent injection, in which a transparent base is coloured by a pigment held in perfect solution, or, at least, so distributed as not to interfere with the passage of light to any considerable extent. For the purpose pure gelatine, coloured with carmine, is generally used; but for ordinary purposes the preparations recommended by Dr. Beale (one a red, and the other a blue fluid) answer very well. They have the advantage of being easily prepared, and may be always kept ready for use. Mix the carmine with a few drops of water, and add five drops of ammonia, next pour in half an ounce of glycerine and agitate. The acid glycerine is then to be gradually added, frequently shaking the bottle during the mixing. If the mixture is not decidedly acid to blue litmus paper, add a few drops of acetic acid to the remaining glycerine and mix it with the fluid. Lastly, mix the alcohol and water and pour them in gradually, agitating the bottle frequently. Dissolve the ferrocyanide of potassium in one ounce of glycerine, add the sesquichloride of iron to the other ounce of glycerine; pour the iron solution gradually into the solution of ferrocyanide, and agitate well. Then add the naphtha mixed with the spirit, and afterwards the water very gradually, shaking the mixture constantly. Apparatus required for injecting are:-a proper syringe, capable of holding about two ounces of fluid, pipes with stopcocks, scalpels, scissors, forceps, bull's-nose forceps, wash-bottle, and needle and thread for passing under a vessel to tie in the nozzle of the pipe. Structures are generally injected by the arteries, the fluid being introduced by a moderate and continued pressure upon the piston of the syringe until it flows out of the larger veins. A small animal, as a frog or mouse, may be injected by the aorta, or an eye or kidney by the artery, by the student as a commencement. After the injection is complete the specimen may be immersed in strong glycerine or absolute alcohol before being dissected. The best time for injection is immediately after death.* * Small animals may be destroyed for dissection by putting them for a few minutes under a bell-glass with a little chloroform. Mammalian animals and also reptiles are best injected by the aorta. Mollusca by making an opening through the integument and putting in a pipe. Fishes may be injected by cutting off the tail and introducing a pipe into the vessel immediately beneath the spine. For insects a pipe may be placed in the abdominal cavity, and the vessels filled from it. Portions of organs are to be filled from one artery, the rest being tied. Lymphatics and lacteals are most readily injected after the blood-vessels have been filled with water. ELEMENTARY TISSUES. CELLS--THEIR NATURE AND FUNCTION. NOTWITHSTANDING the apparent diversity in the structure of the various tissues of which animals and vegetables are constituted, recent microscopic research has demonstrated that all textures originate from cells. The ultimate fibres of muscles are formed of corpuscles arranged in rows; the soft tissue of the liver, and the hard texture of horn are equally constituted of cells; even the seemingly homogeneous filaments of fibrous tissue are associated with occasional nuclei which bear testimony to their cellular origin. In fine, from the first trace of embryonic life to the cessation of animal existence, all the marvellous processes of vitality-the origin, development, reproduction, and decay of the organism-are dependent upon the development and metamorphosis of cells, differing from each other in form and function, but uniformly constituted of a membrane termed the cell wall, so arranged as to form a sac capable of enclosing both fluid and solid contents. In the interior of many cells is a small body of granular structure named the nucleus. Within the nucleus there are to be discerned occasionally one or more smaller bodies called nucleoli. The illustration represents simple cells from the epidermis; 1 and 2 without nucleus, 3 with nucleus, 4 shows nucleus and nucleolus. Fig. 16. 4 SIMPLE CELLS. Many vegetable cells have within them a second cell called by Mohl the Primordial Utricle.' The wall of the outer cell is sometimes quite separate from that of the Primordial Utricle, but occasionally they are so closely connected, as to require the aid of chemical agents (alcohol or hydrochloric acid) to render them apparent. In cartilage cells a Primordial Utricle may often be distinguished. The following woodcut represents three cells from a piece of cartilage showing the Primordial Utricle in three sepa Three human cartilage-cells, magnified 350 times. 1. From the epiglottis, Preparation. The tissues of young plants, or algæ, or specimens of cartilage, may be selected for examination. Minute fragments of either object ought to be placed in a drop of water on the slide and carefully teazed out for the purpose of obtaining the cells separated from their connecting tissues in order to examine them singly; if possible, no more than two or three should be in the field of the microscope at the same time. Cells, especially those of plants, are often connected by an intercellular substance so closely allied to the cell wall as not to be distinguishable from it even by the aid of chemical reagents. Cells vary much in size, and, as a rule, vegetable cells are larger than those of animals. In shape, cells may be spherical, oblong, polygonal, stellate, or fusiform. Cells may coalesce with adjoining cells, and by communicating with them, form tubes; or they may undergo still further modifications and form fibres, bands or spiral vessels. They may be spread out so as to form a membrane, either being immediately united by their edges or connected together by some intervening tissue. Origin of Cells.-1. Cells may arise from a formative fluid, derived from the blood, called the 'blastema.' In the blastema minute granules, termed cytoblasts, appear; a number of which granules becoming grouped together form the nucleus, round which the cell membrane is afterwards developed by the coalescence of a series of other granules. The nucleus may either remain in the centre, or become attached to some part of the cell wall. 2. Cells may arise within parent cells by an endogenous process of development. The first step is the formation of several nuclei by the breaking up of the original nucleus; the second step is the subdivision of the cell's contents into as many portions as there are nuclei, each portion enclosing a nucleus; lastly a new cell wall forms round each part and completes the process. In this way a number of cells may be enclosed within one common investing membrane, as seen in the cleavage of the yolk of the ova of some parasites. Three ova of Ascaris Nigrovenosa. 1. From the second, 2. from the third, Another form of endogenous development occurs in cartilage, commencing by the division of the nucleus into two parts, followed by the passage of a partition from the cell wall of the parent separating the cell into two parts, each part enclosing a nucleus. It often happens that the parent cell wall remains |