15. The tissues of plants, epidermis, ducts, and woody fibres are constantly found in microscopic preparations. They may be studied in delicate sections made with a sharp knife, or by tearing vegetable tissues apart with needles. The down of moths, the hairs of different animals, the fibres of paper, the most common animalcules in water, the dust of shelves, and generally the structures found in all vegetable and animal substances by which we are surrounded, should be studied as a preliminary to any special line of microscopical investigation. [Sidenote: 16. Directions for examining a drop of blood?] 16. THE STUDY OF HUMAN TISSUES.--When this has been done and familiarity with the use of the instrument has been obtained, proceed to the study of the human body, for human physiology is our subject. If the end of the finger be pricked with a pin, a drop of blood may be procured for examination. Place this on one of the glass slides, cover it with a thin piece of glass, press down the cover so as to make a thin layer, and then examine with the magnifying power just mentioned. Do not add water, for that will cause the blood corpuscles to disappear. If the drop of blood is placed under the microscope at once after being drawn from the finger, most interesting phenomena will be observed. The red corpuscles will be seen to arrange themselves in rows, like piles of coin, while the blood is coagulating. The spherical, white corpuscles will {245} be left out of the rows of red disks, and, if the highest power be used, will be seen to change their shape constantly. [Sidenote: 17. Examination of the scales of the mouth? Dandruff?] 17. If you scrape with a dull knife the inside of the cheek, the flattened scales of "pavement epithelium," or of the insensible covering which, analogous to the scarf-skin on the outer surface of the body, lines the cavities of its interior, may be readily studied. They have the appearance of transparent tiles, each enclosing a round or oval body, called its nucleus. Dandruff and the scrapings from the skin of the body are composed of scales like those of the mouth, but they differ somewhat in being hardened by horny matter, and in having a very faint central body or nucleus. [Sidenote: 18. In what, as respects the tissues, do the warm blooded animals differ? Statement of Milne Edwards?] 18. THE TISSUES OF THE INFERIOR ANIMALS.--The warm-blooded animals do not differ in the tissues, or microscopic structures, that compose them, but only in the amount and arrangement of these tissues. Milne Edwards says these tissues "do not differ much in different animals, but their mode of association varies; and it is chiefly by reason of the differences in the combination of these associations in various degrees, that each species possesses the anatomical properties and characters which are peculiar to it." [Sidenote: 19. How to procure materials for the study of the tissues of man?]