Histology , (Gr. a web, and a discourse), the science which describes the anatomical elements and tissues of the body, according to their form and organization. If we take any organ of the body, such as a muscle or a nerve, and subject it to minute dissection, we find that it can be divided into smaller and smaller portions, by simply separating from each other the various parts of which it was composed. Thus a muscle is distinctly fibrous in appearance to the naked eye, being formed of parallel bundles which may be successively separated from each other in dissection, by removing the intervening material. But the possibility of thus dividing an organ into smaller and smaller parts, similar to each other, has its limits; for, after it has been carried to a considerable extent and the parts are reduced to microscopic size, we then come in every instance to certain definite anatomical forms, which can no longer be divided in the above manner. They can still of course be divided or disintegrated by mechanical means; but this will be no longer a separation into similar parts already distinct from each other, but simply an artificial mutilation of its substance.
Such a definite form, to which the organ is reduced in its minutest natural subdivision, is called an anatomical element. It is readily distinguished, as a general rule, under the microscope, by its size, form, color, consistency, and chemical relations. Thus the striped muscular fibre of the voluntary muscles, and the smooth muscular fibre of the internal organs, are anatomical elements. The red globules and the white globules of the blood are two different anatomical elements, both mingled with the plasma of the circulating fluid. The tendons, ligaments, and periosteum contain a minute, white, straight, inelastic but very strong fibre, which is their most abundant and characteristic anatomical element. Other membranes and organs contain a larger, flattened, curled, yellowish, elastic fibre, which communicates to them the property of elasticity in proportion to its own abundance. These two kinds of fibre are also distinguished from each other by their reaction with dilute acetic acid; the white inelastic fibre becoming rapidly swollen and transparent by contact with this reagent, while the yellow elastic fibre is not affected by it.
Other anatomical elements which may be enumerated are bone corpuscles, nerve cells, pavement, columnar and ciliated epithelium cells, glandular cells, adipose vesicles, cartilage cells, capillary tubes, etc. - When two or more kinds of anatomical elements are mingled together and interwoven in a determinate manner, they form a tissue, just as woollen or cotton threads interwoven with each other form a web or textile fabric; and the animal tissues, like artificial fabrics, derive their appearance, qualities, and texture from the number and variety of anatomical elements of which they are made up, and the particular manner in which they are interwoven. It is very rare that a tissue consists of but a single anatomical element. The tissue of the crystalline lens, containing only flattened fibres with finely toothed edges, and that of cartilage, containing only cartilage cells with an intervening hyaline substance, and certain epithelial tissues, are perhaps the sole examples of this in man and the higher animals. Generally speaking, a tissue consists of several anatomical elements, one of which is peculiar to it, the others perhaps common to several tissues.
Thus muscular tissue consists of muscular fibres, arranged in parallel bundles, with ultimate nervous filaments and capillary blood vessels; the bundles themselves being surrounded by a thin layer of connective tissue, and associated into secondary and tertiary bundles of larger and larger size, with the trunks and branches of nerves and blood vessels ramifying between them. The liver contains a peculiar anatomical element, the glandular liver cells; but these are arranged in definite groups, forming the lobules or acini, with the intra-lobular capillary blood vessels, the commencement of the hepatic ducts, and the terminal filaments of the hepatic plexus of the sympathetic nerve. - In the study of histology the different powers of the microscope are employed for different objects. The highest powers are generally requisite for the examination of the ultimate anatomical elements. But in order to learn in what form these elements are associated with each other, or how they are interwoven with still different ones - to ascertain, in short, the structure of the tissue - the lower powers must be employed; since a very highly magnifying lens can only be used when the tissue has been minutely subdivided, and this destroys of course the natural arrangement of its parts.
Certain artificial aids and contrivances are often also of great value in bringing into view peculiarities of texture which would otherwise remain invisible, such as the injection of minute vessels with colored fluids, and the staining of the tissue with certain reagents, such as iodine, nitrate of silver, and the like, which will attack some of its anatomical elements and leave others entirely or comparatively unaffected. Thus two different anatomical forms which were originally so similar in color, consistency, and refractive power that they could not be distinguished by the microscope, may have their outlines made visible and easily recognized in the surrounding parts. These manipulations, however, should be used with great caution and judgment; otherwise deceptive appearances, produced by the action of reagents, such as the shrivelling of membranes, the contraction or enlarging of fibres, or the coagulation of soft material, may sometimes be mistaken for the natural characters of the tissue. - The best works on histology, often called microscopic anatomy, are those of Kolliker, Handbuch der Gewebelehre (Leipsic, 1852; translated by Busk and Huxley, Sydenham society publications, London, 1853, and Philadelphia, 1854); Peas-lee, "Human Histology" (Philadelphia, 1857); and Strieker, Lehre ton den Geweben des Men-schen und der Thiere (Leipsic, 1869-71).