TISSUES, ANIMAL, may be either normal or pathological. The most important of these tissues have already been considered in special articles, and we shall here merely notice the view at present most generally adopted regarding their classification (see HISTOLOGY). The normal tissues are divisible, according to Virchow and his fol lowers, into three groups or categories. We have (1) tissues which consist exclusively of cells, when cell lies close to cell; or (2) tissues in which one cell is regularly separated from the others by a certain amount of intermediate matter, or intercellular substance; or (3) tissues in which the cells have attained specific, higher forms of development, by means of which their constitution has acquired an entirely peculiar type. As illustra tions of the first group of tissues, the simple cellular tissues in the modern sense (eel lular tissue here being quite distinct from areolar or connective tissue), we may take the epithelial formation, such as occur in the epidermis and the nails, and in the epithelium of mucous and serous membranes, in the crystalline lens of the eyes (which is originally a mere accumulation of epidermis), and in the glands. The second group is formed by' the connective tissue; which is composed of intercellular substance, with cells of vari ous forms embedded in it, and includes cartilage, fatty tissue, etc. In the third group, which is somewhat heterogeneous, the structures are usually more or less tubular. This group includes the muscles, nerves, and vessels, and Virchow also places the blood in it. ouch an arrangement as this is quite distinct from, and altogether at variance with, those adopted a comparatively few years ago. This arrangement has reference to general his tology (tissues, properly so called), while that has reference to special histology, or the structure of organs in which a combination of various tissues may enter. Thus, the
osseous tissue of general histology consists of bone cells + calcified intercellular substance, while bone as an organ consists of osseous tissue 4- medullary tissue periosteum vessels + nerves; similarly, nervous tissue is by no means identical with cerebral mat ter, which additionally contains membranes, vessels, etc.
Morbid tissues may be classified upon exactly the same plan as the physiological or normal tissues. The belief is gradually extending that there is nothing peculiar or spe cific in pathological structures, or, in other words, that every pathological tissue has its physiological prototype, and that "no form of morbid growth arises which cannot in its elements be traced back to some model, which had previously maintained an inde pendent existence in the economy."—Virchow's Cellular Pathology, translated by Chance, p. 60. The distinguished pathologist whose words we have just quoted maintains that there is no other kind of heterology in morbid structures than the abnormal manner in which they arise, and that this abnormity consists either in the production of a struc ture at a point where it has no business, or at a time when it ought not to be produced, or to an extent which is at variance with the typical formation of the body; "but," he adds, " practical experience shows us that it would be altogether incorrect to conclude from the mere correspondence of a pathological tissue with a physiological one that the case would continue to follow a benignant course." The curious bodies provided with large nuclei and nucleoli, which have been described by many pathlogists as "the specific polymorphous cells of cancer," are merely irregularly developed epithelial cells, such as occur, for example, in the lining of the urinary passages; and the apparent heter ology of other morbid growths may be similarly explained.