Biological tissue

“ Each part of the animal body is formed by aggregations of different kinds of cells. The skin and the cavities within the body as well as the glands are composed of epithelial cells, the bones and muscles are produced by special bone and muscle cells. Uniting the various special structures and cell layers in the body, we find the so-called connective tissue consisting of special cells (connective tissue cells) and their product, the connective tissue fibers. ”

“ The various tissues which successively arise in the unfolding organism, are primarily cellular; and in many of them the formation of cells continues to be, throughout life, the process by which repair is carried on. But though cells are so generally the ultimate visible components of organisms, that they may with some show of reason be called the morphological units; yet we cannot say that this tendency to aggregate into special forms dwells in them. In many cases a fibrous tissue arises out of a nucleated blastema, without cell-formation ”

“ As we have seen, the higher plants are made up of a vast number of cells of many kinds. Collections of cells alike in structure and performing the same function we have called a tissue. Examples of animal tissues are the highly contractile cells set apart for movement, muscles; those which cover the body or line the inner parts of organs, the skin, or epithelium; the cells which form secretions or glands and the sensitive cells forming the nervous tissues. ”

“ In the third class come tissues the cells of which have fused by their walls, but whose cell-cavities are not in continuity, such as osseous tissue and cartilage. In the tissues of the fourth class, comprising the most highly specialised of all, not only are the cell-walls continuous but also the cell-cavities ”

“ It will be observed that the blood corpuscles, the epithelial tissues, the ganglionic corpuscles, the ova and the spermatozoa, are all demonstrably nucleated cells, more or less modified. The first form of connective tissue is so similar to epithelial tissue, that it may obviously be regarded as an aggregate of as many cells as it presents nuclei, the matrix representing the more or less modified and confluent bodies of the cells, or products of these. ”

“ It is becoming more and more clear every day that the cells composing the tissues of animals are not isolated units, but that they are connected with one another. I need only refer to the connection known to exist between connective tissue cells, cartilage cells, epithelial cells, &c. ”

“ All living structures, whether animal or vegetable, are built up of cells (which we shall consider in due course) , and these cells are grouped together for different purposes to form different tissues. The tissues are the different materials of which the body is made. There are eight principal tissues in the body: bone, gristle, muscle, nerve, skin, fat, fiber, and connecting tissue. ”

“ We see, then, that variation among these related cells has led to differences in size, in form, in transparency, and in function; while the cell-products have been differentiated into those which are of lifelong value, such as bone, cartilage, connective tissue, horn, chitin, etc., together with a variety of colouring matters; those which are of temporary value, such as the digestive secretions, fat, etc.; and those which are valueless or noxious, such as carbonic acid gas and urea, which are excreted as soon as possible. ”

“ A tissue represents an aggregate of similar cells with the intercellular substances in relation with these as connective tissue, muscular tissue, etc. Where such cell aggregates are localized and where the cells are arranged in structures having definite form and size and performing a definite function, it is customary to designate such structures as organs, as the brain, liver, etc. ”

“ From long years of practical experience I have found that the special cells of each tissue will take up only those constituents which they need for the construction of their respective tissue, as taught by the law of chemotaxis. ”

“ As the cell from which the body is to come begins to develop, the cells formed from it quickly become different one from the other, so that very early it can be distinguished that certain cells are to form the future nervous system, others the future muscles, and so on. One of the earliest of these groups of cells to become distinguishable is that which is to become the germinal tissue in the newly formed individual. ”

“ Regeneration of Tissues. The reparative powers of the tissues of the human body are considerable, although not comparable with those of the lower animals, in the [Pg 36] lowest orders of which the reproduction of an entire limb, or even one-half of the body, may take place. In order to understand the regeneration of tissue, we must first consider briefly the life history of the cells.
A cell consists of a mass of protoplasm, generally enclosed in a cell membrane, and containing a nucleus and nucleolus. ”

“ Besides these most individualized cells, so characteristic of every organ that a cytologist can at once distinguish cells that form the epithelium, intestines, brain, muscle, glands, etc., there always remain other far less differentiated or more primitive cells, chiefly leucocytes or white blood corpuscles and the connective tissue cells. ”

“ For it is undoubtedly a very different matter whether a cell bears within it the elements which compel it in the course of growth to develop an organ, for instance a leg, of quite definite size, sculpture, number of joints, and so on, or whether it only bears the somewhat vague power of determining that connective tissue or fatty tissue is to be produced. ”

“ As the body is composed of individual organs, and each organ of separate tissues, so each tissue is made up of minute cells. Each cell is a little world by itself, too small to be seen by the naked eye, but open to the microscope. It has its own form and constitution as much as a special organ in the body. It absorbs from the blood such food as suits its purposes. Moreover, the number of cells in an organ is as constant as the number of organs. As the organs expand with the growth of the body, so the cells of each tissue enlarge, but shrink again with age and the decline of life. ”

“ Throughout life, millions of these cells retain their primary characters, and constitute the white corpuscles of blood, "phagocytes," and connective tissue corpuscles; others again, engage in the formation of material round themselves, and lie, in such cases, as gristle and bone, embedded in the substance they have formed; others again, undergo great changes in form and internal structure, and become permanently modified into, for instance, nerve fibres and muscle substance. ”

“ There can be no doubt that blood corpuscles, liver cells, and ova are all nucleated cells; nor any that the third, fourth, and fifth somites of the abdomen are constructed upon the same plan; for these propositions are mere statements of the anatomical facts. But when, from the presence of nuclei in connective tissue and muscles, we conclude that these tissues are composed of modified cells ”

“ Of the Composition of the Tissues and of the Secretions in Health and Disease.—Having described thus individually the constituents of the tissues, we shall now present such results as have been hitherto obtained as to the quantitative composition of the organized tissues formed by their reunion, their secretory products and morbid alterations.
The skin of animals is a congeries of finely constructed organs, sensitive and secretory, imbedded in a peculiar tissue, which is one of those most yielding gelatine, whence the process of tanning skins. ”

“ This clew consisted of the final demonstration that all organic action is in the last resort a question of cellular activities, and, specifically, that all abnormal changes in any tissues of the body, due to whatever disease, can consist of nothing more than the destruction, or the proliferation, or the alteration of the cells that compose that tissue. ”

“ If we take a slice of the bone and examine it, we shall find that it is very like this diagram of a section of the bone of an ostrich, though differing, of course, in some details; and if we take any part whatsoever of the tissue, and examine it, we shall find it all has a minute structure, visible only under the microscope. All these parts constitute microscopic anatomy or 'Histology.' These parts are constantly being changed; every part is constantly growing, decaying, and being replaced during the life of the animal. The tissue is constantly replaced by new material ”

“ For instance, in a given tissue or organ, not all cells are capable of dividing into two daughter cells. In the epidermis, which is the thin outer layer of the skin, only cells in the deepest portion can divide. ”

“ I close the present one with one more protest against the entirely absurd terms 'par-enchyma,' for common cellular tissue, 'pros-enchyma,' for cellular tissue with longer cells;—'cambium' for an early state of both, and 'diachyma' for a peculiar position of one! [46] while the chemistry of all these substances is wholly neglected, and we have no idea given us of any difference in pith, wood, and bark, than that they are made of short or long—young or old—cells! ”

“ The cells of bone differ from those of cartilage mainly in the different consistency of the substances secreted by the cells to lie between them; skin cells are soft-walled masses lying close together; even blood is a tissue, although it is fluid and its cells are the corpuscles which float freely in a liquid serum. Thus an organism proves to be a complex mechanism composed of cells as structural units, just as a building is ultimately a collection of bricks and girders and bolts, related to one another in definite ways. ”

“ The materials which compose animal bodies are endowed with properties which differ in every different tissue: for instance, we say that the muscular tissue has the property of contracting. These properties of tissues develop functions: for example, the contraction of the ciliary muscle permits correct vision; but it is evident that if the tissue does not keep up the process of nutrition, i. ”

“ The organs invariably derived, in the triploblastic forms, from the mesoblast, are the vascular and lymphatic systems, the muscular system, and the greater part of the connective tissue and the excretory and generative (?) systems. On the other hand, the nervous systems (with a few possible exceptions) and organs of sense, the epithelium of most glands, and a few exceptional connective-tissue organs, as for example the notochord, are developed from the two primary layers. ”

“ An organ may be composed of several tissues. Thus the heart has contractile tissue in its muscular walls, epithelial tissue lining its cavities, and skeletal tissue forming its framework. Still, notwithstanding their aggregation [21] into tissues and organs, it remains true that the body of one of the higher animals is composed of cells, together with certain cell-products, horny, calcareous, or other. The simplest animals, called protozoa, are, however, unicellular, each organism being constituted by a single cell. ”

“ The constituent cells of the tissues of the plant retain their original form and individuality, while, in the animal, they undergo the greatest modifications, and their individuality is usually entirely lost. A vegetable tissue or organ may easily be shown to be made up of a mass of nearly similar cells, each of which is independent and sharply defined; while the tissues of animals present the greatest differences in structure and appearance, and sharply-defined individual cells are seldom to be seen. ”

“ Sometimes a number of cells are laid end to end, and, by the absorption of the transverse partitions, form a continuous tube, as in the sap vessels of plants, or in muscular and nervous fibre; and when the cells are thus woven together, they are called cellular tissue, which, in the human body, forms a fine net-like membrane, enveloping or connecting most of its structures. In pulpy fruits, the cells may be easily separated, one from the other; and within the cells are smaller cells, commonly known as pulp. Among the cells contents of some plants are beautiful crystals, called raphides. ”

“ It seems probable, however, that these cells, separate as they seem, are in some cases united to each other by minute protoplasmic filaments. In the higher animals the cells in different parts of the body take on different forms and perform different functions. Like cells with like functions are also aggregated together into tissues. Thus the surfaces of the body, external and internal, are bounded by or lined with epithelial tissue; the bones and framework of the body are composed of skeletal tissue; nervous tissue goes to form the brain and nerves; contractile tissue is found in the muscles ”

“ So is protoplasm, though in an almost infinitely less degree. Is muscular tissue eminently contractile, serving for the production of the varied and complicated movements of all parts of the body? Protoplasm is also capable of slight spontaneous motions of its entire mass. Are the various glands of the body actively secretory and excretory? So is protoplasm within the narrow limits of its chemical necessities. Equally, also, with the highest tissues and organisms, it reproduces its kind. ”

“ The different materials found by such a dissection correspond closely to the substances, called tissues, which make up the human body. The main tissues of the body, as ordinarily named, are the muscular tissue, the osseous tissue, the connective tissue, the nervous tissue, the adipose tissue, the cartilaginous tissue, and the epithelial and glandular tissue. ”

“ When we take into consideration the large number of nitrogenous products formed and present in the various tissues and organs of the body, products of proteolysis and of tissue changes; when we consider how these products circulate through the organism, in blood and lymph; how they come in more or less immediate contact with the different cells of the body prior to their decomposition or elimination, we can not avoid being impressed with the part they may play in stimulating and modifying tissue or other changes. ”