CHAPTER IV

THE TISSUES AND THEIR FUNCTIONS

It might naturally be inferred that higher organisms are composed of many different kinds of cells. Complicated biological mechanisms like ourselves with our varied activities of digesting, absorbing, eliminating, moving about, and thinking, require many kinds of cellular units. And these different kinds of units are intimately related to the number of different things which our bodies can do. We have specialized muscle cells through the contraction of which we execute our numerous movements; secretory cells which aid in digesting our food; other cells which absorb the digested products; and nerve cells which are concerned in conducting stimuli, and in thinking, which involves a special and highly elaborate kind of conduction of stimuli. We cannot get very far in understanding either the structural make-up or the workings of an organic body without becoming acquainted with its different kinds of cells.

Cells in all complex organisms are massed together into groups called tissues, the knowledge of which constitutes the subjectmatter of the science of histology. In the animal body the kinds of tissues generally distinguished are epithelial, connective, muscular, and nervous, some writers adding blood and lymph ́as a separate variety. Epithelium is a very common type of tissue in which the cells are arranged in layers. These layers may be one or more cells thick, and the cells are usually polyhedral in outline as the result of mutual pressure. Sometimes epithelium consists of much flattened cells (squamous epithelium), as in the shed outer skin of the frog and the outer cuticular cells of our own skin. Epithelial cells are sometimes cuboidal, or more frequently columnar in shape. In the variety called ciliated epithelium the exposed surface is furnished with

numerous hairlike processes called cilia which rapidly beat to and fro, thus creating a current in the fluid by which the cells are bathed. The currents caused by cilia frequently carry off foreign particles, as is the case with the lining epithelium of our windpipe. Clams, oysters, and a host of other marine animals get their living by the industrious beating of the ciliary epithelium which carries to the mouth the small organisms which these creatures utilize for food. The currents created by the cilia on the gills of molluscs and other animals aid in the process of respiration; in some cases these currents carry the products of elimination to the 이이이

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FIG. 11-Varieties of epithelium: a, b, flattened, or squamous epithelium, the cells seen from the side in a, and from above in b; c, cuboidal epithelium; d, epithelium several cells in thickness-the deepest cells are columnar, but the others become more flattened as they are pushed outward; e, columnar ciliated epithelium.

outside of the body. In some animals, especially in the larval state, cilia afford the means of swimming through the water. The beating of cilia is worked in to perform a variety of useful functions; in fact most animals which have cilia are dependent upon them for their life.

Epithelium is a kind of covering tissue used for lining various organs both within and without. In our own bodies it not only covers the entire outer surface, but forms the inner layer of the alimentary canal and all the glands attached thereto. Epithelium lines the body cavity and the organs which project into it, such as the liver, kidneys, and pancreas. It coats the heart and the blood vessels inside and out, and constitutes the secreting cells of glands which discharge their contents through a duct. In almost every place where there is an exposed surface there is a coating of this tissue.

Epithelium performs a variety of functions. It is frequently

modified for the purpose of absorption, as in a considerable part of the intestine. It is the tissue usually involved in secretion, which is its function in the glands of the stomach and intestine, and the tubules of the liver, kidneys, and pancreas. In some places it is sensory in function, as in the olfactory epithelium of the nasal cavities and the auditory epithelium of the inner ear. In many animals it subserves the function of reproduction in giving rise to the sex cells.

In stratified epithelium, where there are several layers of cells, the new cells are formed in the deeper layers and gradually become pushed toward the periphery as they get older. As they pass outward, the cells become thinner and flatter, and in the outer skin they become dead, dried, and corneous and are finally cast off. In those animals which we speak of as shedding their skin, the whole outer cuticle may be cast off in a single intact layer, as in snakes; in the frog the cuticle comes off in large pieces, while in mammals the outer dead cells are shed gradually and unobtrusively in much smaller masses. In all these forms the cells are actively renewed in the deeper layers, and are gradually pushed outward, to die and be cast away (Fig. 11).

Epithelial cells are closely fitted together, each cell being separated from its neighbors by a thin film of what is called intercellular cement substance. Sometimes this substance appears to be crossed by strands which have been considered to be protoplasmic bridges connecting one cell with another. The cells of epithelium form continuous sheets in which there are normally no gaps; and if on account of injury a surface becomes denuded of its epithelial coating the latter is quickly regenerated. This covering of exposed surfaces results not merely from the formation of new cells, but more from the fact that epithelial cells creep over the surface from neighboring areas. These cells are not like so many passive building stones, as we might be prone to consider them; they are like active organisms endowed with the power of movement and response to stimulation. This property has been demonstrated by growing these cells outside the body and observing their behavior. The method of growing

tissues outside the body was perfected a few years ago by Harrison, Carrel, and others, and it has made possible the discovery of many interesting facts. The tis

sue to be studied is placed in a hanging drop of blood plasma or other suitable medium suspended on the lower surface of a thin cover glass over the cavity of a hollow glass slide.

FIG. 14—Epithelial cells of a tadpole cultivated in a hanging drop. Strands of cells have crept out along fibers of cotton wool, t, t; c, a group of cells more highly magnified.

It is thus possible to observe the tissue with a microscope and follow its changes. If one prepares in this way a bit of epithelium

from the skin of a frog, or better, a tadpole, the cells may be seen to creep out from the margin of the tissue as a very thin sheet which actively extends over

all available solid objects with which it comes in contact. Bits of sterile cotton fibers or other objects put in contact with epithelium may receive a complete coating of this tissue. In fact, epithelium exhibits a remarkable proclivity to spread over all sorts of surfaces. It is doubtless owing in large part to this peculiar trait of its behavior that epithelium is so generally distributed as an investing layer over so many surfaces of bodily organs.

FIG. 15-Fibrous connective tissue showing cells amid the fibers.

In connective tissue the cells are separated by a matrix of supporting substance which is usually much greater in bulk than the cells themselves. The general function of connective tissue

is to bind together and sustain the parts of the body. In white fibrous tissue there is a clear matrix in the form of a layer, or often a spongy network, containing fibers which give it strength. Scattered about in the matrix are cells which are generally irreg