In =epithelial tissue= there is little intercellular substance, the cells being close together and arranged generally as a skin or membrane covering external or internal surfaces. When there are several layers of cells, the deepest are columnar in shape and the others become more and more flattened and scale-like as they approach the surface, where they are gradually rubbed off and replaced by the growth of new cells from below. This stratified epithelium, as it is called, is found wherever a surface is exposed to friction, as in the skin and in the mucous membrane of the mouth, pharynx, and esophagus, and in that of the vagina and the neck of the uterus. In simple epithelium, where there is only a single layer of cells, the cells may be pavement or hexagonal, columnar, glandular, or ciliated, according to their different functions. The flat pavement cells occur where a very smooth surface is required, as in the heart, lungs, blood-vessels, serous cavities, etc. None of these surfaces communicate directly with the external surface of the body and the name endothelium is substituted for epithelium. The columnar form of cell in the intestine facilitates the passage of leucocytes between the cells. In glandular epithelium the cells vary according to the gland in which they occur, their protoplasm being filled with the material the gland secretes. Finally, ciliated epithelium is composed of columnar cells with cilia or little hair-like processes upon their free surface which serve to send secreted fluids and other matters along the surfaces where they occur, as in the air passages, parts of the generative organs, the ventricles of the brain, and the central canal of the spinal cord. =Connective tissue= has a great deal of intercellular substance. One form, _areolar tissue_, is composed of a loose network of fine white fibers with a few yellow elastic fibers interspersed and with cells lying in the spaces between the fibers. It connects and surrounds the different organs and parts, holding them together, yet allowing free motion, and is one of the most extensively distributed of the tissues. It is continuous throughout. [Illustration: FIG. 2.--Section of bladder epithelium. (Hill.)] Closely allied to the areolar is the _fibrous tissue_, in which the white fibers lie close together and run for the most part in one direction only. This is found in ligaments, joints and tendons, as also in such fibrous protective membranes as the periosteum, dura mater, the fasciæ of muscles, etc. Fibrous tissue is silvery white in appearance and is very strong and tough, yet pliant. It is not extensile. _Elastic tissue_, on the other hand, has a large predominance of yellow elastic fibers and is very extensile and elastic, though not so strong as the fibrous. It is found in the walls of the blood-vessels, especially the arteries, in the walls of the air tubes, in the ligaments of the spine, etc.