CELLS AND TISSUES , 1^ 



digestive enzymes or the waste products of metabolism, such as 

 perspiration. They also form a largo portion of many of the sense 

 organs of the body. In shape, the cells of epithelial tissues as they 

 lie side by side may be fiat, cuboidal, columnar, or even ovoid. 



The Supporting Tissues. These tissues .serve to bind together or 

 support the various parts of the body. They include bone, cartilage, 

 and connective tissue, and they differ from other ti.ssues in that it is 

 the material formed by the cells, rather than the cells themselves, that 

 is of functional importance. In bone or cartilage, for example, the 

 supporting portion or matrix is produced by the cytoplasm of cells 

 and surrounds it. Fat cells are connective ti.ssue cells in which the 

 body of the cell becomes a storehouse for a drop of fat, the living part 

 of the cell being much reduced. Pigment cells are branched irregular 

 structures of a somewhat similar nature. Most characteristic of 

 true connective tissues are the white non-elastic fibers that make a 

 network in certain parts of the body, or form the glistening cords or 

 tendons \\\nQh. connect bones with muscle, or ligaments, which connect 

 bones with bones. Other forms of connecti\e ti.ssue that might be 

 mentioned are the areolar, which forms an elastic padding underneath 

 the skin; and the yellow elastic fibers found in the air tubes of the 

 lungs and the walls of arteries. 



The Muscular Tissues. Motion of certain cells is produced by 

 ameboid movement, or by the lashing of tiny threads of protoplasm, 

 that is, flagella or cilia. But in higher animals movement is brought 

 about by the muscle cells in which the propert}' of contractility is 

 greatly developed. In higher animals, muscles are groups of highly 

 specialized cells bound together by connective tissues. There are 

 three kinds of muscle cells, namely, smooth, striated, and cardiac. 

 Smooth muscle cells are long with an outer contractile fibrillar layer 

 surrounding a central area of semifluid protoplasm containing a 

 nucleus. In vertebrate animals, smooth muscle is found i^articularly 

 in the walls of the blood vessels and the walls of the digesti\-e tract. 

 Striated muscle fibers in higher animals are groups of cells slu)wing n(» 

 cell boundaries and held together by connective ti.ssue. They s1k)w 

 curious cross striations and on the whole in man are under control (»f 

 " the will," hence are called voluntary muscles. A third type o( 

 muscle, the cardiac, is striated, but involuntary in action, making 

 up the tireless muscles of the heart. 



Circulatory Tissue. Although the blood, lymph, and other 

 fluids that serve to transport foods and wastes in the body are <-on- 



