All living bodies are made up of tissues. There is no part, no organ, however soft and yielding, or hard and resisting, which has not this peculiarity of structure. The _bones_ of animals, as well as their flesh and fat, are composed of tissues, and all alike made up of cells. When viewed under a microscope, each cell is seen to consist of three distinct parts, a _nucleolus_, or dark spot, in the center of the cell, around which lies a mass of granules, called the _nucleus;_ and this, in turn, is surrounded with a delicate, transparent membrane, termed the _envelope_. Each of the granules composing the nucleus assimilates nourishment, thereby growing into an independent cell, which possesses a triple organization similar to that of its parent, and in like manner reproduces other cells. [Illustration: Fig. 4. Nucleated cell. From Goeber. 1. Periphery of the cell, or cell-wall. 2. Nucleus. 3. Nucleolus in the center.] A variety of tissues enters into the composition of an animal structure, yet their differences are not always distinctly marked, since the characteristics of some are not unlike those of others. We shall notice, however, only the more important of the tissues. The _Areolar_, or _Connective Tissue_, is a complete network of delicate fibers, spread over the body, and serves to bind the various organs and parts together. The fibrous and serous tissues are modifications of the areolar. The _Nervous Tissue_ is of two kinds: The gray, which is pulpy and granulated, and the white fibrous tissue. The _Adipose Tissue_ is an extremely thin membrane, composed of closed cells which contain fat. It is found principally just beneath the skin, giving it a smooth, plump appearance. [Illustration: Fig. 5. Arrangement of fibers in the Areolar Tissue. Magnified 135 diameters.] The _Cartilaginous Tissue_ consists of nucleated cells, and, with the exception of bone, is the hardest part of the animal frame. The _Osseous Tissue_, or bone, is more compact and solid than the cartilaginous, for it contains a greater quantity of lime. The _Muscular Tissue_ is composed of bundles of fibers, which are enclosed in a cellular membrane. [Illustration: Fig. 6. Human Adipose Tissue.]