*Properties of the Tissues.*—If we inquire how the tissues are able to serve such widely different purposes, we find this answer. The tissues differ from one another both in composition and in structure and, on this account, differ in their properties.(3) Their different properties enable them to serve different purposes in the body. Somewhat as glass is adapted by its transparency, hardness, and toughness to the use made of it in windows, the special properties of the tissues adapt them to the kinds of service which they perform. Properties that adapt tissues to their work in the body are called _essential_ properties. The most important of these essential properties are as follows: 1. Of osseous tissue, hardness, stiffness, and toughness. 2. Of muscular tissue, contractility and irritability. 3. Of nervous tissue, irritability and conductivity. 4. Of cartilaginous tissue, stiffness and elasticity. 5. Of connective tissue, toughness and pliability. 6. Of epithelial tissue, ability to resist the action of external forces and power to secrete. [Fig. 1] Fig. 1—Hand and forearm, showing the grouping of muscular and connective tissues in the organ for grasping. *Tissue Groups.*—In the construction of the body the tissues are grouped together to form its various divisions or parts. A group of tissues which serves some special purpose is known as an _organ_. The hand, for example, is an organ for grasping (Fig. 1). While the different organs of the body do not always contain the same tissues, and never contain them in the same proportions, they do contain such tissues as their work requires and these have a special arrangement—one adapted to the work which the organs perform. In addition to forming the organs, the tissues are also grouped in such a manner as to provide supports for organs and to form cavities in which organs are placed. The various cavities of the body are of particular interest and importance. The three largest ones are the _cranial_ cavity, containing the brain; the _thoracic_ cavity, containing the heart and the lungs; and the _abdominal_ cavity, containing the stomach, the liver, the intestines, and other important organs (Fig. 2). Smaller cavities serving different purposes are also found. [Fig. 2] Fig. 2—Diagram of a lengthwise section of the body to show its large cavities and the organs which they contain.