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—Diagram of a lengthwise section of the body to show its large cavities and the organs which they contain.
Organs and Systems.—The work of the body is carried on by its various organs. Many, in fact the majority, of these organs serve more than one purpose. The tongue[pg 009] is used in talking, in masticating the food, and in swallowing. The nose serves at least three distinct purposes. The mouth, the arms, the hands, the feet, the legs, the liver, the lungs, and the stomach are also organs that serve more than one purpose. This introduces the principle of economy into the construction of the body and diminishes the number of organs that would otherwise be required.
The various organs also combine with one another in carrying on the work of the body. An illustration of this is seen in the digestion of the food—a process which requires the combined action of the mouth, stomach, liver, intestines, and other organs. A number of organs working together for the same purpose form a system. The chief systems of the body are the digestive system, the circulatory system, the respiratory system, the muscular system, and the nervous system.
The Organ and its Work.—A most interesting question relating to the work of the organ is this: Does the organ work for its own benefit or for the benefit of the body as a whole? Does the hand, for example, grasp for itself or in order that the entire body may come into possession? Only slight study is sufficient to reveal the fact that each organ performs a work which benefits the body as a whole. In other words, just as the organ itself is a part of the body, the work which it does is a part of the necessary work which the body has to do.