30 THE STRUOTTJEAL UNITS OF LIVING MATTER 



split in the same manner the appendages show a central 

 cylinder of bone surrounded by solid masses of soft matter 

 called flesh. In addition to these general regions we find 

 that the whole body is covered with a soft layer called skin, 

 with openings at certain points for special structures 

 such as eyes, ears, and mouth. 



Character of the body parts. — When we examine the 

 parts of the body more in detail, we find that they are 

 capable of separation into portions that perform some 

 definite kind of work. For example, in and on the head 

 are parts called eyes, tongue, brain, etc., each of which 

 has some special work to do. Again, the body cavities 

 contain portions called the heart, the lungs, the stomach, 

 etc., all with special work to perform. These specialized 

 portions of matter which may be entirely soft or hard, or 

 contain both soft and hard parts, are called organs. They 

 are usually of a definite shape or outline and position. 

 We may define an organ as a portion of the matter of 

 the body which performs a special kind of work. For 

 example, the eye is an organ for seeing, the ear for hear- 

 ing, the heart for pumping, etc. 



In some cases several organs combine and work to a 

 common end, and we call such a collection of organs a 

 system. For example, the brain, the spinal cord, and 

 the nerves, are organs combined for the control of our 

 actions. Likewise the stomach, liver, intestines, etc., all 

 share in preparing food for the body, and hence constitute 

 a system. We may define a system as a collection of 

 organs so arranged as to accomplish some definite end by 

 their combined actions. Systems are named, usually, 

 according to the end attained, for example, the digestive 

 system, the circulatory system, etc. 



