12 THE INDIVIDUAL ORGANISM 



sequence of body functions. Examples are the digestive, locomotor, 

 circulatory, nervous, and excretory systems. A system can be subdivided 

 into a number of connected and coordinated organs. 



3. Organs are major subdivisions of a system, which accomplish some 

 essential and more or less complex task or tasks in the total functioning 

 of the entire system. To illustrate, the stomach is one of the organs that 

 make up the digestive system. It performs certain essential but incom- 

 plete parts of the whole digestive process. Organs, in turn, are composed 

 of a number of appropriate and coordinated tissues. 



4. Tissues are groups of similar cells (or of cells and their joint prod- 

 ucts) united to fulfill a common function. The stomach, for example, is 

 made up of muscular, secreting, connective, and other types of tissues; in 

 combination these make possible the complex functions of the organ. 

 Each tissue consists of a group of similar cells or of such cells and their 

 common products. 



5. Cells. Here we have reached not the ultimate subdivision of the body 

 but a very real biological unit of structure and functioning. This is so 

 because the cell cannot be divided into any smaller living units. The 

 human body comprises billions of individual cells, specialized in hundreds 

 of different structural and functional ways. But all are alike in fundamen- 

 tal structure and properties, and all are composed chiefly of the peculiar 

 living stuff which we call protoplasm. 



SOME STRUCTURES AND PROPERTIES OF PROTOPLASM 



When one examines a bit of protoplasm beneath a microscope he is apt 

 to be disappointed by how little he can see. The semitransparent, some- 

 what viscid material looks not unlike raw egg white, with little dis- 

 cernible detail or evidence of structure. The tremendous actual complexity 

 is invisible, partly because of the lack of optical contrast between its 

 grosser but still minute parts but chiefly because of the submicroscopic 

 dimensions of its finer details. 



The earlier microscopists, lacking the concepts and techniques of 

 modern chemistry and physics, had to be content with establishing the 

 fact that protoplasm is the common formative material of all plant and 

 animal life and with discovering such of its structures and properties as 

 they could observe with the microscope and the simple chemical and 

 physical techniques then at hand. These early workers noted certain 

 characteristic phenomena that occur in protoplasm as well as in non- 

 protoplasmic matter, but they and biologists in general were especially 

 impressed with the capacity of protoplasm for carrying on certain adap- 

 tive and apparently purposive activities that did not occur in any non- 

 living substance. The properties exhibited by protoplasm thus came to be 

 classified into two groups — the so-called "non vital" and "vital" prop- 



