A STUDY OF LIVING SUBSTANCE 23 



them we take hold of things and make the definite move- 

 ments required in writing, drawing, and sewing. 



Tissues. When we pinch the arm and the hand, we feel 

 the hard bones that form their skeleton. We can raise from 

 the bones the softer fleshy material called muscle. By 

 straightening back our fingers as far as possible, we can see 

 and feel on the back of the hand the tough cords or tendons 

 of connective tissue. Run a clean needle point into the finger ; 

 blood flows and we feel pain. In this way we discover two 

 more of the materials of which our hand is composed, name- 

 ly, blood and nerves. All these parts of the body we have 

 enumerated are known as tissues. For the present, a tissue 

 may be defined as one of the building materials of which an organ 

 is composed (see p. 28). * In the hand we have evidence of 

 the presence of bone tissue, muscle tissue, connective tissue, 

 blood tissue, and nerve tissue. Other kinds of tissue will be 

 discussed in the pages that follow. 



2. CELLS, THE UNITS OF LIVING SUBSTANCE 



When we have considered the characteristics of the tissues, 

 we can go no further in our study of structure without the aid 

 of the compound microscope. With this instrument we dis- 

 cover that the tissues are by no means the simplest parts of 

 an animal. In order to get a clear idea of the units of 

 which living substance is composed, let us turn for a time 

 from the study of the human organism and consider some of 

 the lowest of animal forms. 



The Amoeba. The material for our work is best obtained 

 by securing some of the mud and decaying leaves from the 

 bottom of a pool of water. When we come to examine a 

 drop of this sediment with microscope lenses that magnify 

 two or three hundred times, a wonderful scene of life is 

 revealed. Our attention is fixed upon a multitude of minute 



1 It is important to bear in mind that a tissue is not necessarily or 

 usually a thin membrane like tissue paper ; bony tissue, for instance, 

 has considerable thickness and great strength. 



