38 APPLIED BIOLOGY 



connective tissue. The muscles contain muscular tissue, and 

 the nerves have nervous tissue. The hard part of bones is 

 bony tissue, and the softer tissue at the ends of bones is called 

 cartilage. In the frog's leg, then, we find epithelium, con- 

 nective tissue, muscular tissue, nervous tissue, bone, and 

 cartilage. 



(L) Examine your own hand. Skin or epithelial tissue covers 

 and protects it. Pull up on the skin, and it is found to be bound 

 (by connective tissue) to the underlying muscles, which you can 

 feel and move. You can also feel bones. Lastly, you have the 

 sense of touch or feeling in the hand ; this indicates the presence of 

 nerves. Name the tissues which you find in your hand. Do you 

 find any which were not seen in the frog's leg ? 



The tissues which have just been examined are the kinds 

 of building materials which form not only the frog's legs, 

 but also its whole body. The same kinds are also in our own 

 bodies. If we were to examine any organ in the frog or in 

 the human body, we should find it made up of two or more 

 of the tissues. For example, the heart is largely composed 

 of muscular tissue, but it has nerves, connective tissue, and 

 epithelium; and the stomach has epithelium, muscles, 

 nerves, and connective tissue. Any backboned animal's 

 body is made up of many kinds of materials or tissues which 

 have different appearances and serve different purposes. 

 And just as the materials (iron, stone, brick, wood, etc.) 

 used in building houses may be put together in various com- 

 binations so as to form many different kinds of buildings for 

 different purposes, so the few kinds of building materials 

 or tissues of an animal's body are united to form organs which 

 are quite different in appearance and purpose. A frog's 

 heart does not resemble a leg muscle and their purposes and 

 work are different, but they are chiefly composed of the same 

 kind of tissue (muscular), because muscular activity is needed 

 for movement in legs and in the heart. In like manner, we 

 find epithelium wherever there is a surface, inside or outside, 



