IO 



HUMAN BIOLOGY 



Fig. io. — Three 

 Muscle Fibers 

 from the heart 

 (showing the nu- 

 clei of six cells). 



The Tissues. — As the organs have dif- 

 ferent functions, they must have different 

 structures that they may be adapted to their 

 work. Just as a house must have brick 

 for the chimney, shingles for the roof, 

 and nails to hold the timbers and other 

 parts together, so the body has various 

 tissues to serve different purposes. The 

 bones must not be constructed like the 

 muscles, and the muscles cannot be like 

 the skin. The chief work of the cells is 

 to construct the tissues and repair them. 

 During life changes are constantly going 

 on. Careful little workmen are keeping 

 watch over every part of 

 the body; thrifty little 

 builders are busy in repairing and restor- 

 ing. No sooner is one particle removed 

 than another takes its place. In one di- 

 rection the cells, acting as undertakers, are 

 hurrying away matter which is dead ; in 

 the other direction the unseen builders 

 are filling the vacant places with matter 

 that is living. 



The Seven Tissues. — There are seven 

 kinds of tissues. Two of them, the mus- 

 cular and nervous tissues, are called the 

 master tissues, since they control and ex- 

 pend the energies of the body. The other 

 five tissues are called the supporting tis- 

 sues, since they supply the energy to the 

 master tissues, support them in place, 

 nourish and protect them. 



Fig. ii.— Nerve 

 Cells, showing 

 their branches 

 interlacing. 



