MUSCLES 



373 



666. Contraction of muscles. When a muscle cell is 

 cut or pinched or irritated in any way, it becomes shorter 

 and thicker. This is called a contraction, and is the essen- 

 tial peculiarity of muscles. An end of a motor nerve 

 thread touches every muscle cell and conveys to it orders 

 from the cells of the spinal cord and brain. Each order 

 causes a contraction. 



A muscle cell requires about fa of a second to contract and another 

 fa of a second to become relaxed. So it is impossible to move a limb 

 more than ten times a second. The brain sends about ten orders per 

 second. Thus before the muscle relaxes it receives another order and 

 so remains in a tremulous state of contraction which becomes apparent 

 during excitement or when a great effort is being made. Each con- 

 traction is a change in the shape and not in the size of the muscle. 



I. II. III. 



The three classes of levers, and also the foot as a lever. 



667. Bones as levers. A rigid bar turning about a 

 fixed point or fulcrum is called a lever. When the weight 

 is at one end of the bar and the power at the other end 

 while the fulcrum is between the two, the bar is called a 

 lever of the first class. 



When the weight is between the power and the fulcrum, 

 the bar is called a lever of the second class. 



When the weight is at one end, the fulcrum at the other, 

 and the power between, the bar is called a lever of the 

 third class. 



