THE MUSCULAR SYSTEM 67 



The peristaltic action of plain muscle is seen in the 

 small intestine and in other parts of the alimentary canal. 

 When any part of the tube is stimulated, a circular con- 

 traction results, which slowly passes along in a wavelike 

 manner through the length of the tube. In the diges- 

 tive tract this movement serves to drive the food onward. 



83. Involuntary muscle, as a rule, contracts more slowly 

 than voluntary muscle. It contracts, not with a tetanus 

 like that of voluntary muscle, but with a single, much 

 prolonged contraction. 



84. Mechanism of Movement. The power of the muscles 

 to change their form carries with it the power to change 

 the positions of the bones and other parts of the body to 

 which they are attached, and hence to change the positions 

 of the different parts in respect to one another and to move 

 the whole frame from place to place. 



When a part of the body is moved at a joint, the bone 

 which is moved acts as a lever. A lever is a stiff bar 

 which can be moved round a fixed point, or fulcrum. 

 Three classes of levers are known to the science of 

 mechanics, depending upon the position of the fulcrum 

 with reference to the weight to be moved and the power 

 which produces the motion. In the first class the fulcrum 

 is between the weight and the power, as in using a crowbar 

 to lift the edge of a stone. In the second class the ful- 

 crum is at one end, the power at the other, and the weight 

 between them. In the third class the fulcrum is at one 

 end, the weight at the other, and the power is applied 

 between the two. All three forms of levers are found in 

 the human body, though the levers of the third class are 

 the most numerous (Fig. 45). 



85. Lever of the First Class. The action of a lever of 

 the first class is seen in the straightening of the bent arm. 



