ELEMENTARY PHYSIOLOGY 



passes in front of the other. A forward motion is thus given to 

 the body, which is maintained without the 

 expenditure of much muscular force. It 

 will be observed that both feet are never 

 off the ground at the same time in walking ; 

 also that, as the forward motion of each 

 leg is a swinging motion, it follows the 

 law of pendulum motion the longer the 

 pendulum the more slowly it swings. 

 Hence the natural step of a child is 

 quicker than that of a tall man. 



Running differs from walking in that 

 the heels are never brought to the 

 ground, and both feet are off the ground 

 at -the same time for a brief period at 

 each stride. The contraction of the 

 muscles is also much more rapid, and, in 

 addition, the sudden straightening of the 

 legs, by the contraction of the muscles 

 of the thighs, adds greatly to the force 

 with which the body is propelled forward. 

 Jumping, resembles running, as far as 

 the action of individual muscles is con- 

 cerned, but both legs act simultaneously 

 instead of alternately, so that the body 

 is thrown forward to a greater distance. 



Fig. 54. Diagram showing 

 the Action of the chief 

 Muscles which keep the 

 Body erect. 



Muscles which tend to keep the 



body from falling forward. 

 i, muscles of the calf; 2, of the 

 back of the thigh ; 3, of the 

 spinal column. 



Muscles which tend to keep the 

 body from falling backward. 



i. muscles of the front of the leg ; 

 2', of the front of the thigh: 3', of 

 the front of the abdomen; 4', of 

 the front of the neck. 

 The arrows indicate the direction 

 in which these muscles act. 



QUESTIONS ON LESSON VII. 



1. Describe the general character and appear- 



ance of muscle. 



2. What is the difference between voluntary 



and involuntary muscles ? Give examples 

 of each. 



3. Describe the manner in which muscles are 



connected with bones. 



4. Describe the action of any one voluntary 



muscle, showing how its form changes 

 during this action. 



5. What is a tendon ? Where are tendons to be 



found ? 



6. Define the terms origin and insertion as- 



applied to muscles. 



7. Why are voluntary muscles generally ar- 



ranged in pairs which act in opposition 

 to each other ? 



8. How is the erect posture of the body main- 



tained ? 



