7 6 



PHYSIOLOGY FOR BEGINNERS 



same time for a moment in each step. The muscles contract 

 more powerfully and quickly, the thrust 

 forward of the body being accomplished not 

 only by the contraction of the muscles of 

 the calf, but also by the powerful action 

 of the extensor muscles of the leg, 

 straightening the bent knee of the forward 

 leg. 



The Erect Position. The erect posi- 

 tion of the body is maintained by the con- 

 traction of certain muscles, and by the 

 tension of certain ligaments at the joints. 

 A dead body cannot be made to stand 

 erect without artificial support. In a man 

 standing upright the feet form the fixed sup- 

 port, and the muscles which keep him from 

 falling forwards or backwards act from the 

 feet as fixed points and keep in the right 

 position the segments of the body above 

 them. The leg is kept over the ankle, so 

 that the body does not fall forwards or back- 

 wards, by the action of the muscles passing 

 from the back and of those passing from 

 the front of the leg to the foot. The set in 

 front, acting from the foot as the fixed at- 

 tachment, would tilt the leg forwards, and 

 the set behind would tilt it backwards. 

 These two sets contract at the same time 

 so as just to balance each other, and so 

 the leg is kept erect. 



The knee is kept straight in a more 

 simple way, the chief muscles acting being 

 those in front of the thigh, the extensor 

 muscles, which by contracting keep the 

 knee from bending, and so prevent the body 

 from falling backwards. The leg cannot 



FIG. 37. Diagram indi- 

 cating some of the 

 muscles which tend to 

 keep the body erect. 



1., muscles of calf ; IF., 

 of back of thigh; III., 

 of spine. 



i, muscles of front of leg ; 



2, of front of thigh ; 3, . 



of abdominal wall; 4, 5, be extended beyond the straight line, or 

 of front of neck. The the knee be bent the wrong way, on account 

 arrows indicate the o f (h e ligaments of the joint, so that in 



j g nQt necessary for the flexor 



direction of action of 

 the muscles. 



