IN HUMAN PHYSIOLOGY. 149 



muscle * of the front of the thigh) acts upon the knee- 

 cap ; and the position of the weight is represented by 

 that of the center of gravity of the thigh and leg, which 

 will lie somewhere between the end of the knee and the 

 hip. HUXLEY. 



n. Describe the motions of the bones when we are using 

 a gimlet. 



The radius rolls on the ulna at the elbow, while the 

 ulna rolls on the radius at the wrist. The two combined 

 produce a free, rotary motion. 



12. Why do we tire when we stand erect ? 



(See Physiology, page 49.) 



Because so large a number of muscles must be in con- 

 stant action to maintain this position. 



13. Why does it rest us to change our work ? 



We thereby bring into use a new set of muscles. 



14. Why and when is dancing a beneficial exercise ? 



When dancing is performed out-of-doors, or in a well- 

 ventilated room and at proper hours, it is doubtless a 

 beneficial exercise, since it employs the muscles and 

 pleasantly occupies the mind. Late at night, in a heated 

 room, with thin clothing and exciting surroundings, it is 

 simply a dangerous dissipation, ruinous to the health, alike 

 of body and soul. 



* This muscle is attached above to the haunch-bone or ileum, and below 

 to the knee-cap. The latter bone is connected by a strong ligament with the 

 tibia. 



