3/6 APPLIED PHYSIOLOGY 



spine and legs are held rigid. If one set overacts, it pulls the body to 

 one side and tends to upset it. Then the opposing set contracts and 

 rights the body. In standing, the two sets continually act in this way. 

 Walking is due to a regular action of the flexor and extensor muscles 

 of the leg, in such a way that there is always one foot upon the ground. 

 In running, the whole body is completely removed from the ground 

 at every step. 



673. Face muscles. The expression of the face is due 

 to flat muscles which are attached to the skin. A circular 

 muscle surrounds the mouth and each eye, while other 

 muscles radiate from their edges. The contractions of 



Illustration of the change of expression produced by the muscles 

 of the mouth. 



these muscles cause the mouth and eyes to assume a great 

 variety of positions. Even the nose may be moved by 

 muscles, and in rare cases the ears also. The different 

 shapes of the mouth and eyes which these muscles produce 

 are reliable indications of the feelings of the mind. 



674. Muscular power. The power which the muscles 

 use is derived from the oxidation both of food and of their 

 own substance. In their work they use about one fourth 

 of the total heat produced in the body. 



A horse can drag about two thirds of its own weight, while an ant 

 can drag 40 times its own weight, and a grasshopper can leap 300 times 

 its own length. In proportion to their size, all insects seem to be far 

 stronger than man. The strength of a muscle depends upon its thick- 



