SKELETAL SYSTEMS AND MOVEMENT 



507 



animals such as the dog can express a hm- 

 ited degree of emotion, but in vertebrates 

 below the mammals, the face is expression- 

 less. 



The frontal elevates the eyebrows and 

 causes the transverse wrinkles of the fore- 

 head; the orbicularis oculi closes the eye- 

 lids; the masseter is a muscle of mastication 



Fronta 



Orbicularis ocu 



Platysma 



Occipital 



Masseter 



Orbicularis oris 



Trapezius 



Figure 375. Superficial muscles of the face and neck of man. The facial muscles are of special 

 interest because they make possible the expression of emotions such as joy and anger. (After 

 Gerrish.) 



and raises the lower jaw, closing the mouth; 

 the orbicularis oris closes the lips; and the 

 platysma is a neck muscle that draws the 

 lower jaw and the corners of the mouth 

 downward. 



Physiology of muscle 

 contraction 



The stimuli that bring about muscular 

 contraction may be nervous, chemical, or 

 physical. The reaction is a shortening and 

 thickening of the muscle, but no alteration 

 in volume. Chemical changes of a very com- 

 plicated nature occur, but the exact nature 



of these is not completely understood. How- 

 ever, it is known that when a muscle con- 

 tracts, it functions as a machine which con- 

 verts stored potential chemical energy to 

 mechanical energy. The efficiency of the 

 process of changing chemical energy into 

 mechanical energy is only about 25 to 40 

 per cent. The greater part of the energy 

 liberated is in the form of heat; everyone 

 knows that exercising the muscles makes 

 9ne warmer. About four-fifths of all the 

 body heat is derived from this source. 



It has been demonstrated that in the con- 

 traction of a muscle, oxygen is used, carbon 

 dioxide is released, glycogen disappears, lac- 



