48 



PHYSIOLOGY OF MUSCLE AND NERVE 



Theories of Muscular Contraction. — The contracting striated 

 muscle also presents certain microscopic changes which have been em- 

 ployed in the formulation of several theories regarding the manner 

 in which the contraction is brought about. To begin with, it should 



w 



r 



^ 



I ^ 



W 



F 



w 



Fig. 



20. — Different Systems of Levers. 

 F, fulcrum; P, power; W, weight. 



be remembered that the different elements composing a muscle, do not 

 contract simultaneously, but successively, those nearest the seat of the 

 stimulation being activated first. Consequently, the contraction 

 progresses over the muscle in the form of a wave which is directed 

 toward the part farthest removed from the point stimulated. The 



Fig. 21. — Different Positions Given to the Foot in Illustration of the Three 



Systems of Levers. 



details of this wave of contraction may be studied under the micro- 

 scope in fresh preparations of muscle from the legs or wings of insects. 

 Moreover, if a muscle is dropped into alcohol or osmic acid, a series 

 of waves are evoked in its component fibers which are then fixed as 

 nodular or fusiform swellings. 



