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 



to 



I 



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. 



