XIX. MUSCLES (continued). 



Theie Action. 



Our study of the structure of the muscles has taught us 

 that movement is produced by the contraction of the 

 muscles which are connected to 

 movable bones or so arranged as 

 to inclose cavities. Further, we 

 have learned that this power of 

 contraction is located in tiny, 

 microscopic fibers or cells, the 

 sum of whose combined contrac- 

 tions is the movement of the en- 

 tire bundle. Finally, we have seen 

 that all parts of these tiny cells 

 are not equally endowed with this 

 power, but that it is limited 

 to certain contractile substances 

 found in the cell fibers. 



Study of a frog's muscle. — (See 

 Ex. IV.) If we dissect away the bel- 

 lied muscle known as the gastroc- 

 nemius or calf muscle of a frog's 



Fio. 121 -Frog's leg, dissected ^^1^^^ j ^^ gj^g^jj f^^^ j^j^ 

 to show the muscle and nerve ° 



connections ; g, gastrocnemius threadlike flbcr lying cloSO to ItS 



muscle: sc, sciatic nerve : arf, « rni ' n^ i i • . 



pe, etc., other muscles. surtacc. ihis fiber branches mto 



many separate threads, and these, 



in turn, enter the body of the muscle. By means of certain 



stains it is possible to follow these branches and find that 



278 



