THE MUSCLES. 



43 



ing that voluntary muscle has. But it will not do this in 

 obedience to the will. Cold contracts, and heat relaxes it. 

 The involuntary muscle in the walls of the intestine con- 

 tracts when food comes in contact with it. Many other 

 influences produce the same effect, but the will which 

 controls the voluntary muscle has no authority over the 

 involuntary. Involuntary muscle never contracts as rap- 

 idly as voluntary muscle sometimes does. It moves in a 

 sluggish way. 



STRUCTURE OF MUSCLE. 



SECTION III. 1, If we examine a little shred of vol- 

 untary muscle with a microscope which 



will magnify four hundred 



times, we shall see that the 



small fibers which are visi- 

 ble to the naked eye are 



made up of still smaller 



libers, and that these have 



dark lines crossing them. 



These microscopic fibers are 

 gathered into bundles, and these again 

 into larger bundles. The muscle is 

 made up of many such bundles, sur- 

 rounded and bound together by fibrous 

 membranes. We can readily see this 

 in a piece of beef. The fibers con- 

 stitute what we call the grain of the 

 meat. *** 26 - 



T FIBERS or INVOLUNTART 



2, Involuntary muscle, when ex- MUSCLE. -i. Fiber soaked iu 

 amined with a microscope, is also aceticacid - 2. Nucleus, 

 seen to consist of fibers. But they are much shorter than 



E" 

 55 



a 



1 



