xiv MUSCLES AND EXERCISE 193 



seen in beefsteak, corned beef, dried beef, lean mutton, 

 boiled chicken, etc. When carefully examined, a piece 

 of dark meat of the chicken, or corned beef, that has been 

 " boiled to rags," shows many bundles of fleshy threads 

 known as fibers, closely 

 packed together and 

 held in place by a very 

 thin sheath. Each fiber 

 when "teased out" with 

 a fine needle will be 

 found to consist of still 

 smaller threads of flesh 



FiG. 109. Showing several bundles of 



Called fibrtlS. A Single musc le fibers cut across. / / single 



fibril, mUSCle fiber, Or bundles. (From Huxley.) 



muscle cell, is covered with a very delicate membrane 

 inclosing a soft semifluid substance known as myosin. 



The involuntary muscle differs from the voluntary in 

 that it is plain instead of striped, has few capillaries and 

 therefore less color, and is sluggish in its action. 



135. How Muscles Work. A muscle cell has the 

 power to contract, or to become shorter and thicker 

 under the influence of a stimulus. It changes its shape, 

 but not its bulk. Since the muscle cells are placed side 

 by side to form the thickness of a muscle, and end to 

 end to form its length, it follows that the muscle as a 

 whole grows shorter and thicker when its fibers contract. 

 The longer a muscle, the greater the amount of contrac- 

 tion possible. 



A muscle will not act unless it is stimulated in some 

 way. Each muscle cell has a little nerve fiber going to 



