I2O 



ELEMENTS OF PHYSIOLOGY 



the food is mixed ; in the intestines the muscles keep the 

 partly digested food in motion ; the muscles in the limbs 

 enable us to move and work ; the 

 heart is chiefly muscle ; the muscles 

 in the chest and trunk enable us to 

 breathe ; those in the larynx are used 

 in talking. Muscles have been divided 

 into two classes, voluntary and invol- 

 untary. The first class is under the 

 control of the will, either at all times 

 or part of the time : the second is 

 never under the control of the will ; 

 their work goes on quite independ- 

 ently of the will and even during 

 sleep. Can you assign to their proper 

 classes the muscles that have been 

 named in this paragraph ? 



207. Structure of Muscles. In 

 studying the master tissues we 

 studied muscular tissue in general, 

 and learned how a muscle is able 

 to shorten. The contraction of a 

 muscle is caused by the contraction 



showing the capillary vessels f the individual fiber-cells which 

 compose it. Each fiber shortens in 



length and becomes proportionally thicker ; the sum total 

 of the contractions of these fiber-cells taking place at the 

 same time, makes up the contraction of the whole muscle. 

 The number of fibers lying side by side determines the 

 thickness of the muscle, and the amount of strength with 

 which it can contract ; while the number of fibers lying end 

 to end determines the amount of shortening or contraction 

 of which the muscle is capable. When a muscle is habitu- 

 ally used, it becomes larger, firmer, darker, and stronger. 

 208. Voluntary and involuntary muscles are not con- 



, 



FIG. 104. A Portion of 



Three striated Muscle 



