332 Our Surroundings 



It should be noted that, although voluntary and involuntary 

 muscles differ from each other in size and in structure, the volun- 

 tary muscles may act involuntarily, as in the case of move- 

 ments made quickly on the sudden appearance of danger. The 

 muscles of the chest which help in respiration are apparently 

 under the control of the will, and it is possible to stop breathing 

 for a time, but this cannot be continued long. The voluntary 

 control is soon overcome and the breathing process is involun- 

 tarily resumed. 



Structure of a Muscle. Examination of a voluntary muscle 

 shows that it is composed of layers and bundles of small fibers. 

 The white network is connective tissue. The fibers are made up 

 of still smaller, thread-like parts of cellular structure. These parts 

 are often two inches long, but so small that they can be observed 

 only by the aid of a microscope. Under the microscope the fibers 

 show a cross-striping. 



Involuntary muscles are called plain muscle fibers, since they 

 show no cross-striping. The heart muscle, however, is an excep- 

 tion. Although involuntary, its fibers are striped. 



Muscles are, for the greater part, arranged in pairs, so that 

 when motion is produced in one direction by one set, there is 

 another muscle, or set of muscles, which brings the limb back to 

 its former position. They are united to bones by tendons, tough 

 cords of tissue, and require a constant supply of food and fresh 

 air, regular exercise, and periods of rest. 



Effect of Exercise. Exercise has great influence on the 

 growth and condition of the muscles. We know from experience 

 that muscles develop and become strong by use, and that they 

 soften and become flabby and weak by disuse. Moreover, the 

 right kind of exercise not only builds up the muscles themselves 

 but reacts on the body as a whole. It causes the blood to circulate 

 more rapidly, increases the supply of food and oxygen for the 

 cells, and hastens the excretion of waste material. Furthermore, 

 exercise aids in keeping the temperature of the body normal, and 

 promotes the activity of the digestive system. 



The immediate effect of exercise, however, is upon the muscles 

 themselves. By use, they become large, firm, and capable of 



