THE MUSCLES 35 



4. Structure of Unstriped Muscle. The visceral muscles, 

 except those of the diaphragm and heart, are not arranged 

 in definite bundles, nor are they striped. They consist of 

 plain, elongated cells which taper toward the ends. -These 

 cells have the power of shortening in the direction of their 

 long axes. But their action is much slower than that of 

 striped muscular cells. Unstriped muscles, therefore, con- 

 tract with a slow, vermicular (worm-like) motion. Unstriped 

 muscles are much lighter in color than striped muscles. 



5. Heart Muscle. This muscle is dark in color, like skeletal 

 muscles. Its action is entirely beyond the control of the will. 

 The histology of heart muscle closely resembles that of skeletal 

 muscles. Its cells are striped, but they are much smaller than 

 the cells of skeletal muscles ; they usually contain only one 

 nucleus, and their ends are branched. These branches fit 

 closely into one another. 



6. Flexion and Extension. The muscles are, for the most 

 part, so arranged in pairs, or corresponding sets, that when 

 motion is produced in one direction by one set, there is, oppo- 

 site to it, another muscle, or group of muscles, which brings 

 the limb back to its place. When they act alternately, a to- 

 and-fro movement results. When a joint is bent, the motion 

 is called flexion ; and when it is made straight again, it is 

 called extension. W T hen both sets act equally and at the same 



PIG. 13. RAISING THE FOREARM. A, Biceps muscle 



moment, no motion is produced, but the body or limb is main- 

 tained in a fixed position: this occurs when we stand erect. 



4. How do visceral muscles differ from skeletal muscles ? 5. Describe the heart muscle, 

 6. What is flexion ? Extension? 



