THE MUSCLES AND HOW TO USE THEM 



45 



66. Two Great Kinds of Muscles. We shall learn later 

 that all the varied movements of the muscles are carried 

 on under the control of the nervous 

 system. Some of these movements are 

 under the control of our will, while 

 others are managed by the nervous 

 system quite independently of the will. 

 For instance, we know we have it in 

 our power to decide when to move our 

 head, our hands, or our feet. The mus- 

 cles whose duty it is to move them are 

 under the control of our will. We call 

 these voluntary muscles. 



On the other hand, our will has no 

 control over the muscles of the stomach, 

 the heart, or any of 

 the internal organs. 

 The work of these 

 organs goes on night 

 and day without regard to our will. We 

 call these involuntary muscles. 



Experiment 13. To show the general appear- 

 ance of the muscles. Obtain the lower part of a 

 sheep's leg, with most of the lean meat and the FIG. 35. A Portion of 

 hoof left on. One or more of the muscles, with Striped Muscular 

 their bundles of fibers, fasciae, and tendons, are 

 readily made out with a little careful dissection. 



FIG. 34. A Portion 

 of a Striped Mus- 

 cular Fiber. 

 Highly magnified. 



A, fiber separating into 

 disks; B, fibrilke; 

 C, cross-section of a 

 disk. 



Fiber, showing 

 Stripes and Nuclei. 

 Highly magnified. 



Experiment 14. To show the gross structure 

 of muscle. Take a small portion of a large muscle, as a strip of 

 lean corned beef. Pick the bundles and fibers apart with fine needles 

 until they are so fine as to be almost invisible to the naked eye. Con- 

 tinue the experiment with the help of a hand magnifying glass or a 

 microscope. 



