40 THE HUMAN ]30DT. 



CHAPTER III. 

 THE MUSCLES. 



SECTION I. 1, The muscles are organs of motion. 

 They cover the bones, and constitute the " flesh." They 

 are found also in the walls of the alimentary canal, and 

 of the blood-vessels, and in other inward parts. The heart 

 is chiefly muscle. 



2, The muscles with which we are most familiar, act 

 when we mil that they shall act. We can walk, run, or 

 sit still; we can move our arms or our heads as we will; 

 but we can not stop the beating of our hearts in this way. 

 The movements of the intestines are also independent of 

 our will. 



3, Muscles are accordingly divided into two classes, 

 voluntary, or those which are subject to the will; and 

 involuntary, or those which are not subject to the will. 



4, The voluntary muscles constitute about two-fifths of 

 the weight of the body. They are compactly arranged 

 over the skeleton, most of them being attached to a bone 



SUGGESTIONS TO TEACHERS. 1. A piece of fresli beef will illustrate to 

 the naked eye the gross structure of voluntary muscle. With a micro 

 scope magnifying four hundred times, a small fragment teased out with 

 a needle will show the structure of the fibers. The play of the muscles 

 and tendons in the fore-arm is easily seen in a thin person. The action of 

 involuntary muscle can be seen in the iris, whose muscular fibers contract 

 under the influence of light, but are not subject to the will. 



2. Fibrous tissue is seen between the muscular bundles of beef. Ten- 

 dons show in the leg of a fowl. 



