How do the muscles move the bones?--"By shortening or lengthening themselves according to the way the bones are to be moved." Tell how the muscles move your arm at the elbow.--"The muscles in the front part of the arm shorten themselves, to draw my fore-arm toward the shoulder; when I wish to stretch out the fore-arm these muscles lengthen, while another set of muscles shorten, to draw the fore-arm away from the upper arm." What do you say about the muscles because they have the power to shorten and lengthen themselves?--"They are elastic." About how many muscles are there in your whole body?--"About four hundred and fifty." How may these be divided as you study about them?--"They may be divided into the muscles of my head, the muscles of my trunk, and the muscles of my limbs." Of what use are the muscles of your head?--"They cover and move the parts of my head and face." Of what use are the muscles of your trunk?--"They move the parts of my neck and trunk." Of what use are the muscles of your limbs?--"They move the parts of my arms and legs." How can you make your muscles strong?--"By using them." How can you make your muscles weak?--"By not using them." What is necessary to make your muscles strong and healthy?--"They must be used; they must be rested when tired; they must be supplied with pure blood." How should the muscles be used?--"They should be exercised in work or play." How may they be rested?--"I may rest my muscles by changing position; by changing my kind of work or play; or by going to sleep." Explain what you mean by changing your position.--"If I am standing, I must sit or lie down to rest them; if they are tired, because I have been sitting too long, I must rest them by standing, walking, or running." What do you mean by changing the kind of work or play?--"If, in my work or play, my arms become tired, I must do something in which my arms may rest, though other parts of my body may be in exercise." How may you help supply your muscles with good blood?--"By breathing pure air; by taking wholesome food and drink; and by living in the sunlight." How does drinking alcoholic liquors hurt the muscles?--"It makes them weak, and unfit to move the parts of the body." What wonderful muscle moves without your will?--"The heart." How does alcohol hurt the heart?--"It makes it beat too fast." How does "beating too fast" hurt the heart?--"It makes it tired, and sometimes wears it out." See Appendices on Alcohol and Tobacco. * * * * * [Illustration: THE SKIN (very highly magnified).--(From Walker's _Physiology_, 1884.)] A, arteries; V, veins; N, nerves; F, fat cells; E, the outer skin; CL, the color layer; D, the true skin; PT, a perspiratory tube; HF, a hair and hair sac; EP, muscles; SG, oil glands; TC, tactile corpuscles; CT, connective tissue. * * * * * PART VII. FORMULA FOR THE LESSON ON THE SKIN. 1. My skin covers my body.