5. Bad positions and tight or poorly-fitting clothing are common causes of flat chests, round shoulders, and other deformities. 6. Tight or high-heeled shoes deform the feet and make the gait awkward. 7. The bones may be easily broken or put out of joint, or the ligaments may be torn by rough play. 8. Alcohol prevents healthy growth. CHAPTER XX. THE MUSCLES AND HOW WE USE THEM. ~1. The Muscles.~--Where do people obtain the beefsteak and the mutton-chops which they eat for breakfast? From the butcher, you will say; and the butcher gets them from the sheep and cattle which he kills. If you will clasp your arm you will notice that the bones are covered by a soft substance, the flesh. When the skin of an animal has been taken off, we can see that some of the flesh is white or yellow and some of it is red. The white or yellow flesh is fat. The red flesh is lean meat, and it is composed of muscles. ~2. The Number of Muscles.~--We have about five hundred different muscles in the body. They are arranged in such a way as to cover the bones and make the body round and beautiful. They are of different forms and sizes. ~3.~ With a very few exceptions the muscles are arranged in pairs; that is, we have two alike of each form and size, one for each side of the body. ~4. How a Muscle is Formed.~--If you will examine a piece of corned or salted beef which has been well boiled, you will notice that it seems to be made up of bundles of small fibres or threads of flesh. With a little care you can pick one of the small fibres into fine threads. Now, if you look at one of these under a microscope you find that it is made of still finer fibres, which are much smaller than the threads of a spider's web. One of these smallest threads is called a _muscular fibre_. Many thousands of muscular fibres are required to make a muscle. [Illustration: MUSCULAR FIBRES.] ~5.~ Most of the muscles are made fast to the bones. Generally, one end is attached to one bone, and the other to another bone. Sometimes one end is made fast to a bone and the other to the skin or to other muscles. ~6. The Tendons.~--Many of the muscles are not joined to the bones directly, but are made fast to them by means of firm cords called _tendons_. If you will place the thumb of your left hand upon the wrist of the right hand, and then work the fingers of the right hand, you may feel these cords moving underneath the skin. ~7. What the Muscles Do.~--With the left hand grasp the right arm just in front of the elbow. Now shut the right hand tightly. Now open it. Repeat several times. The left hand feels something moving in the flesh. The motion is caused by the working of the muscles, which shorten and harden when they act. ~8.~ All the movements of the body are made by means of muscles. When we move our hands, even when we close the mouth or the eyes, or make a wry face, we use the muscles. We could not speak, laugh, sing, or breathe without muscles.