The tendons pass directly into the periosteum, the thick, strong covering of the bones. So strong is this attachment that the bone will often break before the tendon will pull loose. THE SKIN [Sidenote: The skin as an armor] The entire body is covered with skin, which regulates the heat of the body and acts as an armor against blows and cuts which would otherwise injure the delicate nerves and blood vessels beneath. It also serves to some extent to keep out the germs of disease. The skin appears to be smooth, but if you examine it through a strong magnifying glass you will see that it is divided into little areas. The dividing lines do not run straight, however, and the areas are not square like those you find on a checkerboard. [Illustration: FIG. 81. A section of the skin, highly magnified.] [Sidenote: Sweat glands] After looking at the skin with a strong magnifying glass you will think that you must have seen all its irregularities, but if you will look at it with a powerful microscope you will find out many other things. In the first place you will see many little openings in the skin. These little openings make the ridges which divide the skin into little areas. Some of the openings are _sweat glands_, and there is always some perspiration coming out of them. When you are very warm you can see, without the aid of the glass, the drops of perspiration as they come out on the surface of the skin. When you are not very warm you cannot see these drops of perspiration, but they are nevertheless coming out all the time. When the perspiration comes so slowly that you cannot see it, it is called _insensible perspiration_. [Sidenote: Hair] The whole body is covered with hair. You can see the hair on your head and some of the hair on your arms and the backs of your hands, but most of the hair on the body is so fine that you cannot see it without a microscope. Each of the fine hairs on the body has a root that goes through the skin just as the root from each hair on your head goes through it. [Sidenote: Sebaceous glands] Opening into the little pockets in which the hairs stand, are glands that secrete a kind of oily material. They are called _sebaceous glands_ or _follicles_. Sometimes these follicles become stopped up; then the material they secrete becomes thick and cheesy, and the little black points appear on the skin which we call _blackheads_. The white matter which comes out of these blackheads is merely the secretion of the glands from which the water has been absorbed, leaving the solid or cheesy portion. [Sidenote: Nails] There is a part of the skin that we do not usually think of as skin. We mean the _nails_. The finger nails and the toe nails do not look like the rest of the skin of the body, but they are made of just the same kind of cells. The cells of the nails are flat, dead, and closely packed together. There are no sweat glands, or sebaceous glands, or any hairs in the nails. THE DIGESTIVE SYSTEM