CHAPTER XI
THE SKIN AND KIDNEYS
122. Waste matters.—The food is burned in the cells. As this burning goes on, the smoke goes off by the lungs and the unburned substances, the ashes, go off by the skin and kidneys. The ashes are mostly the minerals of the cells, but there are also some from the burned albumin. All these go back to the blood and are carried to the skin and kidneys.
The skin (×100).
a, b and c epidermis.
d and g tough and thick part of skin.
e sweat gland.
f blood tubes.
h fat pockets.
123. The skin.—The skin covers the whole body. It is strong and keeps the body from being hurt.
124. The epithelium.—The skin is covered with a thin layer of cells like fine scales. These scales are called epithelium, or epidermis. They have no blood tubes or nerves and so have no feeling. You can run a pin under them without feeling pain. They are always growing on their under side and wearing off on their upper side. They keep the nerves and blood tubes of the skin from being hurt.
125. The nails.—The top scales of epithelium at the ends of the fingers become matted together to make the nails. The nails keep the ends of the fingers from being hurt. They can also be used to hold or cut small things. The new parts of the nails form under the skin and push down the older parts. So the nail grows farther than the end of the finger and needs to be cut off. Biting the nails leaves their ends rough. Then they may catch in the clothes and tear into the tender flesh. We ought to keep the nails cut even with the ends of the fingers.