Tendons
There is not room enough on most of the bones for all the muscles to be attached directly to them. Instead of being thus attached directly to the bone, they end in what we call tendons. These tendons are hard and strong, and a very small tendon will lift as much without breaking as quite a large muscle. The muscles are soft and would have to be attached over a large area in order to secure the required strength. The tendons, being so much stronger than the muscles, can be attached to a very small area and yet secure the same amount of power as would result from attaching the muscle itself.
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
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.
Fig. 81. A section of the skin, highly magnified.
Sweat glands