Notes

Every movement of the human body is possible because of the interaction of muscles with the skeletal system. This system is made up of a strong connective tissue called bone. The skeletal system serves as the anchor for all of the body’s movement. It also provides support and protects soft organs inside the body.
Bones are living tissue.
Bones can be sorted into four groups: long bones, short bones, irregular bones, and flat bones. Long bones are found in the arms and legs. Short bones are found in the feet and hands. Irregular bones are found in the spine. Flat bones are found in the ribs and skull.
You might think that bones are completely solid and made up of dead tissue. They actually are made of both hard and soft materials. Like your heart or skin, bones are living tissue. Bones are not completely solid, either; they have spaces inside. The spaces allow blood, carrying nutrients, to travel through the bones. Because bones have spaces, they weigh much less than they would if they were solid.
Types of Bone Tissue
Every bone is made of two types of bone tissue: compact bone and spongy bone. Hard compact bone surrounds the soft spongy bone. Each bone cell lies within a bony web. This web is made up mostly of minerals containing calcium. Click the terms to learn about bone tissue.
Compact Bone
(Compact bone functions as the main supportive tissue of the body. You call this part of the body the skeleton. The outer layer of compact bone is very hard and tough. It covers the outside of most bones.)
Spongy Bone
(Inside the bone, the web of calcium is less dense. This tissue is called spongy bone. Spongy bone is strong but lightweight. It makes up most of the short, flat, and irregular bones found in your body. It also makes up the ends of long bones.)
A Close Look at the Bone: All bone, like the long bone shown here, is made up of compact bone tissue and spongy bone tissie. Drawing of a bone that has been broken so that the inside of the bone can be seen. The outside of the bone is compact bone. It is dense tissue that gives the bone much of its strength. Inside, spongy bone is strong and lightweight. Marrow, part of the spongy bone, produces blood cells. It fills the spaces in spongy bone. Blood vessels inside the bone transport new red blood cells from the marrow and carry materials to and from the bone.
Credit: Houghton Mifflin Company (left); (c) Prof. P.M. Motta/Univ. "La Sapienza", Rome / Photo Researchers, Inc. (right)
Marrow and Blood Cells