Look at the skeleton on the right.
The bones of this skeleton are dry bones: all the organic matter is gone. Only the calcified matrix (the bone stuff) is left.

We are going to choose three bones quite different from each others:
- the bone making the front part of the cranium,
- the bone of the arm,
- one of the lower vertebrae.

Cut through the three bones colored in blue by clicking on them with your mouse. Explore the inside of these bones by clicking on the drawings with your mouse. It will provide you with a good idea of what we are going to study next.

These three bones are made up of a hard substance and of several cavities of different sizes and shapes containing blood vessels and bone marrow. The hard substance is the bone tissue itself. Depending of the size, shape and distribution of the cavities, the bone tissues will be categorized as compact or spongy.

By looking at the figures on the right you can already identify the two bone tissues:

- one that looks like swiss cheese or like a sponge (the cheese stuff and sponge stuff being the bone stuff): this is the spongy bone tissue. It is located inside bones.

- one that looks more dense, more compact: the compact bone. It makes the outer part of bones.

Bone tissues are connective tissues: that means that bones cells are scattered in the matrix, far apart from each others. You also know from a previous lab that the bone cells lying in the matrix are enclosed in individual little chambers called lacunae. Other types of bones cells are found on the bone membranes lining up the outer surface of the bones as well as the bone cavities containing blood vessels and bone marrow.

Bone tissues have both organic and inorganic components:
- organic components: the cells and the organic part of the matrix
- inorganic components: inorganic part of the matrix: the calcium salts

Next we are going to take a closer look at the Matrix, the Bones Cells, the Bone Membranes and the two types of Bone Tissue.