Between the bodies of the vertebr‘ (except the first two) there are placed tough gristly plates or discs which are attached closely and firmly to the bodies of the vertebr‘. They are soft in their inside, and their edges are so firmly attached to the bodies of the vertebr‘, that the vertebr‘ can be broken rather than separated from them. They are called the intervertebral discs, because they are between the vertebr‘. Besides that, the vertebr‘ are joined together by very strong fibrous structures, which we call ligaments; these pass right down in front of them, and also down behind their bodies, inside the spinal canal, thus binding them all together.

Now what is the use of all this contrivance?

Firstly, the protection of the spinal cord. This is frequently given as the fifth or sixth object of the existence of the vertebral column. Why I call it the first is, because in animals that have no spinal cord, there is no vertebral column, and in animals that have a vertebral column there is a spinal cord.

The old naturalists divided animals into two classes, vertebrate animals, and invertebrate animals - animals that have a vertebral column, and animals that have not.

You may think that it is a very extraordinary thing that the animal kingdom should be divided into two great parts, merely because of the presence or absence of a set of bones, but the reason is the one I have just mentioned, viz. that animals that have vertebral columns have spinal cords and brains, and animals that have not vertebral columns have not spinal cords and brains. This division of the old naturalists, although it merely rested originally upon the possession or not of this set of bones, is a sound one, and we have kept it.

The first use, then, of the vertebral column is the protection of the spinal cord. In the next place, it supports the head, the chest, and the upper extremities, which are attached to it; and it is supported by the lower extremities.

I want now to point out to you the reasons why it is constructed in the way I have described.

In the first place, the fact that it is made up of a large number of bones, separated by strong elastic discs, or rather connected by these compressible discs, gives it a certain possibility of movement, and this movement is attained without injury to the spinal cord.

Then another reason why the vertebral column is not made up of one bone, or why the spinal cord is not protected by one solid tube of bone, is, that it is very important that the spinal cord should not receive shocks, and it is very important, too, that the brain should not receive sudden shocks. Any shock communicated to a series of bones like this, separated by fibrous discs, containing softer matter in their interior, is lost before it gets very far, such shock being divided up as it were into a number of shocks, which get less and less the farther they go. We can jump on the ground without any severe shock being communicated to the spinal cord, and thence to the brain.

We will now pass on to the consideration of the bones of the head: they form the Skull, which is supported on the top of the spinal column. In the cranium, the part of the head in which the brain is contained, there are eight bones: the one in front goes by the name of the frontal bone; the two flat ones at the sides and top, because they are as it were the walls of the skull, are called the parietal bones; at the sides, lower down, are the two temporal bones, in which the internal organs of hearing are placed; the bone at the back part of the base of the cranium goes by the name of occipital; it is the bone which rests upon the spinal column. (The two other bones of the cranium need not be noticed here.)

There is a large hole in the occipital bone which is continuous with the hole that passes down through all the rings at the back of the vertebr‘; through that hole in the occipital bone the spinal cord comes up into the skull and joins the brain. So we see that in the cranium there is a large cavity which contains the brain, and is continuous through a hole with the spinal canal formed by the rings at the back of the vertebr‘.

We may therefore say that vertebrate animals have a separate cavity in their body, containing the brain and spinal cord, and that the rest of their body is outside of that, while animals which have no brains or spinal cords have no such additional cavity.

In the skull, besides the cranial cavity, there is the face, in which there axe fourteen bones.

The bones of the cranium and face, with one exception, are fastened together tightly, bone to bone, i.e. immovably fastened together; and the bones of the cranium form an arch over the brain, the strongest possible construction for the protection of the brain.

There are two ways in which they are fastened together - either the bones have irregular edges, and are joined so that the projections of one edge fit into the notches of the other, - a very secure connection, called, from giving the appearance of stitches, a suture - or else the edges are bevelled off so that the edge of one bone fits over the edge of the other in one place, and under it in another. The one bone in the skull which is not fastened immovably to the rest of the bones, is the lower jaw.

We now come to the bones of the chest.