Bone, is in the first place, the hardest substance in the body (with the exception of the teeth), and contains three parts of mineral matter to only one part of organic or living substance. This composition gives great solidity to bony structures.

We call the assemblage of bones in the body the skeleton, and the uses of it are these: - The first is to protect the soft parts; the second to support them, and so keep the shape of the body; and the third to afford levers by means of which the parts of the body may be moved. All bones are constructed so that they shall be as light as possible, compatible with strength: they are either flat, like some bones of the skull, or long, like some of the bones in the limbs, or irregular in shape. Flat bones are made (in order to have the greatest strength with the least weight) of two plates of solid hard bone, with sponge-like bone between them. If you had a flat bone of one solid piece of the same weight as one of the bones of the skull, it would not be strong enough; and, on the other hand, if you had a flat solid bone as thick as one of the bones of the skull, it would be too heavy. So, too, the long bones are not solid pieces of hard bone; they have hard compact bone on the outside, and on the inside soft spongy bone, and in the middle of this a cavity containing marrow, which we call the medullary cavity. In this way the long bones have the greatest amount of strength combined with lightness. Their section is, as a rule, more or less circular; they are, in fact, hollow cylinders, the strongest part of which is outside, a softer, more spongy part inside, and a hollow in the middle.

We will now refer to the bones that form the Spinal Column, which we call the back-bones. It is not correct to speak of the back-bone, as this spinal column consists originally of thirty-three separate bones, each of which is called a vertebra, two or more being called vertebr‘. They each consist, in the first place, of a solid piece of bone called the body; a solid disc-shaped piece of bone, more or less oval in section, and harder, like any other bone, on the outside than on the inside. This body has what are called processes of bone projecting out from it backwards; they meet together a little distance from it, and form an arch. This arch, therefore, leaves a hole or ring between the body and the processes, so that you have in each vertebra, a body, processes, and a ring. From the back of the arch a process starts, called the spinous process, backwards, and it is from these spinous processes of the vertebral column that the old anatomists gave the name of the spinal column to this set of bones, and so the nervous cord which passes through the rings gets the name of the spinal cord. These vertebr‘ also have upon their processes places for joining them each with the vertebra above and the vertebra below, called articular or joining processes. When they are fitted together by means of these joining processes, as they are in the human body, the rings form a canal, and that canal is the spinal canal, which contains the spinal cord. The canal communicates at its upper end with the cranial cavity, in which is the brain.

Now, to consider the parts of the vertebral column. These thirty-three vertebr‘ are divided in the following way. There are seven in the neck between the head and the chest, called cervical or neck vertebra, and it is a very curious fact, that all animals that belong to the same class as we do (viz., the mammalia), with two exceptions, have each seven cervical vertebr‘ in the neck, whether they have, as in the case of the giraffe, very long necks, or, as in that of the whale, very short ones.

After these seven neck vertebr‘, there come twelve in the back, called dorsal vertebr‘, to which the ribs are attached. They belong especially to the thorax or chest; below them there come five large vertebr‘, called lumbar, or the vertebr‘ of the loins. The bodies of the vertebr‘ get larger as we go down from the head to the last of the lumbar vertebr‘. Then come five more, which in the child are separate from one another, and which in the adult grow together into one bone, called the sacrum.

After that there is in the adult, a little bone, called the coccyx, from its resemblance to the beak of the cuckoo; it is formed by the joining together of four small bones, which correspond to the bones of the tail in most other animals. That makes up thirty-three bones.

The first two cervical vertebr‘ have strange peculiarities. The first of them is called the atlas, because it carries the head. It has no body at all, properly speaking; but is merely a ring of bone, and the place of its body is taken by a curious process called the odontoid or tooth-like process, which projects upwards from the body of the second vertebra or axis. It sticks up into the ring of the atlas; these two vertebr‘ are attached to the head by fibrous bands. This contrivance enables the head to be turned round upon the vertebral column without moving the rest of the vertebral column. The spinal cord, passing through the spinal canal, passes also through the ring of the atlas, and it is necessary in the turning of the head that the spinal cord should not be pressed upon, so there is a strong fibrous band, called the transverse ligament, which stretches from side to side and divides that ring into two parts. In the front part is the odontoid process of the axis, occupying the position that the body of the atlas would have done, and in the hinder part of the ring there is the spinal cord, so that this ring, although it is merely one ring, is divided by the transverse ligament into two parts.