This section is from the book "Health", by W. H. Coefield.
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 vertebrae. 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 vertebrae 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 vertebrae are divided in the following way. There are seven in the neck between the head and the chest, called cervical or neck vertebrae, 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 vertebrae 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 vertebrae, there come twelve in the back, called dorsal vertebrae, to which the ribs are attached. They belong especially to the thorax or chest; below them there come five large vertebrae, called lumbar, or the vertebrae of the loins. The bodies of the vertebrae get larger as we go down from the head to the last of the lumbar vertebrae. 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 vertebrae 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 vertebrae 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.
Between the bodies of the vertebrae (except the first two) there are placed tough gristly plates or discs which are attached closely and firmly to the bodies of the vertebrae. They are soft in their inside, and their edges are so firmly attached to the bodies of the vertebrae, that the vertebrae can be broken rather than separated from them. They are called the intervertebral discs, because they are between the vertebrae. Besides that, the vertebrae 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 thé 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 vertebrae; 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 vertebrae.
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 are 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.