This section is from the book "Health", by W. H. Coefield.
When the skin of a part of the body is irritated, a message is started along the nerves up to the spinal cord; it goes into the grey matter which we call the nervous centre of the spinal cord; that grey matter is like a telegraph office: it takes note of this message, and then it sends messages along the fibres, which go through the anterior roots of certain spinal nerves to the muscles to which these nerves go, and causes them to contract.
Now, the spinal cord at its upper part joins the brain, and the part of the brain that it joins appears like a direct continuation of the spinal cord; that part goes by the name of the medulla oblongata or prolonged marrow.
Several very important nerves, which I shall have to speak of bye and bye, rise from it.
The fibres of the anterior columns of white matter, along which the stimuli which cause the muscles to contract travel, cross one another in the medulla.
The little central canal of the cord opens out into a wide cavity on the posterior, or rather upper surface of the medulla oblongata. This wide cavity is covered by the middle part of the small brain or cerebellum, which is at the lower part of the back of the head, and has two halves which are precisely similar.
In the brain there are certain cavities which go by the name of ventricles, and the cavity just underneath the small brain and above the prolonged marrow is called the fourth ventricle; then at the upper part of the prolonged marrow, nerve fibres cross over from one side of the small brain to the other, forming the bridge or pons, and the fibres which have run up the spinal cord, right through the prolonged marrow, run underneath these fibres that cross over and between them, and then emerge beyond in two bundles. These bundles go by the name of the legs of the brain; one bundle goes to each side of the great brain or cerebrum, and their fibres run on through certain large bodies in the lower part of the brain, which we call the ganglia at the base of the brain, consisting partly of grey matter and partly of white matter, so that you see now that these ganglia at the base of the brain are connected by fibres, which run right down through the spinal cord into the spinal nerves of the different parts of the body; and from what I said just now you will see that the fibres which form the right anterior column of the cord come from the left side of the brain, and vice versa ; so that injury to one side of the brain causes paralysis of the other side of the body. Surrounding the ganglia at the base of the brain, there is the large mass of brain proper. Now this large mass of substance, unlike the spinal cord, which is white outside and grey inside, has the grey matter outside and the white matter inside, and this is true, both of the large brain and the small brain. The surface of this grey matter and also the quantity of it is increased very considerably by a device, viz. by the doubling of the surface of the brain into a large number of folds, which go by the name of the convolutions of the brain, and are separated from one another by furrows. The two halves of the great brain, like the two halves of the small brain, are connected together by the fibres which run across. These fibres form a thick, hard, white body, that joins the right side of the brain to the left, and so these two sides of the brain are in continual communication with one another. If it were not for the band of white fibres and one or two other structures, one-half would be separated from the other.
The grey matter on the outside of the brain is connected, by means of the white matter beneath it, with the ganglia at the base of the brain, and these ganglia are connected by means of fibres which pass through the two legs of the brain, with the prolonged marrow and the spinal cord, and so by means of the nerves with the different parts of the body.
From the brain, just as from the spinal cord, nerves start; these were divided by the old anatomists into nine pairs, because they pass out of the skull by nine openings on each side; the old anatomists considered that the nerves that pass through one hole belong to one another, and so they counted nine pairs of nerves. Now they are divided into twelve pairs. I am not going to tell you all the pairs of these nerves, but I will tell you of one or two of the more important ones.
The first pair of these cranial nerves are called the olfactory nerves; they go to that part of the nose in which the organ of smell is situated; they are the nerves by which the stimuli which cause the sensation of smell are conveyed to the brain.
These nerves do nothing else; no muscles are caused to contract by means of them, and no other kind of sensation than that of smell is conveyed by them, so you see here we have a different class of nerves, all the spinal nerves having two properties; muscles were moved by means of them, and sensations were conveyed by means of them. The second pair of nerves start one from each side of the brain; they join one another, or rather are connected together by a band of nerve fibres, and then pass forwards, one to each eyeball These are the nerves which belong to the sense of sight; they are called the optic nerves, because they convey the sense of sight to the brain, they do nothing else whatever. These first two pairs of nerves are examples of purely sensory nerves. The third pair are the nerves, by means of which most of the muscles vrhich move the eyes are made to contract. The fourth and sixth pairs supply those muscles which move the eyeball, and which are not supplied by the third pair. In the third, fourth, and sixth pairs of nerves we have examples of nerves which only influence muscles.
The fifth pair of nerves are the largest pair of nerves which start from the brain, and they resemble spinal nerves in that each of them has two roots,-a motor root and a sensory root, with a ganglion upon the latter; they are mixed nerves, motor and sensory. They supply the skin of all the face, the mucous membrane of the mouth and nose, the teeth, and certain muscles of the upper and lower jaws.