These nerves that I have been speaking about are made up of bundles of fibres, nerve fibres, and each of these nerve fibres can be shown to consist of a little tube with a little thin cord running down its centre inside of it. The nerves are made up of these bundles of nerve fibres, and you will see at once that these nerve fibres bear a strict analogy in their construction to the telegraph wires which are laid down under the streets, and consist of copper wire with a coating of gutta-percha. The white matter of the spinal cord is made up also of these nerve fibres running the whole length of the spinal cord. The grey matter is made up of large irregular star-shaped bodies, which go by the name of nerve cells. Now I have made a comparison with the electric telegraph, and you will see from what I have already told you, how far that comparison holds; it holds to a very large extent.

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 con-nected, by means of the white matter beneath it, with the ganglia at the base of the brain, and these ganglia axe 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.