This is the system which regulates and controls the action of all the other organs of the body. In us, and in the higher animals generally - i.e., vertebrate animals -there are two nervous systems, one of which corresponds to the nervous system in the lower or invertebrate animals. That we call the great sympathetic nervous system.

I have told you already a little about it. It consists of two chains of nerve-cords running down, one on e ach side of the vertebral column. These nerve-cords have knots upon them at intervals; knots, which we call ganglia, and nervous cords pass from these ganglia and form networks about the great organs or viscera in the thorax and in the abdomen.

In connection with these networks of cords there are other knots or ganglia, and from these smaller nerves are given off to the viscera and to the walls of the blood-vessels.'

This great sympathetic nervous system, which I tell you corresponds to the only nervous system in invei-tebrate animals, like insects, snails, etc., controls the actions of the body, which we cannot influence by means of our wills, such as those of the involuntary muscles of the walls of the stomach, intestines, and blood-vessels, and so it has especial control over the organs of vegetative existence.

The other nervous system consists of the brain tod the spinal cord, and the nerves connected with them.

The brain and spinal cord are situated in a special cavity, consisting, you will remember, of the cranial cavity, and the canal, which runs down behind the bodies of the vertebrae, being formed by the rings, which are themselves formed by the arches at the back of the vertebrae. This canal we call the spinal canal, and it is inside that canal that the spinal cord is lodged.

The spinal cord and brain, besides having the bony walls of this cavity in which they are lodged for their protection, have certain membranes surrounding them. The first of these membranes is hard, tough, and fibrous. It lines the bony cavities, lying next to the bone. Then there is a very fine thin membrane, which closely invests the brain and the spinal cord throughout, and dips into all the irregularities of the surface both of the brain and spinal cord. This fine membrane carries a network of very fine blood-vessels, which are the vessels to supply parts of the brain and spinal cord with blood. Between these two membranes is a third, which is a closed sac of very fine membrane called the arachnoid membrane, from its resemblance to a spider's web, and which secretes fluid in its interior. So you see these great nervous structures are protected in a most remarkable manner. In the first place, in a bony cavity of their own, which is lined with a tough fibrous membrane, so that even if part of the bone should be broken, this protects the nervous substance, which is itself invested closely with a fine membrane, and between these two there is a closed sac which secretes fluid in its interior, and keeps the whole soft and moist These are the investing structures of the brain and spinal cord.

In describing this nervous system, we will consider the spinal cord first.

The spinal cord begins at the top of the spinal canal, where it joins a part of the brain, and it extends downwards to the vertebrae of the loins in the spinal canal, and then it subdivides into a quantity of strands, a kind of leash as it were. The spinal cord and the brain are similar on the two sides; they can be divided into two equal and similar halves; one side is just like the other side. If you take the spinal cord and cut it across and look at the section of it, you find that it is more or less oval in section, and that it has two fissures, one at the front, and one at the back,-the posterior and anterior fissures, as they are called; the posterior fissure is rather deeper than the anterior one, and reaches nearly to the middle, so that they almost cut it into two halves. And the next thing you see is that it does not look quite the same all through; it is not composed of one and the same substance all through; the greater part of it is made up of a white substance, the white matter of the spinal cord; but there is a greyish-red substance in it, disposed similarly in the two halves in such a way that in the section of it it looks somewhat crescent-shaped in each half; this goes by the name of the grey matter of the spinal cord.

These two crescent-shaped patches, as it were, of grey matter are joined across the middle, where the two fissures do not meet, by more of this grey substance, and in the middle of that there is a little canal which runs the whole length of the spinal cord and is called the central canal of the cord. That is the rough anatomy of a section of the cord in a very few words.

You will see also that a considerable number of branches, as it were, leave the spinal cord; these branches are called nerves, and because they leave the spinal cord they are called the spinal nerves. They leave the cord similarly on each side, thus forming pairs, so we speak of pairs of spinal nerves, and there are thirty-one of these pairs. The next thing that you notice about these nerves is that each of them springs from the spinal cord in two pieces; these pieces are called the roots of the nerve, so that each spinal nerve has two roots; one of these roots comes from the front and the other from the back of the spinal cord; these two roots join and form a single trunk, which goes by the name of the spinal nerve. Just before these roots join there is a swelling or knot, which we call the ganglion, upon the posterior root.

If you take the section of the spinal cord you find that the posterior root of .the spinal nerve starts from the posterior horn of the crescent of grey matter, and that the anterior root starts opposite to the anterior end of this crescent of grey matter. Upon the posterior root there is the swelling or ganglion before mentioned, and then the anterior root joins it, and they form the trunk of the spinal nerve; and that is how each of these spinal nerves belonging to the thirty-one pairs of spinal nerves is formed.