A brief review of the nervous system and its parts will be necessary before we can proceed with a discussion of the phenomena of nerve action, and decide how much of the phenomena of consciousness may be accounted for by physical causes alone.

7. The Matter of the Nervous System.

The matter of the nerve-system is of two kinds, the one consists of nerve-cells, or round bodies more or less irregular in shape, and gathered or located in groups, constituting ganglia or nerve-centres. The other kind of nerve-matter is of a different color-tubular or fibrous in form, and serves either (1) to connect the nerve-centres with the muscles; (2) those centres with the glands and other vital organs, which are not muscular, or (3) to connect these nerve-centres or ganglia with one another. In this last case they are called commissures. In all others they are called simply nerves.

The ganglia or nerve centres are the centres of force. The fibres connect these centres with other parts of the body, and with one another. They serve merely as conducting agents; although it is true that they have some sensibility and may be excited, if the excitants are applied in the midway between. their two extremities. The telegraphic apparatus furnishes a good illustration of the relation of these two parts of the nervous system. The ganglia-consisting of the nerve-cells, are the batteries, and the fibrous nerves are the wires that conduct or convey the currents from the battery to the register.

8. Two Classes of Nerve Fibres.

These nerve-fibres are called in reference to the direction in which they carry their currents, afferent and efferent-sensor and motor, or finally centripetal and centrifugal. If they carry a current up from the skin, or any external organ or tissue to

the ganglia or nerve-centres they are called afferent, sensory, or centripetal; and if they carry it outward from this centre, they are called efferent, motor, or centrifugal.

Thus, if a sharp pain, like the cut of a knife, should be caused in the fore finger-the effect would pass up the afferent fibres of the nerve-hence called sensory fibres, to the ganglionic or grey matter in the spinal cord. It would then cause an unusual activity in those nerve-cells; and one effect would be that they would transmit down the efferent-fibres-hence called motor-fibres-an influence which would cause the contraction of the muscles concerned, so that the hand would be jerked away. And this result, as we all know, might occur in spite of any effort we could make. The pain might be so severe and so sudden, that no one could hold his hand still.

Now what I have described is called reflex action. And it is of several kinds, according to the objects that cause it; but more especially according to the nerve-centres in which it originates. Of this we shall have much more to say as we proceed with our subject.

Beginning, for the convenience of the thing, with the spinal cord, we have in it both kinds of nerve-matter. In the human being we have a continuous column of gray or nerve-cell matter of an irregular contour, surrounded by white or fibrous matter. The gray matter is shaped something like the letter X, and the white or fibrous matter is considered as consisting of four columns, one in each of the angles-called the anterior, the posterior and the two lateral columns.

From the gray matter there come out two pairs of nerveroots, between each pair of vertebræ. All the posterior roots of each pair coming from the posterior cornua of the gray matter are afferent nerves or sensory fibres. The anterior roots are all efferent or motor fibres. And the two roots or branches, that is the two on each side-one sensory, and one motor, unite, entering the same nerve shaft, soon after leaving the spinal column, and form what appears, at first, to be one

nerve. It is soon divided, however, and subdivided into branches, and in all their ramifications and subdivisions-until the very last separation of the ultimate fibres-the two sensory and motor fibres go together, so that whenever a sensory fibre goes to the skin or any tissue below it, the motor fibres are distributed to the muscles that are in that immediate region. Hence, a sharp pain in one hand, moves the hand—that same hand and not the other; nor yet one of the feet.

9, The Ganglionic Masses.

As we ascend we find that the spinal cord enters the skull through what is called the foramen magnum. It there undergoes many changes, which it will be well for us to note, although we can do it only in the most general way.

First, there appear two olive-shaped ganglia-the exact object and function of which is not known. Von der Kolk has thought that they are intimately connected with emotional expression.

The two lateral columns double over backwards, and on their extremity, there is developed a ganglionic enlargement called the Cerebellum or little brain; the office and function of which is by no means well known, though it is doubtless, in some way connected with animal motion.

Soon after entering the skull, the spinal column enlarges, by an increase of gray matter or nerve-cells, and by commissural nerve-fibres. This enlarged portion of the cord is called the medulla oblongata. The white band of nerve-fibres passing around it in front is called the Pons Varolii. But within this band, the posterior and the anterior columns of the cord, pass up and forward.'

On the upper side, as they bend forward to pass over the sella turcica there arise, in the higher mammalia, two or four protuberances, called the corpora quadrigemina. They are in man, in shape, a little like two pea-nuts side by side, and are inti

mately connected with the optic nerves-and the organs of sight.

A little farther up the posterior column-or columns, for it seems to be nearly divided into two by a deep fissure-appears to terminate in two large egg-shaped masses called the optic thalami.

The anterior column passes farther on towards the forehead and appears to terminate in two pear-shaped bodies called the corpora striata. Farther to the front there extend the olfactory nerves, which pass into the nose and become the organ of the sense of smell. These "olfactory nerves" as they are called, are rather prolongations of the hemispheres themselves, than nerves, tending to separate ganglia.

10. The Hemispheres of the Brain.

But the most important part of the encephalic mass remains to be described-the cerebrum, or the hemisphere which constitute the brain proper. The hemispheres appear at first, in the lower vertebrates to be scarcely anything more than two rudimentary sacs, formed on the end of some of the fibres, that pass out from the optic lobes. But as we ascend in the scale up to man, they enlarge, until we find them in man, two large masses packed side by side extending from the forehead to the occiput. On the outside they consist mostly of gray matter or nerve-cells, but within they are vinter fibrous, with a hollow or ventricle in the centre of each. And as we ascend in the scale, we find, besides several smaller commissures connecting the basilar ganglia, a large band of white nerve-fibres called the corpus callosum, connecting the two hemispheres and serving to keep them in place and position.

In the lower vertebrates, as fish, for example, the spinal column, from beginning to end, from the caudal extremity to the end of the nose, is in one continuous horizontal line. In some cases the cerebellum appears immediately on entering the

skull; and in others this development appears to be but a rudimentary union of the two lateral columns. The corpora quadrigemina and the optic thalami are not distinguishable from one another, and constitute the second bulbous enlargement after entering the skull; the cerebellum where there is one, being the first.

The next mass, distinguishable by the eye, has been regarded as either the corpora striata, with the hemispheres as a mere rudimentary sac, extending over them, or as the hemispheres themselves with the corpora striata at their base, as yet rudimentary and scarcely visible.

But the hemispheres themselves are connected organically at this stage, with the anterior end of the optic lobes, which afterwards become distinct from the corpora quadrigemina as the optic thalami.

In this stage of development the hemispheres are not distinguished from the corpora striata, and are really developments from the optic thalami although extending in a line horizontal with them towards the forehead. But as we ascend in the scale the hemispheres gradually enlarge until, in man, they extend from the forehead to the hindermost part of the skull, running over the olfactory ganglia in front and the cerebellum behind.

And with this development, the spinal column becomes curved so that instead of the one horizontal line in the fishes, we have, in man, a nearly perpendicular column bending over until it becomes horizontal as it presses forward on the base of the skull.

In the lower vertebrates the outer surface of the hemispheres is smooth. But in the fishes it begins to be corrugated or rather wrinkled up and marked off with fissures and convolutions. These fissures and convolutions are pretty uniform in animals of the same species; so much so that they might serve as a means of discriminating one species from another.

In man the hemispheres are connected at the bottom by the corpus callosum on which as a sort of a bridge or arch, they