left halves of the cord, and the two vertical bars representing the gray matter on the right and left of the fissure.

The white matter consists of fibres, some traversing it in a horizontal and others in a vertical direction, and a connecting substance called neuroglia. The gray matter consists of ganglion cells and a homogeneous gray mass in

FIG. 4.-Diagram illustrating the general relationships of the parts of the brain. A, fore-brain; b, mid-brain; B, cerebellum; C, pons Varolii; D, medulla oblongata; B, C, and D together constitute the hind-brain. (Martin.)

which a majority of recent observers find a net-work of fine axis-cylinders running in all directions.

Thirty-one pairs of nerves enter the spinal cord. Each of these nerves, before entering the cord, divides into a dorsal and ventral part which are called respectively the posterior and the anterior roots of the nerve. The posterior root consists of afferent or sensory fibres, the anterior root of efferent or motor fibres.

FOLDS OF THE CORTEX.

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The brain is much larger than the spinal cord, and much more complex in its structure. The whole brain in the adult male weighs on the average about 50 ounces. Figure 4 illustrates in a general way the position of the various parts of the brain. The fore-brain weighs in man It consists chiefly of the

on the average about 44 ounces. cerebrum, which is divided into two parts known as the cerebral hemispheres by a deep fissure which extends through its middle.

Folds of the Cortex. The gray cortical rind which constitutes the surface of the cerebrum is folded upon itself many times as appears from Figure 5. These folds are called gyri or convolutions. Their effect is to greatly increase the surface of the brain. It is estimated that if the cortex of the brain of a person of average intelligence were unfolded it would be found to have an area of about four square feet. The folds of the human cortex are deeper and more numerous, as a rule, than those of the most intelligent animals, and in the brains of the most highly civilized nations than in those of savages.

For reasons which will be stated in a later chapter, the cortex of the cerebrum is the part of the brain which is supposed to be connected in the closest and most intimate way with intelligence. It is, therefore, important for students of Psychology to pay special attention to it.

If we examine the convolutions of different brains, we shall see that they vary greatly in their details, not only in different individuals, but even in the two hemispheres of the same brain. The convolutions have been divided into primary, secondary and tertiary classes according to the strength and clearness and positiveness with which they

are distinguishable. The primary convolutions have been compared to the large mountain ranges whose height and breadth and direction give to an extensive territory its characteristic features; the secondary convolutions to those subordinate ranges which owe their existence to valleys in the mountain range, running in the same direction; the tertiary convolutions to the small spurs that extend into

FIG. 5.-The brain from the left side. Cb, the cerebral hemispheres forming the main bulk of the fore-brain; Cbl, the cerebellum; Mo, the medulla oblongata; P, the pons Varolii; * the fissure of Sylvius. (Martin.)

the valleys from the side of the ranges. The primary convolutions are distributed in the brains of different individuals and in the two lobes of the same cerebrum with a good deal of regularity. With them, all regularity stops. The depressions between the convolutions are called sulci. Corresponding to primary, secondary and tertiary convolutions are, accordingly, primary, secondary and tertiary sulci.

CORTEX A SYSTEM OF ORGANS.

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Cortex a System of Organs. The cortex is a very complex organ—perhaps we ought to say, system of organs. For it is made up of a vast multitude of nervous elements with immovable fibres connecting them with each other and with other parts of the nervous system. We shall the more clearly realize the reasons for regarding — at least in a provisional way—the cortex as a system of organs, if we bear in mind what these connecting fibres are. They may be divided into four classes.

Sensory Fibres and the Cortex. The first class is composed of sensory fibres. They may be described in brief as the fibres which form the last connecting link between the surface of the body where the sensory impulse starts, and the centre. I say the last connecting link. For the nervous impulse "changes cars," so to speak, a number of times on its way from the surface of the body to the cortex. The first change is made when the sensory impulse reaches the cells in the posterior horns of the spinal cord. Sometimes—as in the case of reflex action, hereafter to be described the sensory impulse travels no farther. But generally it travels upward along fibres which run throughout the entire length of the spinal cord to the medulla oblongata, where these terminal fibres bend at right angles and pass into its gray matter. The sensory impulse is interrupted here- "changes cars"--but passes out of the medulla oblongata through a number of other gray masses, until it finally reaches the cortex. These fibres then, the fibres which form the last connecting link between the various parts of the surface and the centre, are the first of the four classes which terminate in the cortex.

Motor Fibres and the Cortex. The second class of

same motor cell with the muscle. The motor fibres