man."

Nevertheless, it may fairly be regarded as adding to the evidence which has convinced physiologists and psychologists that the brain is the organ of the mind.

QUESTIONS ON THE TEXT.

1. Mention some of the facts that prove the dependence of the mind upon the body.

2. Show how essential to consciousness is a plentiful supply of blood to the brain.

3. What is meant by aphasia?

4. State the details of the American crow-bar case.

5. What is the relation between the size and weight of the brain of an animal, and its position in the scale of intelligence?

SUGGESTIVE QUESTIONS.

1. What is meant by the localization of functions?

2. Have any cases of impairment of memory from injury to the brain come under your observation?

LESSON IV.

THE CENTRAL NERVOUS SYSTEM.

IN the preceding chapter we have considered the evidence which seems to prove that the brain is the organ of the mind. Let us in this chapter endeavor to get an idea of that wonderful mechanism of which the brain constitutes the most conspicuous part. Let us try to get an idea of the central nervous system.

We learned in the last lesson that there is a direct connection between the outside of the body and the brain. If your hand comes in contact with a hot stove, you quickly become aware of it through sensations of touch and of pain. There is an equally direct connection between the brain and the muscles that move the hand. As soon as you become conscious of the sensation of pain you snatch your hand away.

Nerves and Tendons. If you dissect the body of one of the higher animals, you will see some of the machinery by means of which such phenomena are brought about. You will see numerous white cords which look like tendons -those dense white cords in which a muscle terminates, and which attach the muscles to the bones of the body. But that these white cords are not muscles, is shown by the fact that many of them are not connected with muscles

at all, and those which are, usually enter the central part of the muscle, instead of being attached to its end as tendons usually are. These cords are nerves.

If you follow them in one direction, they subdivide into smaller and smaller branches until they become too small to be seen without the aid of the microscope. If you follow them in the opposite direction, they become larger and larger through uniting with similar nerves until they enter a much larger mass, whose structure and appearance differ widely from that of the nerves which enter it. This mass

is called a nerve centre.

Nerve Fibres and Nerve Cells. Nerves are composed of one or more nervous elements called nerve fibres, bound together by connective tissue. The chief constituent of a nerve centre is nerve cells. Nerve fibres and nerve cells differ in density, shape and chemical composition. Fibrous nerve matter contains more water than cellular nerve matter, and is therefore less dense than the latter. They differ in their shape. Fibres are long "thread-like connections," while nerve cells have a great variety of forms. "Some are nearly round; others ovoidal, caudate, stellate, or shaped like a flask or the blade of a paddle." Nerve fibres and nerve cells differ in size. Nerve fibres vary from about 100 to 1000 of an inch in diameter, while nerve cells vary from about 20 to 30 of an inch. It is supposed that there are not less than two and a half millions of sensory nerve fibres alone, while man's entire central nervous system is reckoned to have about three thousand million nerve cells.

Nerve fibres are never found apart from nerve cells. Indeed, recent investigation has shown that the fibre is an

NERVE FIBRES AND NERVE CELLS.

27

outgrowth or prolongation of the cell.1 A nerve cell with its prolongation into a nerve fibre constitutes the unit of the nervous system. The essential element of a nerve fibre is called its axis-cylinder. Near the ending of a nerve fibre it is the only constituent of the fibre that is

D.

Р.

-N.

FIG. 1.

A.

- Isolated body of a large cell from the ventral horn of the spinal cord. Human, X 200 diameters. A, fibre or fibrous element; D, dendrons; N, nucleus with enclosures; P, pigment spot. (Modified from Donaldson.)

left; the other elements-the transparent envelope, called the primitive sheath, and the fatty substance, called the medullary sheath, which the primitive sheath encloses and which usually encloses the axis-cylinder- being wanting.

1 The term neuron is applied to the cell with all of its prolongations, of which the fibre is only one. The other prolongations of a cell are called

dendrons.

Two Functions of the Nervous System. - We may regard the nervous system as a mechanism having two great functions to perform: (1) reporting the condition of the outside world to the individual, and enabling him to control his actions accordingly; and (2) binding the various parts of the body into an interdependent whole.

The first function we are too familiar with to make extended illustration necessary. A person suffering from rheumatism, feeling a draught of cold air, gets up and

FIG. 2.-Longitudinal and transverse (4) sections of nerve fibres. The heavy border represents the medullary sheath, which becomes thicker in the larger fibres. Sciatic nerve. Human, X 400 diameters. (Donaldson.)

closes the window. His nerves report the condition of the outside world; his nerves set in motion the machinerythe proper muscles by means of which he closes the window. The one action may be compared with the telephoning to the fire department of a city that a building in a certain part of it is on fire; the other to the sending of engines to extinguish the fire.

The same illustration may be used to illustrate the second function of the nervous system, the binding together of the various parts of the body into one inter