to the student of mental life. The surgeon or physician must know its shape, the names of its parts, and the outlining walls of its ventricles, because he has to operate upon it. Its more detailed inner structure, as shown by the microscope and by modern histological methods, is of chief concern to the student of psychology. It is not the gross appearance but the composition of the nervous system that throws light upon human learning and conduct.

The reader should, however, in order to understand later descriptions, recall from his studies of physiology1 that the nervous system as a whole is divided into (1) the central nervous system, (2) the nerves passing from it to different parts of the body, (3) the sympathetic system and its isolated ganglia in different parts of the body and (4) the nervous apparatus of the end-organs (eyes, ears, etc.). The central nervous system is further divided into the brain and spinal cord. The brain is further divided into the cerebrum, cerebellum, medulla oblongata and other parts. The cortex of the cerebrum is the gray matter composing its outside layer.

In the descriptions of the figures, the name refers to the source from which the figure was copied. Barker refers to L. F. Barker's, 'The Nervous System and Its Constituent Neurones;' Edinger to L. Edinger's, 'Nervöse Centralorgane,' 5 Auflage;' Kölliker to A. Kölliker's, 'Handbuch der Gewebelehre des Menschen, Zweiter Band, 6 Auflage; Lenhossék to M. v. Lenhossék's, 'Der Feinere Bau des Nervensystems, 2 Auflage; Starr refers to the reproductions of M. Allen Starr's series of photographs of the brain's finer structure in his 'Atlas of Nerve Cells'; Van Gehuchten refers to A. Van Gehuchten's 'Anatomie der Système Nerveux de l'Homme.' Roman numerals refer to the volume, the first arabic numeral to the page, and the second arabic numeral to the number of the figure in the original.

1 The student who has never studied human anatomy and physiology should read the chapters on the nervous system in some standard text book of human physiology.

A.

B.

FIG. 2. A. The brain and spinal cord, viewed from the side, in their relation to the general structure of the body. One-seventh natural size. B. The brain and spinal cord, viewed from the front. Three-seventeenths natural length. After Van Gehuchten, I, 2, 1 and 2.

FIG. 3. A (above). The cerebrum, viewed from the top. Two-fifths natural length. After Van Gehuchten, I, 80, 60.

FIG. 3.

B (below). The cerebrum, viewed from the left side. Two-fifths natural length. After Van Gehuchten, I, 87, 69.

FIG. 4. A section through the cerebrum, showing the appearance to the naked eye of the white and gray matter and the relation of the cortex to the inner substance of the cerebrum. After Edinger, 254, 173.

§ 24. Finer Structure

The Nervous System Equals the Sum of Its Neurones. The nervous system proper (exclusive, that is, of the blood vessels and lymph which permeate it and the tissues which act as connective and supporting structures) is composed of units of structure called neurones or nerve cells. For instance, the optic nerve is essentially a bundle very fine thread-like bodies of protoplasm placed side

of

Thickness is overestimated rela

FIG. 5.

Rough sketches of six neurones.

tively to length.