A, Left Hemisphere of Cerebrum. B, Corpus Callosum. C, Optic Thalamus. D, The Pons Varolii. E, Upper extremity of the Spinal Cord. F, The Arbor Vitæ.] [Sidenote: 11. Medulla oblongata? Cranial nerves? Their shape and position?] 11. From the under surface of the cerebrum, and from the front margin of the cerebellum, fibres collect together to form the _medulla oblongata_ (Fig. 43, MA), which, on issuing from the skull, enters the spinal column, and then becomes known as the spinal cord. From the base of the brain, and from the sides of the medulla originate, also, the _cranial nerves_, of which there are twelve pairs. These nerves are round cords of glistening white appearance, and, {155} like the arteries, generally lie remote from the surface of the body, and are well protected from injury. [Illustration: FIG. 43.--THE BASE OF THE BRAIN.] [Sidenote: 12. The spinal cord? Of what composed? How divided? Each half?] 12. THE SPINAL CORD.--The spinal cord, or "marrow," is a cylindrical mass of soft nervous tissue, which occupies a chamber, or tunnel, fashioned for it in the spinal column (Fig. 44). It is composed of the same substances as the brain; but the arrangement is exactly reversed, the white matter encompassing or surrounding the gray matter instead of being encompassed by it. The amount of the white substance is also greatly in excess of the other material. A vertical fissure partly separates the cord into two lateral halves, and each half is composed of two separate bundles of fibres, which are named the anterior and posterior columns. {156} [Illustration: FIG. 44. A, Cerebrum. B, Cerebellum. D, D, Spinal Cord.] [Sidenote: 13. Uses of these columns? Importance of this part of the nervous system? How protected?] 13. These columns have entirely different uses, and each of them unites with a different portion of the nerves which have their origin in the spinal cord. The importance of this part of the nervous system is apparent from the extreme care taken to protect it from external injury. For, while a very slight disturbance of its structure suffices to disarm it of its power, yet so staunch is its bony enclosure, that only by very severe injuries is it put in peril. The three membranes that cover the brain are continued downward so as to envelope and still further shield this delicate organism. [Sidenote: 14. The spinal nerves? The posterior root? The nerves, how arranged? Their office?]