The most prominent part of the nervous system is that inclosed within the skull and the vertebral column. The spinal cord runs all through this column up to the skull. Entering into the skull, it thickens and forms what is called the bulb (medulla oblongata). It then divides into several bodies, which are referred to as the subcortical centers, because they are located below the cortex, which is the surface layer of the cerebrum, or large brain. These subcortical centers contain the central ends of neurons which are links of chains of afferent neurons coming from the higher sense organs and from the sensory points of the skin and the internal organs. Chains of efferent neurons, on the other hand, take their origin in the subcortical centers, reaching at their peripheral ends the motor points of the body, that is, the muscle fibers of our skeletal muscles and of the muscle tissues contained in the alimentary canal and the other internal organs. Above and partly surrounding the subcortical centers are the large brain and the cerebellum or small brain. The ganglion cells of the neurons contained in the cerebrum and cerebellum are all located near the surface or cortex. There seems to be a peculiar advantage--not yet perfectly understood--in having the gray matter spread out over the surface of the cerebrum and cerebellum in as thin a layer as possible. To this end the surface of the cerebrum is much increased by the formation of large folds, separated by deep fissures (see figure 10). In the cerebellum the folds are more numerous and exceedingly fine, and they do not have the appearance of being the product of fissuration. The surface of the cerebrum is estimated to be equal to a square with a side eighteen inches long. Without the fissures the surface would be only about one third of this. The mixture of ganglion cells and fibers making up the gray matter of the brain is illustrated in figures 11 and 12. Both are sections of the cortex of the cerebrum. In figure 11 the cell bodies alone are stained and thus made visible; in figure 12 the fibers alone are stained. [Illustration: FIG. 10.--FRONTAL SECTION OF THE RIGHT CEREBRAL HEMISPHERE.] From what has been said thus far it is clear that certain areas of the cortex must be connected with certain groups [Illustration: FIG. 11.--SECTION OF THE CEREBRAL CORTEX. Only the cell bodies are stained.] [Illustration: FIG. 12.--SECTION OF THE CEREBRAL CORTEX. Only the fibers are stained.] of sensory points or motor points of the body much more directly than with others. This is confirmed by histological, pathological, and experimental investigations. For the eyes and the ears, for the muscles of arms and legs, hands and feet, even the several fingers and toes, the corresponding areas of the cortex--that is, the areas with which there is direct connection--are definitely known. Figure 13 conveys an idea of the relation between certain parts of the brain and the sensory and motor organs of the body.