Page:Popular Science Monthly Volume 26.djvu/754

734 above the medulla is what is known as the pons Varolii. There are masses of cell-matter scattered at irregular intervals through the pons, but it is made up principally of longitudinal and transverse fibers. The longitudinal fibers connect the medulla with the cerebrum, while the transverse fibers unite the halves of the cerebellum. This organ, the cerebellum, is made up of two hemispheres or lateral lobes and a median or central lobe. The cell-matter lies on the outside, the fibers are within. The external surface of the organ has a foliated appearance, caused by its subdivisions into multitudes of thin plates by numerous fissures. This sub-division allows great increase of cell-matter by numerous fine convolutions, and the matter is further augmented through penetration within of arborescent processes of cell-substance.

The next portion of nerve-matter to be noticed is the cerebrum. This makes up more than four fifths of the contents of the encephalon. The cerebrum is egg-shaped, but flattened on its under side, and lies in the cranium with its small end forward. It is divided into halves or hemispheres by a great longitudinal fissure. These halves, however, are connected by a middle portion of nerve-substance called the corpus callosum. The surface of the cerebrum is molded into numerous convolutions marked off from one another by furrows. The cell-matter of the cerebrum is external, it follows the convolutions, and is from one twelfth to one eighth of an inch in thickness.

The hemispheres of the cerebrum have been divided into lobes called the frontal, parietal, occipital, and temporo-sphenoidal lobes. These divisions are in part arbitrary, while in part they rest upon certain primary fissures, such as the fissure of Sylvius and the fissure of Rolando. It should be borne in mind that these different regions of the cerebrum are not distinct departments physiologically independent. The convolutions are exteriorly connected among themselves and also with convolutions of neighboring lobes. They have, besides, interior connections through bundles of fibers which pass from one convolution to the base of an adjacent convolution. If we remove the encephalic mass and look at it from beneath, we see the medulla as a