267.  The Cerebrum.  The three principal masses which make up the brain when viewed as a whole are: 

  1.  The cerebrum, or brain proper.
  2.  The cerebellum, or lesser brain.
  3.  The medulla oblongata.

The cerebrum comprises nearly seven-eighths of the entire mass, and fills the upper part of the skull.  It consists of two halves, the right and left cerebral hemispheres.  These are almost separated from each other by a deep median fissure.  The hemispheres are united at the bottom of the fissure by a mass of white fibers passing from side to side.  Each of these hemispheres is subdivided into three lobes, so that the entire cerebrum is made up of six distinct lobes.

The cerebrum has a peculiar convoluted appearance, its deep folds being separated by fissures, some of them nearly an inch in depth.

It is composed of both white and gray matter.  The former comprises the greater part of the mass, while the latter is spread over the surface in a layer of about 1/8 of an inch thick.  The gray matter is the portion having the highest functions, and its apparent quantity is largely increased by being formed in convolutions.

The convolutions of the cerebrum are without doubt associated with all those higher actions which distinguish man’s life; but all the convolutions are not of equal importance.  Thus it is probable that only the frontal part of the brain is the intellectual region, while certain convolutions are devoted to the service of the senses.

The cerebrum is the chief seat of the sensations, the intellect, the will, and the emotions.  A study of cerebral injuries and diseases, and experiments upon the lower animals, prove that the hemispheres, and more especially the gray matter, are connected with mental states.  The convolutions in the human brain are more prominent than in that of the higher animals, most nearly allied to man, although some species of animals, not especially intelligent, have marked cerebral convolutions.  The higher races of men have more marked convolutions than those less civilized.

A view of the under surface of the brain, which rests on the floor of the skull, shows the origin of important nerves, called the cranial nerves, the cerebellum, the structure connecting the optic nerves (optic commissure), the bridge of nervous matter (pons Varolii) connecting the two hemispheres of the cerebellum, and lastly numerous and well-marked convolutions.

268.  The Cerebellum.  The cerebellum, or lesser brain, lies in the back of the cranium, and is covered over in man by the posterior lobe of the cerebrum.  It is, at it were, astride of the back of the cerebro-spinal axis, and consists of two hemispheres joined by a central mass.  On its under surface is a depression which receives the medulla oblongata.  The cerebellum is separated from the cerebrum by a horizontal partition of membrane, a portion of the dura mater.  In some animals, as in the cat, this partition is partly bone.