The cerebellum is connected with other parts of the nervous system by strands of white matter on each side, radiating from the center and divided into numerous branches.  Around these branches the gray matter is arranged in a beautiful manner, suggesting the leaves of a tree:  hence its name, arbor vitae, or the tree of life.

The functions of the cerebellum are not certainly known.  It appears to influence the muscles of the body so as to regulate their movements; that is, it serves to bring the various muscular movements into harmonious action.  The mechanism by which it does this has not yet been clearly explained.  In an animal from which the cerebellum has been removed, the functions of life do not appear to be destroyed, but all power of either walking or flying straight is lost.

[Illustration:  Fig. 115.—­A Vertical Section of the Brain.

  A, frontal lobe of the cerebrum;
  B, parietal lobe;
  C, parieto occipital lobe with fissure between this lobe and
  D, the occipital lobe;
  E, cerebellum;
  F, arbor vitae;
  H, pons Varolu;
  K, medulla oblongata;
  L, portion of lobe on the opposite side of brain.

The white curved band above H represents the corpus callosum.]

Disease or injury of the cerebellum usually produces blindness, giddiness, a tendency to move backwards, a staggering, irregular gait, and a feeling of insecurity in maintaining various positions.  There is no loss of consciousness, or other disturbance of the mental functions.

269.  The Membranes of the Brain.  The brain and spinal cord are protected by three important membranes, known as the meninges,—­the dura mater, the arachnoid, and the pia mater.

The outer membrane, the dura mater, is much thicker and stronger than the others, and is composed of white fibrous and elastic connective tissue.  It closely lines the inner surface of the skull, and forms a protective covering for the brain.  Folds of it pass between the several divisions of the brain and serve to protect them.

The arachnoid is a thin membrane which lies beneath the dura mater.  It secretes a serous fluid which keeps the inner surfaces moist.

The pia mater is a very delicate, vascular membrane which covers the convolutions, dips into all the fissures, and even penetrates into the interior of the brain.  It is crowded with blood-vessels, which divide and subdivide very minutely before they penetrate the brain.  The membranes of the brain are sometimes the seat of inflammation, a serious and painful disease, commonly known as brain fever.