myelin  sheath),  causing  them  to  appear  gray. Beneath  this  layer  is  the  medulla.  This  is  often called the white matter of the brain, because the nerves  are  myelinated  (covered  with  a  myelin sheath  and  an  outer  covering  called  the  neuri- lemma),  which  gives  them  their  white  appearance (fig.   3-40). The  cortex  of  the  cerebrum  is  irregular.  It bends on itself in folds called convolutions, which are  separated  from  each  other  by  grooves  and fissures.  The  deep  sagittal  cleft,  a  longitudinal fissure,   divides   the   cerebrum   into   two hemispheres.  Other  fissures  further  subdivide  the cerebrum  into  lobes,  each  of  which  serves  a localized,  specific  brain  function  (fig.  3-41).  For example,  the  frontal  lobe  is  associated  with  the higher  mental  processes  such  as  memory,  the parietal lobe is concerned primarily with general sensations, the occipital lobe is related to the sense of sight, and the temporal lobe is concerned with hearing. The cerebellum is situated posteriorly to the brain  stem,  which  is  made  up  of  the  pens,  mid- brain, and medulla oblongata, and inferior to the occipital lobe. It is concerned chiefly with bring- ing  balance,  harmony,  and  coordination  to  the motions  initiated  by  the  cerebrum. Two smaller divisions of the brain, vital to life, are  the  pens  and  the  medulla  oblongata.  The  pens consists chiefly of a mass of white fibers connec- ting   the   other   three   parts   of   the   brain—the cerebrum,  cerebellum,  and  medulla  oblongata. The medulla oblongata is the inferior portion of  the  brain,  the  last  division  before  the  beginning of the spinal cord. It connects to the spinal cord at  the  upper  level  of  the  first  cervical  vertebra (C-1). In it are the centers for the control of heart action,   breathing,   circulation,   and   other   vital processes  such  as  blood  pressure. The outer surface of the brain and spinal cord is covered with three layers of membranes called the meninges. The dura mater is the strong outer layer; the arachnoid membrane is the delicate mid- dle  layer;  and  the  pia  mater  is  the  vascular  inner- most layer that adheres to the surface of the brain and  spinal  cord.  Inflammation  of  the  meninges is   called   meningitis.   The   type   depends   upon whether  the  brain,  spinal  cord,  or  both  are affected. Cerebrospinal  fluid  is  formed  by  a  plexus  (net- work) of blood vessels in the central ventricles of the brain. It is a clear, watery solution similar to blood  plasma.  The  total  quantity  bathing  the spinal cord is about 75 ml. It is constantly being produced  and  reabsorbed.  It  circulates  over  the 3-31 surface  of  the  brain  and  spinal  cord  and  serves as a protective cushion as well as a means of ex- change  for  nutrients  and  waste  materials. Spinal Cord The spinal cord is continuous with the medulla oblongata  and  extends  from  the  foramen magnum,  down  inside  the  atlas,  to  the  lower border of the first lumbar vertebra, where it tapers to  a  point.  The  cord  is  surrounded  by  the  bony walls  of  the  vertebral  canal  (fig.  3-42).  It  is  un- sheathed in the three protective meninges and sur- rounded by adipose tissue and blood vessels. The cord does not completely fill the vertebral canal, nor does it extend the full length of it. The nerve roots serving the lumbar and sacral regions must pass some distance down the canal before mak- ing their exit. Figure  3-42.—Spinal  cord.