those parts of the nervous system,
exclusive of the nerves themselves, which are contained within the
cranium, or skull-bones; they are the _Cer'e-brum_, _Cer-e-bel'lum_,
and _Me-dul'la Ob-lon-ga'ta_. These are invested and protected by the
membranes of the brain, which are called the _Du'ra Ma'ter_,
_A-rach'noid_, and _Pi'a Ma'ter_.

[Illustration: Fig. 120. 1, 1, The scalp turned down. 2, 2, 2, The cut
edge of the bones of the skull. 3, The external strong membrane of the
brain (dura mater,) suspended by a hook. 4, The left hemisphere of the
brain, showing its convolutions.]

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730. Of what does the cerebro-spinal centre consist? How is the
nervous system divided? 731. What does the term brain designate? Name
them. How are they protected? Describe fig. 120.

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732. The CEREBRUM IS divided into two hemispheres, by a cleft, or
fissure. Into this cleft dips a portion of the dura mater, called the
_falx cer'e-bri_, from its resembling a sickle. The apparent design of
this membrane is to relieve the one side from the pressure of the
other, when the head is reclining to either side. Upon the superior
surface of the cerebrum are seen undulating windings, called
_con-vo-lu'tions_. Upon its inferior, or lower surface, each
hemisphere admits of a division into three lobes--the anterior,
middle, and posterior. (Fig. 122, 123)

[Illustration: Fig. 121 A section of the skull-bones and cerebrum. 1, 1,
The skull. 2, 2, the dura mater 3, 3, The cineritious portion of the
cerebrum. 4, 4, The medullary portion. The dark points indicate the
position of divided blood-vessels. 5, 5, The lateral ventricles.]

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732. How is the cerebrum divided? What is the use of the falx cerebri?
What is seen upon the superior surface of the brain? Its inferior?

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733. When the upper part of the hemispheres is removed horizontally
with a sharp knife, a centre of white substance is brought to view.
This is surrounded by a border of gray, which follows the depressions
of the convolutions, and presents a zigzag outline. The divided
surface will be seen studded with numerous small red points, which are
produced by the escape of blood from the division of the minute
arteries and veins. The gray border is called the cortical, or
_cineritious_ portion, while the white central portion is called the
_medullary_. The two hemispheres are connected b