the interior
of the skull and spinal column, and likewise sends processes inward,
for the support and protection of the different parts of the brain. It
also sends processes externally, which form the sheaths for the
nerves, as they quit the skull and spinal column. The dura mater is
supplied with arteries and nerves.

[Illustration: Fig. 124. A vertical section of the cerebrum, cerebellum,
and medulla oblongata, showing the relation of the cranial nerves at
their origin. 1, The cerebrum. 2, The cerebellum, with its arbor vitae
represented. 3, The medulla oblongata. 4, The spinal cord. 5, The corpus
callosum. 6, The first pair of nerves. 7, The second pair. 8, The eye. 9,
The third pair of nerves. 10, The fourth pair. 11, The fifth pair. 12,
The sixth pair. 13, The seventh pair. 14, The eighth pair. 15, The ninth
pair. 16, The tenth pair. 10, The eleventh pair. 18, The twelfth pair.
20, Spinal nerves. 21, The tentorium.]

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738. Describe the dura mater. What is its use? Explain fig. 124.

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739. The ARACHNOID, so called from its extreme tenuity, is the serous
membrane of the brain and spinal cord, and is, like other serous
membranes, a closed sac. It envelops these organs, and is reflected
upon the inner surface of the dura mater, giving to that membrane its
serous investment.

740. The PIA MATER is a vascular membrane, composed of innumerable
vessels, held together by cellular membrane. It invests the whole
surface of the brain, and dips into its convolutions. The pia mater is
the nutrient membrane of the brain, and receives its blood from the
carotid and vertebral arteries. Its nerves are minute branches of the
sympathetic, which accompany the branches of the arteries.

741. The CRANIAL NERVES, that connect with the brain, are arranged in
twelve pairs. They are called: 1st. The _Ol-fact'o-ry_. 2d. The
_Op'tic_. 3d. The _Mo-to'res Oc-u-lo'rum_. 4th. The _Pa-thet'i-cus_.
5th. The _Tri-fa'cial_. 6th. The _Ab-du-cen'tes_. 7th. The _Por'ti-o
Du'ra_. 8th. The _Por'ti-o Mol'lis_. 9th. The _Glos'so-pha-ryn'gi-al_.
10th. The _Pneu-mo-gas'tric_. 11th. The _Spi'nal Ac'ces-so-ry_. 12th.
The _Hy'po-glos'sal_.

742. The OLFACTORY NERVE (first pair) passes from the cavity of the
skull through many small openings in a plate of the _eth'moid_ bone.
(This plate is called _crib'ri-form_, from its resemblance to a
sieve.) This nerve ramifies upon the membrane that lines the na