Fig. 124.

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 vitæ 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.

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 nasal passages. It is the softest nerve of the body. (Fig. 136.)