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.) 743. The OPTIC NERVE (second pair) passes from the interior of the cranium, through an opening in the base of the skull, (_fo-ra´men op´ti-cum_,) to the cavity for the eye. It pierces the coats of the eye, and expands in the retina. 744. The MOTORES OCULORUM (third pair) pass from the brain, through an opening of the _sphe´noid_ bone, (_sphe-noid´al fis´sure_,) to the muscles of the eye. -=-=-=-=-=-=-=-=-=-=-=-= 739. Describe the arachnoid membrane. 740. What is said respecting the pia mater? 741. How many pairs of cranial nerves? Name them. 742. Describe the olfactory nerve. 743. The optic nerve. 744. Describe the motores oculorum. -=-=-=-=-=-=-=-=-=-=-=-= 745. The PATHETICUS (fourth pair) passes from the brain, through the sphenoidal fissure, to the superior oblique muscle of the eye. [Illustration: Fig. 125. The distribution of the third, fourth, and sixth pairs of nerves, to the muscles of the eye. 1, The ball of the eye and rectus externus muscle. 2, The upper jaw. 3, The third pair, distributed to all the muscles of the eye, except the superior oblique, and external rectus. 4, The fourth pair passes to the superior oblique muscle. 6, The sixth pair, is distributed to the external rectus muscle.]