Human anatomy, including structure and development and practical considerations . losi), and on either sidealong the stem of the Sylvian fissure (cisterna fissurae lateralis). Within the medianregion of the cisterna basalis lie the large arterial trunks forming the circle of vessels are invested with delicate sheaths of arachnoid, which accompany thesmaller branches until they enter the vascular membrane to become pial vessels. The arachnoid also contributes sheaths to the cranial nerves as they pass fromtheir superficial origins to the points where they pierce the dura, these she
Human anatomy, including structure and development and practical considerations . losi), and on either sidealong the stem of the Sylvian fissure (cisterna fissurae lateralis). Within the medianregion of the cisterna basalis lie the large arterial trunks forming the circle of vessels are invested with delicate sheaths of arachnoid, which accompany thesmaller branches until they enter the vascular membrane to become pial vessels. The arachnoid also contributes sheaths to the cranial nerves as they pass fromtheir superficial origins to the points where they pierce the dura, these sheaths o\-er-lie those derived from the pia and, as do the latter, accompany the nerve-trunks for a THE MEMBRANES OF THE BRAIN. 1205 variable but usually short distance beyond their emergence from the dural sac. Thearachnoid sheath is especially well marked along the optic nerve, which it follows ;isfar as the eyeball, and completely subdivides the space between the pial and dural Fui. 1039. Pacchionian l)ody Dnia, reflected nicdiallv Cerebral vein Cerebral vein Iacchionian body. Portion of superior surface of right hemisphere covered by pia an<i arachnoid ; dura has been parti) separatedand reflected towards mid-line to expose Pacchionian bodies and cerebral veins, which are seen entering superiorlongitudinal sinus. sheaths into a subdural and a .subarachnoid perineural compartment, directly contin-uous with the corresponding intracranial spaces. As previously noted, the cerebro-spinal fluid secreted within the ventriclesescapes through the openings in the roof of the fourth ventricle—foramen of Magendieand the foramina of Luschka (page 1100)—^into the subarachnoid space. Afterfilling the cisterna magna and the other large spaces on the basal surface of the brainand surrounding the spinal cord, the fluid finds its way into the smaller spaces onthe exterior of the cerebrum. In this manner the entire mass of nervous tissue isenveloped by a more or less extensive cushion of
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