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Definition of arachnoid. 1  1 : of or relating to a thin membrane of the brain and spinal cord that lies between the dura mater and the pia mater. 2  2 : covered with or composed of soft loose hairs or fibers.
sheath. a tubular case or envelope. arachnoid sheath the delicate membrane between the pial sheath and the dural sheath of the optic nerve. carotid sheath a portion of the cervical fascia enclosing the carotid artery, internal jugular vein, vagus nerve, and sympathetic nerves supplying the head.connective tissue sheath of Key and Retzius endoneurium. tubular case or envelope. arachnoid sheath the delicate membrane between the pial sheath and the dural sheath of the optic nerve. carotid sheath a portion of the cervical fascia enclosing the carotid artery, internal jugular vein, vagus nerve, and sympathetic nerves supplying the head.