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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.
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.