The sheath of the optic nerve was studied in rhesus monkeys, humans, and rabbits. The sheath (i.e., dura mater and arachnoid mater) was normally found to be loose near the eyeball, with a much bigger space between the optic nerve and the sheath than anywhere else in its course, consequently presenting bulbous appearance just behind the eyeball. The space was narrowest in the region of the optic canal, with thick fibrous bands stretching from the dura mater to the pia mater of the optic nerve-so that the space assumed the character of a trabecular meshwork of closely knit fibers in the canal, with the subarachnoid and subdural spaces reduced to almost a capillary size in this region. Communication between the subarachnoid spaces of the cranial cavity and of the sheath was almost always seen; however, the extent of communication in the optic canal showed wide variation from one specimen to another. The significance of our findings in the interpretation of some clinical and pathological findings and processes is discussed.