These parts form really a prolongation outwards of the cerebrum, and are surrounded by processes from the membranes of the brain. The dura mater forms a sheath around the nerve as it passes to the eyeball. This sheath is composed of dense connective tissue, and is continuous with the sclerotic, which is composed of similar tissue. Within this sheath the arachnoid is continued as a loose covering as far as the eyeball, where also it becomes continuous with the sclerotic. The pia mater closely invests the nerve and sends processes amongst its fibres. Between the external sheath or dura mater and the optic nerve there are lymphatic spaces continuous with those of the soft membranes. The nerve itself is composed of medullated nerve fibres running in bundles and enclosed in connective tissue. On reaching the eyeball the nerve bundles pierce the sclerotic, which here forms a perforated membrane (Laimina cribron). At the same time they lose their medullary sheath and are distributed as non-medullated fibres inside the globe. From the optic nerve entrance, which forms a round disc, the fibres radiate outwards around the eyeball where they are in contact with the vitreous. They are very transparent, as the rays of light require to penetrate them in order to reach the rods and cones which are on their outer surface. The nerve fibres, with the rods and cones and the granular layers, constitute the retina.