<P> The human eye is relatively rare for having an iris that is small enough for its position to be plainly visible against the sclera . This makes it easier for one individual to infer where another individual is looking, and the cooperative eye hypothesis suggests this has evolved as a method of nonverbal communication . </P> <P> The sclera forms the posterior five - sixths of the connective tissue coat of the globe . It is continuous with the dura mater and the cornea, and maintains the shape of the globe, offering resistance to internal and external forces, and provides an attachment for the extraocular muscle insertions . The sclera is perforated by many nerves and vessels passing through the posterior scleral foramen, the hole that is formed by the optic nerve . At the optic disc the outer two - thirds of the sclera continues with the dura mater (outer coat of the brain) via the dural sheath of the optic nerve . The inner third joins with some choroidal tissue to form a plate (lamina cribrosa) across the optic nerve with perforations through which the optic fibers (fasciculi) pass . The thickness of the sclera varies from 1mm at the posterior pole to 0.3 mm just behind the rectus muscle insertions . The sclera's blood vessels are mainly on the surface . Along with the vessels of the conjunctiva (which is a thin layer covering the sclera), those in the episclera render the inflamed eye bright red . </P> <P> In many vertebrates, the sclera is reinforced with plates of cartilage or bone, together forming a circular structure called the sclerotic ring . In primitive fish, this ring consists of four plates, but the number is lower in many living ray - finned fishes, and much higher in lobe - finned fishes, various reptiles, and birds . The ring has disappeared in many groups, including living amphibians, some reptiles and fish, and all mammals . </P> <P> The eyes of all non-human primates are dark with small, barely visible sclera . </P>

The fibrous covering of the eye consists of the white outer layer