![]() ![]() The bony area that makes up the human eye socket provides exceptional protection to the sclera. Dogs do not seem to use this form of communication with one another and only look for visual information from the eyes of humans. Animal researchers have also found that, in the course of their domestication, dogs have also developed the ability to pick up visual cues from the eyes of humans. It is believed that the conspicuous sclera of the human eye makes it easier for one individual to identify where another individual is looking, increasing the efficacy of this particular form of nonverbal communication, called cooperative eye hypothesis. It is theorized that this adaptation evolved because of our social nature as the eye became a useful communication tool in addition to a sensory organ. This is not just due to the white color of the human sclera, which many other species share, but also to the fact that the human iris is relatively small and comprises a significantly smaller portion of the exposed eye surface compared to other animals. Human eyes are somewhat distinctive in the animal kingdom in that the sclera is very plainly visible whenever the eye is open. Sometimes, very small gray-blue spots can appear on the sclera, a harmless condition called scleral melanocytosis. The sclera, like the cornea, contains a basal endothelium, above which there is the lamina fusca, containing a high count of pigment cells. The middle (third in number provided the first layer being the anterior and outermost and the sixth layer being the posterior and the inner most), thickest layer is also called the stroma. The cornea, unlike the sclera, has six layers. Moreover, the cornea bears more mucopolysaccharide (a carbohydrate that has among its repeating units a nitrogenous sugar, hexosamine) to embed the fibrils. The sclera is opaque due to the irregularity of the Type I collagen fibers, as opposed to the near-uniform thickness and parallel arrangement of the corneal collagen. From outer to innermost, the four layers of the sclera are: The collagen of the sclera is continuous with the cornea. The eyes of all non-human primates had been thought to be dark with small, barely visible sclera, but recent research has suggested that white sclera are not uncommon in chimpanzees, and are also present in other mammals. The ring has disappeared in many groups, including living amphibians, some reptiles and fish, and all mammals. 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, crocodilians, various reptiles, and birds. In many vertebrates, the sclera is reinforced with plates of cartilage or bone, together forming a circular structure called the sclerotic ring. ![]() 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. The sclera's blood vessels are mainly on the surface. The thickness of the sclera varies from 1 mm at the posterior pole to 0.3 mm just behind the insertions of the four rectus muscles. 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. 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 sclera is perforated by many nerves and vessels passing through the posterior scleral foramen, the hole that is formed by the optic nerve. It is continuous with the dura mater and the cornea, and maintains the shape of the eyeball, offering resistance to internal and external forces, and provides an attachment for the extraocular muscle insertions. The sclera forms the posterior five-sixths of the connective tissue coat of the human eyeball. Other mammals with white or pale sclera include chimpanzees, many Sumatran orangutans, some gorillas, and bonobos. The cooperative eye hypothesis suggests that the pale sclera evolved as a method of nonverbal communication that makes it easier for one individual to identify where another individual is looking. In humans, and some other vertebrates, the whole sclera is white or pale, contrasting with the coloured iris. People with dark skin can have naturally darkened sclerae, the result of melanin pigmentation. In the elderly, fatty deposits on the sclera can make it appear slightly yellow. In children, it is thinner and shows some of the underlying pigment, appearing slightly blue. In the development of the embryo, the sclera is derived from the neural crest. The sclera, also known as the white of the eye or, in older literature, as the tunica albuginea oculi, is the opaque, fibrous, protective outer layer of the human eye containing mainly collagen and some crucial elastic fiber. ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |