Eyes are organs that detect light and convert it into electro-chemical impulses in neurons. The simplest photoreceptor cells in conscious vision connect light to movement. In higher organisms the eye is a complex optical system which collects light from the surrounding environment, regulates its intensity through a diaphragm, focuses it through an adjustable assembly of lenses to form an image, converts this image into a set of electrical signals, and transmits these signals to the brain through complex neural pathways that connect the eye via the optic nerve to the visual cortex and other areas of the brain. Eyes with resolving power have come in ten fundamentally different forms, and 96% of animal species possess a complex optical system. Image-resolving eyes are present in molluscs, chordates and arthropods.
The simplest "eyes", such as those in microorganisms, do nothing but detect whether the surroundings are light or dark, which is sufficient for the entrainment of circadian rhythms. From more complex eyes, retinal photosensitive ganglion cells send signals along the retinohypothalamic tract to the suprachiasmatic nuclei to effect circadian adjustment.
Dire Dawa is one of two chartered cities (astedader akabibi) in Ethiopia (the other being the capital, Addis Ababa). This chartered city is divided administratively into two woredas, the city proper and the non-urban woreda of Gurgura.
Dire Dawa lies in the eastern part of the nation, on the Dechatu River, at the foot of a ring of cliffs that has been described as "somewhat like a cluster of tea-leaves in the bottom of a slop-basin." With a latitude and longitude of 9°36′N41°52′E / 9.600°N 41.867°E / 9.600; 41.867, this city is the second largest in Ethiopia.
The city is an industrial centre, home to several markets and the Aba Tenna Dejazmach Yilma International Airport. Haramaya University is 40 kilometers away from the city.