<P> Mammals normally have a pair of eyes . Although mammalian vision is not so excellent as bird vision, it is at least dichromatic for most of mammalian species, with certain families (such as Hominidae) possessing a trichromatic color perception . </P> <P> Dimensions vary only 1--2 mm among humans . The vertical axis is 24 mm; the transverse being larger . At birth it is generally 16--17 mm, enlarging to 22.5--23 mm by three years of age . Between then and age 13 the eye attains its mature size . It weighs 7.5 grams and its volume is roughly 6.5 ml . Along a line through the nodal (central) point of the eye is the optic axis, which is slightly five degrees toward the nose from the visual axis (i.e., that going towards the focused point to the fovea . </P> <P> The structure of the mammalian eye can be divided into three main layers or tunics whose names reflect their basic functions: the fibrous tunic, the vascular tunic, and the nervous tunic . </P> <Ul> <Li> The fibrous tunic, also known as the tunica fibrosa oculi, is the outer layer of the eyeball consisting of the cornea and sclera . The sclera gives the eye most of its white color . It consists of dense connective tissue filled with the protein collagen to both protect the inner components of the eye and maintain its shape . </Li> <Li> The vascular tunic, also known as the tunica vasculosa oculi or the "uvea", is the middle vascularized layer which includes the iris, ciliary body, and choroid . The choroid contains blood vessels that supply the retinal cells with necessary oxygen and remove the waste products of respiration . The choroid gives the inner eye a dark color, which prevents disruptive reflections within the eye . The iris is seen rather than the cornea when looking straight in one's eye due to the latter's transparency, the pupil (central aperture of iris) is black because there is no light reflected out of the interior eye . If an ophthalmoscope is used, one can see the fundus, as well as vessels (which supply additional blood flow to the retina) especially those crossing the optic disk--the point where the optic nerve fibers depart from the eyeball--among others </Li> <Li> The nervous tunic, also known as the tunica nervosa oculi, is the inner sensory layer which includes the retina . <Ul> <Li> Contributing to vision, the retina contains the photosensitive rod and cone cells and associated neurons . To maximise vision and light absorption, the retina is a relatively smooth (but curved) layer . It has two points at which it is different; the fovea and optic disc . The fovea is a dip in the retina directly opposite the lens, which is densely packed with cone cells . It is largely responsible for color vision in humans, and enables high acuity, such as is necessary in reading . The optic disc, sometimes referred to as the anatomical blind spot, is a point on the retina where the optic nerve pierces the retina to connect to the nerve cells on its inside . No photosensitive cells exist at this point, it is thus "blind". Continuous with the retina are the ciliary epithelium and the posterior epithelium of the iris . </Li> <Li> In addition to the rods and cones, a small proportion (about 1 - 2% in humans) of the ganglion cells in the retina are themselves photosensitive through the pigment melanopsin . They are generally most excitable by blue light, about 470--485 nm . Their information is sent to the SCN (suprachiasmatic nuclei), not to the visual center, through the retinohypothalamic tract which is formed as melanopsin - sensitive axons exit the optic nerve . It is primarily these light signals which regulate circadian rhythms in mammals and several other animals . Many, but not all, totally blind individuals have their circadian rhythms adjusted daily in this way . The ipRGCs have other functions as well, such as signaling the need for changing the diameter of the pupil in changing light conditions . </Li> </Ul> </Li> </Ul>

A well labelled diagram of a mammalian eye