Mammalian eye

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. The dimensions of the eyeball 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). The structure of the mammalian eye has a laminar organization that 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. 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. 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.