This section is from the book "The Human Body: An Elementary Text-Book Of Anatomy, Physiology, And Hygiene", by H. Newell Martin. Also available from Amazon: The Human Body.
The Globe Of The Eye is on the whole spheroidal, but consists of segments of two spheres (see Fig. 88), a portion of a sphere of smaller radius forming its anterior transparent part, and being set on to the front of its posterior segment, which is part of a larger sphere. In general terms it may be described as consisting of three coats and three refracting media.
The outer coat 1 and 3, Fig. 88, consists of the sclerotic and the cornea, the latter being transparent and situated in front; the former is opaque and white and covers the back and sides of the globe and part of the front, where it is seen between the eyelids as the white of the eye. Both are tough and strong, being composed of dense connective tissue.
The second coat consists of the choroid, 9, 10, and the iris, 14. The choroid consists mainly of blood-vessels supported by loose connective tissue, which in its inner layers contains many dark brown or black pigment granules.* Towards the front of the eyeball, where it begins to diminish in diameter, the choroid separates from the sclerotic and turns in to form the iris, or that colored part of the eye which is seen through the cornea; in the centre of the iris is a circular aperture, the pupil, through which light reaches the interior of the eyeball.
The third or innermost coat of the eye, the retina, 15, is its essential portion, being the part in which the light produces those changes that give rise to nervous impulses in the optic nerve. It lines the posterior half of the eyeball.