At birth the eyeball is nearly spherical and has a diameter of about 17 '5 mm. By the age of puberty this has increased to 20 or 21 mm., after which it rapidly reaches its adult size. The eyeball (fig. 558) consists of three concentric tunics or coats, contained within which are three transparent refracting media. The three tunics are (1) an outer fibrous coat, the sclero-cornea, consisting of an opaque posterior part, the sclera, and a transparent anterior portion, the cornea; (2) an intermediate vascular, pigmented, and partly muscular tunic, the tunica vasculosa oculi, com prising from behind forward the Chorioid, the ciliary body, and the iris; (3) an internal nervous tunic, the retina. The three refracting media are named, from before backwards, the aqueous humour, the crystalline lens, and the vitreous body.