Human eye of the size of the pupil, regulation and suppression of the hormone melatonin and entrainment of the body clock. The eye is not shaped like a perfect sphere, rather it is a fused two-piece unit, composed of the anterior segment and the posterior segment. The anterior segment is made up of the cornea, iris and lens. The cornea is transparent and more curved, and is linked to the larger posterior segment, composed of the vitreous, retina, choroid and the outer white shell called the sclera. The cornea is typically about 11.5 mm (0.3 in) in diameter, and 1/2 mm (500
Composition of the human body are averages of different numbers reported by different references. The adult human body averages ~53% water. This varies substantially by age, sex, and adiposity. In a large sample of adults of all ages and both sexes, the figure for water fraction by weight was found to be 48 ±6% for females and 58 ±8% water for males. Water is ~11% hydrogen by mass but ~67% hydrogen by atomic percent, and these numbers along with the complementary % numbers for oxygen in water, are the largest contributors to overall mass and atomic composition figures. Because of water content, the human body