The eye is made up of three coats, which enclose the optically clear aqueous humour, lens, and vitreous body. The outermost coat consists of the cornea and the sclera; the middle coat contains the main blood supply to the eye and consists, from the back forward, of the choroid, the ciliary body, and the iris.
Tritium is a beta-emitting radioactive isotope of hydrogen. Its nucleus consists of one proton and two neutrons, making it three times as heavy as a hydrogen nucleus (with its one proton) and one-and-a-half times as heavy as deuterium (which contains one proton and only one neutron).
The Earth can be divided into four main layers: the solid crust on the outside, the mantle, the outer core and the inner core. Out of them, the crust is the thinnest layer of the Earth, amounting for less than 1% of our planet's volume.
