Posterior segment anatomy
he posterior segment comprises the remaining internal structures which lie posterior to the anterior segment. Note that this excludes the ciliary body which is considered to be part of the anterior segment. Seems obvious but you'd be surprised how many medical students trip over that at least once.
Vitreous (hyaloid) body
In cross section, the vitreous body appears as an almost doughnut-like structure with a potential tube-like space extending from the anterior hyaloid face to the posterior hyaloid just anterior to the macula. The vitreous body is encased in a clear hyaloid membrane akin to the outer membrane seen in the white of an egg. And, as in the egg white, can sometimes have strands which will show up as "floaters".
With advancing age the gel liquefies, forming pockets of aqueous fluid. The collagen fibrils tend to be coalesce into thick strands as the process continues. Eventually, if they become large enough and lie in the visual axis, they can be seen as grayish floaters. These floaters may disappear only to reappear over days, months, or years. As the process continues, the floaters tend to fall below the visual axis or line of vision. Rapid eye movement, however, may cause them to reappear as the motion of the thinner fluid causes them to swirl around. This tends to happen especially in the morning upon awakening and getting out of bed.
Associated with the degeneration of the vitreous and as it becomes more mobile within the back of the eye, are sometimes seen lightning flashes bright flashes of light seen in the peripheral vision. This occurs due to the movement of the hyaloid face across the nerve fiber layer. By itself this entopic light phenomenon is harmless. However, if it is associated by a curtain partially or completely blocking vision, is a serious association with a retinal detachment. This danger is sufficiently serious that all individuals first experiencing such lightning phenomenon associated with floaters (or 'blackbirds') should seek consultation with an ophthalmologist.
The retina lies on a black membrane (another anti-halation coating the back layer of the iris is the other) which in turn overlies a very leaky collection of blood vessels called the choroid. The bog-like choroid supplies the inner retinal sensory cells with nutrients and oxygen while the retinal arteries and veins, coming in from the center of the optic nerve and overlying the retina, supply the rest. The movement of nutrients into the sensory cells is done by active transfer across cell membranes; that's why vision deteriorates when the retina is detached and no longer in contact with the choroid.