Saline is used for two purposes. One it is sometimes used to wash procedure areas. Second it is used to keep a patient hydrated.
In terms of saline being used for hydration, because water diffuses through permeable membranes to areas of higher to lower concentrations they water would naturally do that in the body. This is super dangerous, if a lot of water enters the body without salt to keep the concentration balance in check. A person may overfill cells with water, and can cause dangerous organ swelling and degradation. That is the purpose of saline.
Depth perception is the ability to see the world in 3 dimensions and by doing so to be able to estimate the distance between objects.
Retinal disparity<span> is defined as the way that your left eye and your right eye view slightly different images. </span>
Convergence refers to the eyes' disposition to rotate inward toward each other in a coordinated manner in order to focus effectively on nearby objects. <span>
</span>Disparity and convergence are the two important things that help us to judge distance. W<span>hen looking at a close-up object, your eyes angle inwards towards each other (you become slightly cross-eyed). The extra effort used by the muscles on the outside of each eye gives a clue to the brain about how far away the object is. This is convergence.</span>