To expand a bit on Ignacio's answer: he's refering to pre-stressed concrete. The idea is to put the metal rods (rebars) under tension then allow the concrete to set around them. Once the concrete has set you release the tension on the rebars and the rebars then exert a compression force on the concrete. This allows the concrete to deal better with stretching forces. As Ignacio says, un-prestressed concrete is weak under tension.

You also see unstressed metal grids being used when concrete is being poured. I'm not sure what benefit you get from these except that they stop cracks in the concrete from compromising the whole structure.

What is the purpose of iron bars in concrete?
Retinitis pigmentosa (RP) patients typically become blind after a period of years in which their eye sight slowly deteriorates due to photoreceptor degeneration. Generally RP patients develop night-blindness first, then tunnel vision, after which finally the central eye sight diminishes leaving them with basically no eye sight, bar some crude light perception. The wiki page on RP has more information when needed. Typically, the process from diagnosis of RP to (near) complete loss of eye sight takes years to decades.

However, Chebat et al. (2007) make mention that their study group, consisting of congenitally blind subjects, includes one person with RP (see table 1 in the linked article). Hence, this implicates that this particular subject was blind from birth due to effects of RP. 

Is anyone familiar with newborns blinded by the effects of RP?


Reference
Chebat et al. Neuroreport 2007;18:1901-4
