Multiple variable in the same plot would cause a messy plot due to the different scales. I recommend to create multiple plots like this:
x<-NULL
for (i in 1:10)
x<-cbind(x, runif(10000, 5, 10))
par(mfrow=c(2,5))
for (i in 1:10)
hist(x[,i], xlab=i)
par(mfrow=c(1,1)) # don't forget to set back the layout