Of all the symbols of Christ in the Old Testament the Brazen Serpent certainly is the most "shocking" one. The serpent, symbol of sin and of Satan, a picture of Christ! Not a real serpent, but the brazen likeness of it was lifted upon the desert. So did Christ come in the likeness of sinful flesh taking upon himself the pains of sin, without sin. As the serpent was lifted up on the pole, so he was lifted up on the cross, as as the likeness of the serpent was made of metal as a lasting memorial, so did the death of the Saviour become a lasting memorial in the Holy Sacrifice of the Mass.
Note: All of these symbols can be seen on the outside walls of the Hesburgh Library.