Because the head of the brutes is bent down towards the ground and looks upon the earth, but the head of man was made erect towards the heaven, his eyes tending upward. For it becomes us to seek what is above, and with our sight to pierce beyond earthly things.
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).
Also cites: Hom. 9. in Hex.