Of the Outer Limbs of Man, and Their Use
Again, the breadth of the breast, being elevated, and exposed to the eyes, displays a wonderful dignity of its condition; of which this is the cause, that God appears to have made man only, as it were, reclining with his face upward: for scarcely any other animal is able to lie upon its back. But He appears to have formed the dumb animals as though lying on one side, and to have pressed them to the earth. For this reason He gave them a narrow breast, and removed from sight, and prostrate towards the earth. But He made that of man open and erect, because, being full of reason given from heaven, it was not befitting that it should be humble or unbecoming. The nipples also gently rising, and crowned with darker and small orbs, add something of beauty; being given to females for the nourishment of their young, to males for grace only, that the breast might not appear misshapen, and, as it were, mutilated. Below this is placed the fiat surface of the belly, about the middle of which the navel distinguishes by a not unbecoming mark, being made for this purpose, that through it the young, while it is in the womb, may be nourished.
Source: On the Workmanship of God (New Advent)