Lucretius, De Rerum Natura
William Ellery Leonard, Ed.

     And as said before,
That seed is roused in us when once ripe age
Has made our body strong...
As divers causes give to divers things
Impulse and irritation, so one force
In human kind rouses the human seed
To spurt from man. As soon as ever it issues,
Forced from its first abodes, it passes down
In the whole body through the limbs and frame,
Meeting in certain regions of our thews,
And stirs amain the genitals of man.
The goaded regions swell with seed, and then
Comes the delight to dart the same at what
The mad desire so yearns, and body seeks
That object, whence the mind by love is pierced.
For well-nigh each man falleth toward his wound,
And our blood spurts even toward the spot from whence
The stroke wherewith we are strook, and if indeed
The foe be close, the red jet reaches him.
Thus, one who gets a stroke from Venus' shafts-
Whether a boy with limbs effeminate
Assault him, or a woman darting love
From all her body- that one strains to get
Even to the thing whereby he's hit, and longs
To join with it and cast into its frame
The fluid drawn even from within its own.
For the mute craving doth presage delight.