It's produced mainly by the corrugator muscle, which lowers & pulls together the eyebrows
The orbital portion is thicker and of a reddish color; its fibers form 
a complete ellipse without interruption at the lateral ... For example, the 
corrugator supercilii pulls the eyebrows to the bridge of the nose, making a roof 
over the... [DOC] [TLE] Procerus muscle - Wikipedia [PAR] The procerus is a small pyramidal slip of muscle deep to the superior orbital 
nerve, artery and vein.