The orbital muscle of Müller.

The orbital muscle consists of smooth muscle fibres interspersed with blood lacunae and is therefore both contractile and erectile. Contraction is under sympathetic control, whereas erection appears to be under parasympathetic (pterygopalatine) influence. Apart from its possible effect on the position of the eyeball in the orbit, the muscle seems to be mainly concerned with directing facial venous blood to or away from the cavernous sinus which acts as a heat exchanger for internal carotid blood.