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A) Guinea pig’s skull base showing the tympanic bullas (lower view) (millimeter scale); B) Rat’s skull base showing the tympanic bullas (lower view) (millimeter scale); C) Lateral left-side view of a young guinea pig’s skull (millimeter scale); D) Lateral left-side view of a young-adult rat’s skull - (millimeter scale). 1 - Bulla; 2 - External auditory canal; 3 - Squamous bone; 4 - Posterior air Sinus notch; 5 - Anterior air sinus; 6 - Para-occipital apophysis.
