Hypothesis The bottom or opposing wall of the uticulo-endolymphatic (Bast’s) valve is the moving and thus functional part of the valve. Background 3 1 5 10 3 4 2 20 6 15 17 Material and methods Specimen preparation Two healthy female albino guinea pigs (Harlan Laboratories, UK), weighing 450 g, were used. Animal care and use were approved by the Experimental Animal Committee of Groningen University, protocol No. 2883, in accordance with the principles of the Declaration of Helsinki. The animals were terminated by lethal administration of sodium pentobarbital. After decapitation the bullas were dissected and fixated in a 10% formalin solution, neutrally buffered. Then the bullas were rinsed in aqua-dest. Decalcification in ethylenediaminetetraacetic acid 10% solution (EDTA; Sigma, ED5SS, pH 7.4) took place at a temperature of 50°C in a microwave oven (T/T MEGA microwave histoprocessor, Milestone) in eight sessions of 6 h. After decalcification the bullas were again rinsed with aqua-dest and dehydrated in a graded seven-step ethanol series (30%, 50%, 70%, 90%, 96%, 100%, and 100%). 14 −4 15 16 OPFOS imaging system 1 6 Z X X Y Fig. 1 Orthogonal-Plane Fluorescence Optional Sectioning (OPFOS) set-up. (BE: beam expander, FS: field stop, CL: cylindrical achromat, O: object, OTS: object translation stage, FTS: focusing translation stage, OL: objective lens with colour filter, CCD = charge coupled device camera)  http://bio3d.colorado.edu/imod Results 2 2 2 2 2 Fig. 2 a 7 c b b a c  3 4 Fig. 3 3D-Reconstruction of Bast’s valve as seen from inside the utricule  Fig. 4 3D-Reconstruction of walls of Bast’s valve and the utricular duct. The shape of the utricular duct at the utricular side is that of a flattened funnel. This funnel runs into a very narrow duct  5 Fig. 5 3D-Reconstruction of part of the outer walls of the utricle and utricular duct. Note the small caliber of the utricular duct in relation to the size of surrounding structures  Discussion 3 9 2 6 Fig. 6 Light microscopy image of lip and bottom of Bast’s valve  13 13 2 6 5 5 7 Fig. 7 black arrows grey arrows  3 2 6 12 19 7 11 18 4 5 8 7 6 Conclusion The use of OPFOS imaging techniques and graphical 3D-reconstruction of the uticulo-endolymphatic (Bast’s) valve and its surroundings has given some additional insight in its functioning. It is most likely that opening or closure of the valve occurs through movement of the flexible base away from or toward the relatively rigid valve lip.