ap'erture. 1

An opening. 2. Of a lens, the diameter of the portion of the lens exposed to the light, angular a. The angle included between the extreme rays proceeding from the object to the lens or the angular breadth of the train of light entering the lens, auriculoventricular a. The opening of communication between an auricle and the corresponding ventricle of the heart, mastoid a. Syn.: meatus ad cellulas mas to ideas. The opening of communication between the cavity of the tympanum and the mastoid cells. umbilical a. The opening in the intestinal canal of the embryo at its union with the umbilical vesicle. [Lat., dpertura, from aperire, to open.]