In anatomy, isthmus refers to a constriction between organs. This is a list of anatomical isthmi: Aortic isthmus, section of the aortic arch Cavo-tricuspid isthmus of the right atrium of the heart, a body of fibrous tissue in the lower atrium between the inferior vena cava, and the tricuspid valve Isthmus, the ear side of the eustachian tube Isthmus, narrowed part between the trunk and the splenium of the corpus callosum Isthmus, formation of the shell membrane in birds oviduct's Isthmus lobe, lobe in the prostate Isthmus of cingulate gyrus Isthmus of fauces, opening at the back of the mouth into the throat Isthmus organizer, secondary organizer region at the junction of the midbrain and metencephalon Isthmus tubae uterinae, links the fallopian tube to the uterus Kronig isthmus, band of resonance representing the apex of lung Thyroid isthmus, thin band of tissue connecting some of the lobes that make up the thyroid Uterine isthmus, inferior-posterior part of uterus Isthmus Anatomical isthmus