| isthmus prostatae | The narrow middle part of the prostate anterior to the urethra. Synonym: isthmus prostatae. (05 Mar 2000) |
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| isthmus rhombencephali | A constriction in the embryonic neural tube delineating the mesencephalon from the rhombencephalon, the anterior portion of the rhombencephalon connecting with the mesencephalon. Synonym: isthmus rhombencephali, isthmus of His. (05 Mar 2000) |
| isthmus tubae auditivae | The narrowest portion of the auditory tube at the junction of the cartilaginous and bony portions. Synonym: isthmus tubae auditivae, isthmus of eustachian tube. (05 Mar 2000) |
| isthmus tubae uterinae | The narrow portion of the uterine tube adjoining the uterus. Synonym: isthmus tubae uterinae. (05 Mar 2000) |
| isthmus uteri | An elongated constriction at the junction of the body and cervix of the uterus. Synonym: isthmus uteri, Guyon's isthmus, orificium internum uteri, os uteri internum, ostium uteri internum. (05 Mar 2000) |
| Kronig's isthmus | The narrow straplike portion of the resonant field that extends over the shoulder, connecting the larger areas of resonance over the pulmonary apex in front and behind. (05 Mar 2000) |